ZOO 121:Classification of animal kingdom

ZOO 121:Classification of animal kingdom

1.      Phylum Protozoa

The Protozoa are a diverse group of unicellular eukaryotic organisms. They were defined as single-celled organisms with animal-like behaviors, such as motility and predation. The terms protozoa and protozoans are also used informally to designate non-photosynthetic protists, such as ciliates, amoebae and flagellates.

Protozoa are almost are free-living types occuring in soil, wet sand, and in fresh, brackish, and salt waters. Habitats of endoparasites vary. Some are intracellular, such as malarial parasites in vertebrates, which are typical Coccidia in most of the cycle. Other parasites, such as Entamoeba histolytica, invade tissues but not individual cells. Most trypanosomes live in the blood plasma of vertebrate hosts. Many other parasites live in the lumen of the digestive tract or sometimes in coelomic cavities of invertebrates, as do certain gregarines. Many Protozoa are uninucleate, others are binucleate or multinucleate, and the number of nuclei also may vary at different stages in a life cycle. Protozoa range in size from 1 to 10⁶ micrometers. Protozoa have not developed tissues and organs.

Characteristics: Microscopic organisms, ranging in size from 10 to 52 micrometers. Many protozoan species are symbionts, some are parasites and  predators of bacteria, algae and other protists. Unicellular, binucleate or multinucleate eukaryotes with varied type of nutrition including photosynthesis,holozoic, parasitism or saprozoic. Asexual and sexual reproduction. Flagella and cilia, where present have the 9+2 structure. Movement are by flagella, cilia or psuedopodia. In some forms the cells are prouped togrther into colonies.

Feeding/ Nutrition

In protozoan feeding, either phagotrophic (holozoic) or saprozoic (osmotrophic) methods predominate in particular species. In addition, chlorophyll-bearing flagellates profit from photosynthesis; Certain species have not been grown in darkness and may be obligate phototrophs.

-Phagotrophic: ingestion of food, followed by digestion in vacuoles, is characteristic of Sarcodina, ciliates, and many flagellates. Formation of food cups, or gulletlike invaginations to enclose prey, is common in more or less ameboid organisms, such as various Sarcodina, many flagellates, and a few Sporozoa. Entrapment in a sticky reticulopodial net occurs in Foraminiferida and certain other Sarcodina. A persistent cytostome and gullet are involved in phagotrophic ciliates and a few flagellates. Many ciliates have buccal organelles (membranes, membranelies, and closely set rows of cilia) arranged to drive particles to the cytostome. Particles pass through the cytostome into the cytopharynx (gullet), at the base of which food vacuoles (gastrioles) are formed. Digestion occurs in such vacuoles.

-Saprozoic feeding involves passage of dissolved foods through the cortex. It is uncertain to what extent diffusion is responsible, but enzymatic activities presumably are involved in uptake of various simple sugars, acetate and butyrate. In addition, external factors, for example, the pH of the medium, may strongly influence uptake of fatty acids and phosphates. -Osmotrophy is process of absorbing nutrients through their cell membranes; or they may feed by phagocytosis, either by engulfing particles of food with pseudopodia (as amoebae do), or taking in food through a mouth-like aperture called a cytostome). All protozoa digest their food in stomach-like compartments called vacuoles). In addition to nuclei, food vacuoles (gastrioles) in phagotrophs, chromatophores and stigma in many phytoflagellates, water-elimination vesicles in many Protozoa, and sometimes other organelles, the cytoplasm may contain mitochondria, Golgi material, pinocytotic vacuoles, stored food materials, endoplasmic reticulum, and sometimes pigments of various kinds.

Pellicle: The pellicle is a thin layer supporting the cell membrane in various protozoa, such as ciliates, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. The pellicle varies from flexible and elastic to rigid. Although somewhat stiff, the pellicle is also flexible and allows the protist to fit into tighter spaces. In ciliates and Apicomplexa, it is formed from closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the euglenoids and the ciliate Paramecium. In some protozoa, the pellicle hosts epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili).

Motility: Organisms traditionally classified as protozoa are abundant in aqueous environments and soil. The group includes flagellates (which move with the help of whip-like structures called flagella), ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of foot-like structures called pseudopodia). Some protozoa are sessile, and do not move at all.

Ecological role Protozoan

As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Transfer of bacterial and algal production to successive trophic levels, Predators. They also control bacteria populations and biomass to some extent. Decomposer decomposition of organic matter, digests cellulose in the rumen of cows and termite guts, and Play a role in nutrient mobilization.

Protozoan infection/Disease

A number of protozoan pathogens are human parasites, causing diseases such as malaria (by Plasmodium), amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), amoebic dysentery etc.

Reproduction/ Life cycle       

Protozoa reproduce asexually by binary fission or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation. Fission, involving nuclear division and replication of organelles, yields two organisms similar in size.

Budding produces two organisms, one smaller than the other. In plasmotomy, a multinucleate organism divides into several, each containing a number of nuclei. Schizogony, characteristic of Sporozoa, follows repeated nuclear division, yielding many uninucleate buds.

 Simple life cycles include a cyst and an active (trophic) stage undergoing growth and reproduction. In certain free-living and parasitic species, no cyst is developed. Dimorphic cycles show two active stages while polymorphic show several active stages. Some protozoa have life phases alternating between proliferative actively feed stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

Classification

Protozoa have been divided traditionally on the basis of their means of locomotion.

Flagellates (e.g., Giardia lamblia, Trichomonus, Trypanosoma,Leishmania)

Sarcodina/Rhizopodia (Amoeboids) (e.g., Entamoeba histolytica)

Sporozoans (e.g.Plasmodium/ malaria parasites)

Ciliates (e.g., Balantidium coli & paramecium)

Classes of protozoa

1) Sarcodina:- These are protozoa with organs of locomotion as pseudopodia which means false foot. Protoplasm divided into ectoderm and endoderm. Posses pseudopodia. One nucleus. Holozoic nutrition. Feed by phagocytosis- engulfing of food by pseudopodia.No sexual reproduction. Live in soil fresh water and damp places. Example Amoeba

2) Mastigophora;- These are protozoa with organs of locomotion as flagella. The numbers of flagella are different in different protozoa. Some contain chloplast eg euglena. Reproduce by binary fission. Found in ponds or pollutted water.Body is covered by thick flexible pellicles( Giardia, Trypanosoma)

3) Cilliata:- These are protozoa which have organs of locomotion as cilia. Cillia are small hairs like structure seen. These are many in numbers on the body of animals. Example Paramecium

4) Sporozoa:- These are protozoa which have no organs of locomotion. The protozoa phylum has been divided into four subphyla according to revised classification of protozoa: 1)    Sarcomastigophora,2)    Sporozoa, 3)    Cindospora & 4)    Ciliophora

2. THE PHYLUM PORIFERA/THE SPONGES

Characteristics of Porifera

Phylum Porifera is a group of simple animals that includes the sponges. Porifera have no internal organs, nervous tissue, circulatory system, or digestive systems, making them the most primitive of the multi-cellular animals. To support and protect their soft bodies, sponges produce skeletons of calcium carbonate, silica, or a soft organic material called spongin. To obtain food, the sponges filtered the water around them as it passed through tiny pores located on their outer walls.

1)No definite symmetry.
2)Body multicellular, few tissues, no organs.
3)Cells and tissues surround a water filled space but there is no true body cavity.
4)All are sessile, (live attached to something as an adult).
5)Reproduce sexually or asexually, sexual reproduction can be either gonochoristic or hermaphroditic.
6)Has no nervous system.
7)Has a distinct larval stage which is planktonic.
8)Lives in aquatic environments, mostly marine.
9)All are filter feeders.
10)Often have a skeleton of spicules.

Morphology

Have 4 very loosely differentiated cells:

Pinacocytes- outer cells covering sponge; equivalent of epiderm

Porococytes- cells which line the pores of the sponge; through which water is drawn

Choanocytes- similar to choanoflagellates; collared cells with flagella which create water current and collect food matter or sticky contractile collar; may also produce sperm.

Amoebocytes- amoeba-like cells found throughout the sponge; store, digest and transport food, excrete wastes, secrete skeleton and also may give rise to buds in asexual reproduction; there are several different types:

Large Amoebocytes- distribute food to other cells of sponge; move by way of pseudopods

Archeocytes- undifferentiated sponge cells that can give rise to more differentiated cells such as pinacocytes, porocytes or oocytes.

Scleroblasts- produce spicules; two types

Calcoblasts- make calcium carbonate spicules

Silicoblasts- make silicious spicules

There are two basic openings in the sponge:

Ostia- or incurrent pores that open into a central cavity called the spongocoel; it is lined with choanocytes or collar cells.

Osculum- the opening of the spongocoel to the outside; this is the opening by which water leaves the sponge.

Sponge types

There are 3 basic architectural types of sponges

Asconoid- most primitive and simplistic in structure have radial symmetry are tube shaped. Typically vase shaped like sponges. The asconoid imposes definite size limits to sponges due to the problem of water flow the spongocoel contains such a large volume of water that it is hard to push it our rapidly if there are no enough choanocytes.

Syconoid Sponges- this is the next level of complexity in sponge architecture; it is derived from the asconoid structure except the walls are invaginated- allowing for greater surface area over which water can pass. Radial symmetry; the syconoid structure helps to rectify some of the water movement problem by increasing the surface area and decreasing the spongocoel volume- so there are more choanocytes to water volume these sponges able to get bigger than asconoid.

Leuconoid Sponges- This is the highest level of complexity in sponges; have lost radial symmetry and are very irregular in shape and may attain large sizes they arise from syconoid sponges in which the invaginated canals are even further invaginated and folded to from small flagellated chambers further increase in surface area makes these sponges highly efficient in moving and filtering water spongocoel is gone except for canals that lead to the osculum- or there may be a series of excurrent openings the largest sponges are leuconoid types because they have eliminated the spongocoel so that all the water entering the sponge is moved through small canals all lined with flagella is the most hydrologically efficient method

Higher classification of sponges

There are 4 classes of sponges:

Calcarea:- found in shallow coastal waters, all are marine.

Hexactinellida:- glass sponges, chiefly live in 500-1000 meter depth are syconoid sponges, all are marine.

Demospongiae:- spicules are silicious if present otherwise skeleton is made of spongin or both variously shaped some are huge all are leuconoid all but two families are marine- Spongillidae and Metaniidae- are freshwater with about 300 freshwater species; in North America are about 27 species in 11 genera (most belong to Spongillidae) this is the group from which we get our commercial sponges Sclerospongiae have silicious spicules and sponging also have an outer covering composed of calcium carbonate are leuconoid sponges.

Reproduction

Sexual Reproduction:-gametes formed by amoebocytes, there are both hermaphroditic and dioecious species; most freshwater species are dioecious most hermaphroditic species produce eggs and sperm at different times so they do not self fertilize sperm is released into environment via osculum and is brought in by another sponge via ostia fertilization takes place in parent sponge. zygote is expelled - called an amphiblastula larva or parenchymula larva, it is planktonic when it reaches a certain size it drops to bottom and begins to develop.

  Asexual Reproduction (two types)

1. Budding- fragmentation of body wall, buds appear as outgrowth on sides of sponge

when they reach a certain size they drop off and settle to bottom to form a new sponge

2. Gemmules- occurs only in freshwater sponges gemmules are groups of food laden amoebocytes that deposit a hard covering of spicules around them formation is triggered by environmental conditions such as decreased temperatures they allow the sponge to pass the winter or periods of drought after which the outer covering breaks open and a new sponge develops

Nutritional Ecology:- All are filter feeders; Choanocytes produce currents that transport water in and out of sponges; Amoebocytes distribute food throughout sponge many have algae symbionts, which makes them green- these supply organic carbon and oxygen to host sponge, sponge supplies habitat and carbon dioxide to algae

3.      PHYLUM CNIDARIA/COELENTERATES

General Characteristicts

1) The coelenterates are radial symmetrical, tentacle bearing (All are aquatic, mostly marine but a few are fresh water forms).

Hydra.

2) They may be solitary or colonial.

3) They may be sedentary or free-swimming.

4) Head and segmentation is absent.

5) Tentacles encircle the mouth in one or more whorls. They are used for food capture, and defense.

6) These are all diploblastic animals. They show ectoderm and endoderm. In between jelly like mesoglea is present.

7) The cnidoblasts are important defensive and offensive cells. They are useful for food capture.

8) Undifferentiated free interestitial cells are found among the epithelial cells.

9)Mouth is present. Anus is absent.

10) Mouth leads into a central cavity called 'Coelenteron. Hence the name Coelenterate'.

11) Coelenterates are acoelomate. Because there is no true body cavity, or coelom.

12) Respiratory, circulatory and excretory systems are absent.

13) These are radial symmetrical animals.

14) Nervous system is diffused type.

15) Polyp is a nutritive zooid and fixed zooid.The medusa is a free swimming zooid and sexual zooid.

16) Nutrition is intercellular and intracellular.

17) Locomotion in medusa is by muscles.

18) In Coelenterates Polymorphism tendency is seen in some examples.

19) Asexual reproduction is by budding.

20) Sexual reproduction takes place by the development of gonad and sex cells. Planula larva is seen.

Classification of Coelenterate (Cnidaria):

Divided into three classes

Class I: Hydrozoa:

1) Hydrozoa animals are multicellular, diploblastic animals.

2) They show mouth opening, and anus is absent.

3) They show both polyp and medusa forms. Medusa is a reproductive zooid. Polyp is a fixed stage.

4) In medusa the gasto-vascular-system is transversed by canals. In medusa definite sense organs like statocyst, nervous system, and muscular system are well developed.

5) Polymorphic tendency is well developed.

6) Gonads are seen.

7) Alternation of generations is seen in the life history of these animals.

8) Velum is present on the medusa (Craspedote); (Examples: Hydra, Bougainvillea &Obelia).

 

Class II: Scyphozoa:

1) Represented by medusoid forms.

2) Sense organs are tentaculocysts.

3) Gastrovascular system shows stomach and 4 gastric pouches. In the gastric pouches gastric filaments are present.

4) Velarium is present with endodermal canal (Acraspedote).

5) Gonads are endodermal in origin

6) Medusa arises by strobilisation.

Examples: Aurelia -Jelly fish

Class III: Anthozoa:

1) They exhibit only polyp forms.

2) Medusa stage is absent.

3) Mesentries are present, they bear nematocyst.

4) Gonads are endodermal.

5) Corals coral reefs are common.

This class is divided into 2 sub classes.

Examples: Sea anemones, Corals e.g. Meandrina (brain coral), Black coral, orange pipe coral, dead man's fingers,

Distinguishing features

Cnidarians form an animal phylum that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs. Cnidarians are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey. In some species, cnidocytes can also be used as anchors. Like sponges, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no intermediate jelly-like layer. Cnidarians have traditionally been labelled diploblastic, along with sponges. Both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer. As a result, some recent text books classify ctenophores as triploblastic, and it has been suggested that cnidarians evolved from triploblastic ancestors.

	Sponges	Cnidarians	Ctenophores	Bilateria
Cnidocytes	No	Yes	No
Colloblasts	No		Yes	No
Digestive and circulatory organs	No			Yes
Number of main cell layers	Two, with jelly-like layer between them		Two or Three	Three
Cells in each layer bound together	cell-adhesion molecules, but no basement membranes except Homoscleromorpha.	inter-cell connections; basement membranes
Sensory organs	No	Yes
Number of cells in middle "jelly" layer	Many	Few		(Not applicable)
Cells in outer layers can move inwards and change functions	Yes	No		(Not applicable)
Nervous system	No	Yes, simple		Simple to complex
Muscles	None	Mostly epitheliomuscular	Mostly myoepithelial	Mostly myocytes

Basic body forms Cnidarians

Medusa (left) and polyp (right)

Aboral end, Oral end, Mouth,

     Exoderm

     Gastroderm (Endoderm)

     Mesoglea

     Digestive cavity

Adult cnidarians appear as either swimming medusae or sessile polyps, and many hydrozoan species are known to alternate between the two forms.

Polymorphism

Polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism. It is a characteristic feature of Cnidarians, particularly the polyp and medusa forms, or of zooids within colonial organisms like those in Hydrozoa. In Hydrozoans, colonial individuals arising from individuals zooids will take on separate tasks.  For example, in Obelia there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusae.

Cnidocytes

These "nettle cells" function as harpoons, since their payloads remain connected to the bodies of the cells by threads. Three types of cnidocytes are known:

Nematocysts inject venom into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.

Spirocysts do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.

Ptychocysts are not used for prey capture — instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the order Cerianthria, tube anemones.

Nervous system and senses

Cnidaria have no brains or even central nervous systems. Instead they have decentralized nerve nets consisting of sensory neurons that generate signals in response to various types of stimulus, such as odors, motor neurons that tell muscles to contract, and "cobwebs" of intermediate neurons to connect them. As well as forming the "signal cables", intermediate neurons also form ganglia that act as local coordination centers. The cilia of the cnidocytes detect physical contact.

Feeding and excretion

Cnidarians feed in several ways: predation, absorbing dissolved organic chemicals, filtering food particles out of the water, and obtaining nutrients from symbiotic algae within their cells. Corals depend almost completely on their endosymbionts and on absorbing dissolved nutrients. Cnidaria give their symbiotic algae carbon dioxide, some nutrients, a place in the sun and protection against predators. Predatory species use their cnidocytes to poison or entangle prey, and those with venomous nematocysts may start digestion by injecting digestive enzymes. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking.

4.      The Phylum Platyhelminthes "Flatworms"

Etymology:- From the Greek platy for flat and helminthes for worms, Hence Flat Worms.

Characteristics of Platyhelminthes (Unifying Characteristics)

1)Bilaterally symmetrical.
2)Body having 3 layers of tissues (Tryplobastic) with well-developed organ system (Organs and organelles.
3)Body contains no internal cavity (Acoelomate).
4)Possesses a blind gut (i.e. it has a mouth but no anus)
5)Has Protonephridial excretory organs instead of an anus; generally of flame cells and ducts
6)Nervous system of longitudinal fibres rather than a net ( includes a brain cephalisation).
7)Generally dorsoventrally flattened.
8)Reproduction mostly sexual as hermaphrodites ( hermaphroditic).

 9)Mostly they feed on animals and other smaller life forms.
10)Some species occur in all major habitats, including many as parasites of other animals.

11. Parasitic of free-living metazoan flatworms

Class Turbellaria

Mostly free living, primarily aquatic and marine; Bottom dwellers; Mostly carnivorous; Photoreceptive ocelli (primitive eyes); Auricles -flanges on side of head;

Gut - blind sac with two-way flow; Examples –Planaria

Class Trematatoda (flukes)

1. Entirely parasitic; 2. Body thickened into protective tegument; 3. Posses suckers;

4. Gut parasites;5. Secrete enzyme interfering proteins; 6. Inactivate digestive enzymes e.g

Monogenetic and Digenetic Forms; Monogenetic = single host ie: Polystoma of frog hosts

Digenetic = two or more hosts e: Schistosoma, Clonorchis, Fasciola hepatica

Class Cestoda (tapeworms)

Entirely parasitic; No digestive or nervous system; Live from digested materials; Body structures

Anterior scolex with suckers and barbs; False segments called proglottids, each proglittid a self reproducing unit ie: Taenia ssp.

Characteristics that makes platyhelmenthes to be Parasite

Reduction of sense of organs and nervous system; Loss of locomotion organs

Organs of adhesion developed; Complicated life; Tremendous egg production

Protection from enzyme action, loss of cuticle; Modified respiration, can survive anaerobic conditions. Loss of digestive tract; replaced with reproductive organs. 

Name of Disease	Causative Organism	Vector	Definitive test
Snail Fever (Shistanomiasis)	Shistosoma mansoni	Freshwater Snail	Urine/Fecal Sample Look for eggs
	Shistosoma japonicum
	Shistosoma haematobium
Beef Tapeworm	Taenia saginata	Cattle/sheep/goats	Stool Sample look for eggs (proglottids)
Pork Tapeworm	Taenia solium	pig	Stool Sample look for eggs (proglottids)
Fish Tapeworm	Diphyllobothrium latum	fish	Stool sample look for eggs (proglottids)

5.      Phylum: Nematoda

Definition: Phylum Nematoda are un-segmented vermiform animals with ante­rior lateral chemosensory organs or amphids and with a persistent blastocoel or pseudocoelom; dorsal and ventral nerve cords in epidermis and excretory system of renette cells or tubules.

 

General Characteristic Features of Phylum Nematoda:

1. Body of Phylum Nematoda is un-segmented, bilaterally sym­metrical, elongated and tapering at both ends.

2. Triploblastic animals with perivisceral cavity is more extensive than that of Platyhelminthes.

3. Body of of Phylum Nematoda is generally covered with thick, flexible multi-layered collagenous cuti­cle and often bears cuticular setae (hairs), spines or annulations.

4. Cuticle moulted periodically.

5. Epidermis or hypodermis syncytial; i.e., the nuclei are not separated from each other by cell membranes.

6. Only longitudinal body-wall muscles; no circular body-wall muscles.

7. Body cavity of of Phylum Nematoda is pseudocoel filled with parenchyma in most cases.

8. Alimentary canal provided with distinct mouth and anus (complete digestive tract). Muscular pharynx and the inner surface of the gut usually not lined by cilia. Extracellular digestion.

9. Mouth of of Phylum Nematoda is surrounded by six lips.

10. Blood vascular system and respiratory system are absent in of Phylum Nematoda.

11. Haemoglobin sometimes present in the pseudocoelomic fluid.

12. Excretory system without nephridia and flame cells. In the class Adenophorea glandular renette cells with a duct or in the class Secernentea excretory canal system without flame cells act as excre­tory system.

13. Dorsal and ventral nerve cords in the epidermis.

14. Chemosensory organs are small cuticular projections called amphids which are situated on the lips, derived from cilia and opening to the exterior through a small pore, and lined with modified non-motile cilia called sensillae.

15. Sexes of of Phylum Nematoda are separate (gonochoristic).

16. Tubular gonads are present in of Phylum Nematoda.

17. Amoeboid sperm cells.

18. Fertilization is internal in of Phylum Nematoda.

19. Determinate cleavage (mosaic).

20. of Phylum Nematodaare eutelic animals.

21. Generally complex life history.

22. They are free-living or phytoparasitic or zooparasitic.

 

Classification of Phylum Nematoda:

 Class 1. Adenophorea or Aphasmida (Gk. Adenophorea = gland-bearing; Gk. Aphasmida – without phasmids):

1. Most species possess caudal adhesive glands and epidermal glands.

2. Phasmids (caudal papillae bearing pores connecting with glandular pouch called phasmids which are thought to be chemosensory in function) are absent.

3. Amphids are post labial and variously shaped such as pouch-like or tube-like, rarely pore-like.

4. Coelomocytes well developed.

5. Excretory organs are only renette cells but without collecting tubules.

6. Males usually without caudal alae.

7. Usually two testes in males.

8. Mostly marine, and include both free- living and parasitic species. The free- living species include both terrestrial, freshwater, and major marine forms.

Seven Orders of class Adenophorea or Aphasmida( not common)

Order 1. Enoplida, Order 2. Dorylaimida, Order 3. Mermithida, Order 4. Chromadorida

Order 5. Desmoscolecida, Order 6. Monohysterida, Order 7. Araeolaimida

Class 2. Secernentea or Phasmida:

Characteristics:

1. Caudal phasmids present; 2. Labial amphids pore-like; 3. Excretory system canal-like and com­paratively more complex; 4. Epidermal and caudal adhesive glands absent; 5. Males with a single testis; 6. Mostly parasitic; 7. Free-living species are largely terres­trial. It includes certain important orders:

Order 1. Rhabditida: 1. Cuticle smooth and ringed.; 2. Sensory bristles in 2 rings; the outer ring 4, 6 or 10 bristles and inner ring of 6 bristles.; 3. Pharynx with a posterior bulb.; 4. Copulatory spicules with gubernaculum in males. ; 5. Free-living and parasitic nematodes. Examples: Rhabditis, Heterodera, Bunonema.

Order 2. Strongylida: 1. Lips absent.,2. Males with two spicules.; 3. A true copulatory bursa is present.; 4. Buccal capsule well developed.; 5. Pharynx without bulb.; 6. Vertebrate parasites.

Examples: Ancylostoma duodenale (Hook­worm), Strongylus, Trichostrongylus (Hair worm), Ne-cator.

Order 3. Oxyurida: 1. Small to moderate in size.; 2. Copulatory spicules present in males.

3. Tail of females long, narrow and pointed.; 4. Mouth provided with 3-6 simple lips.

5. Pharynx with a valvular posterior bulb.; 6. Caudal alae present.; 7. Invertebrate and vertebrate parasites; Examples: Oxyuris, Enterobius vermicularis (Pinworm), Heterakis gallinae, Thelastoma, Aspiculuris.

Order 4. Ascaridida:  1. Large stout nematodes residing in the intestine of vertebrates as para­sites.

2. Mouth provided with 3 prominent lips.; 3. Males possess 2 equal or almost equal copulatory spicules.; 4. Buccal capsule absent.; 5. Pharynx with or without a posterior bulb.

6. Tail of females blunt.; 7. Oviparous.; Examples: Ascaris (Ascaris lumbricoides, Ascaris megalocephala, Ascaris suillas), Parascari, Toxocara.

Order 5. Spirurida: 1. Thread-like, moderate to large size.; 2. Females larger than males.; 3. Mouth usually provided with two lateral lips.; 4. Pharynx without bulb.; 5. Males with two unequal copulatory spicules but do not have bursa or caudal alae.; 6. Oviparous or viviparous.  7. Parasites of vertebrates and with a blood sucking invertebrate as inter­mediate host.; -Examples: Spirura, Wuchereria bancrofti (Filaria), Loa loa (Eye worm), Brugia, Onchocerca, etc.

Order 6. Trichuroida (= Trichinelloidea): 1. Anterior part of the body whip-like.; 2. Mouth without lips.; 3. Pharynx slender.; 4. Males with one or without spicule.; 5. Males are either with bursa or with cirrus.; 6. Life cycle simple—no intermediate host.; 7. Commonly known as whip worms.; Examples: Trichuris (Parasites of mam­mals), Trichinella spiralis (Trichinia worm).

Order 7. Camallanida: 1. Thread-like; females larger than males.; 2. Lips absent.; 3. Buccal capsule large or absent.; 4. Spicules of males of same size.; 5. No bursa in males.; 6. Adult females with degenerated bursa.; 7. Mostly oviparous.; 8. Parasites of vertebrates.; Examples: Camallanus, Procamallanus, Dracunculus medinensis (Guinea-worm), Philometra.

6.      Phylum: Annelida (Segmented Worms)

Characteristics of Phylum Annelida
1)Bilaterally symmetrical and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a true coelom, often divided by internal septa.
4)Body possesses a through gut with mouth and anus.
5)Body possesses 3 separate sections, a prosomium, a trunk and a pygidium.
6)Has a nervous system with an anterior nerve ring, ganglia and a ventral nerve chord.
7)Has a true closed circulatory system.
8)Has no true respiratory organs.
9)Reproduction normally sexual and gonochoristic or hermaphoditic.
10)Feed a wide range of material.
11)Live in most environments.

12.  Segmentation

13. Chaetae (Hairs made of Chitin

14.  Cell Layers

15. Head develops first (Protostomal)

 3 Major Classes

1. Polychaeta (Poly – keet - a); -a. Poly = Many, Chaetae = hairs

2. Oligochaeta (Oligo – keet – a); - a. Oligo = few

3. Hirudinea:- Leeches  (Ectoparasites)

Importance of Segmentation/Segments

1. Allows movement with maximum protection.

2. Each segment contains repetition of excretory, movement organs.

3. Each is separated from others by a septum.

4. Segmentation provides an evolutionary framework to build upon.

5. Other major phyla with segmentation are Arthropoda, Chordata.

Class Polychaetes

1. Live in the ocean (marine).

2. Many chaetae (chitinous hairs)

3. Movement

a. Each segment has paired parapodia for swimming/crawling.

b. May burrow, using peristalsis

4. Feeding

a. Deposit feeders (eat mud, digest organic content)

b. Raptorial predators

c. Herbivores/Scavengers

d. Filter feeders

i. With mucous net

ii. With feeding arms

5. Reproduction

a. Dioecious -Mostly have two sexes: male, female.

b. Segmentation allows regeneration.

c. Also allows Epitoky! (i). Posterior (tail) end of worm grows enlarged gonads and eyes; parapodia become modified for swimming.

ii. Often, the posterior end will then break off and swim away to reproduce! It is called an epitoke

iii. Usually on one or two days of the year, all epitokes of a species will congregate

at the surface, with males swarming around females, shedding sperm.

Females, once fertilized, shed all eggs into water.

iv. Epitokes of the Palolo worm are a delicacy in Samoa.

Class: Oligochaeta

1. Live in freshwater, land, ocean

2. Few chaetae (chitonous hairs)

3. Movement

a. Most burrow with peristalsis. Muscular contractions of body, using both longitudinal and circular muscles.

b. Use chaetae as anchors.

4. Feeding

a. Deposit feeders (earthworms)

b. Herbivores/scavengers

5. Reproduction

a. Hermaphroditic

b. Worms reproduce by holding clitella (singular is clitellum) together, and exchanging sperm.

i. Clitellum provides mucus for transfer of sperm, also creates a cocoon for eggs, a few days later.

Class Hirudinea

1. Mostly freshwater, some marine. Like to live in ponds.

2. No chaetae

3. Movement

a. Have suckers on both ends

b. Move like inchworms

4. Feeding

a. 3/4 blood-sucking ectoparasites

b. 1/4 predators

c. 3 Triangular teeth slice skin, proboscis is forced in, along with anticoagulant, blood is

sucked through proboscis by pumping of pharynx.

5. Reproduction

a. Hermaphroditic

 

The annelids are bilaterally symmetrical, triploblastic, coelomate, have parapodia for locomotion. Traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species; leeches as a sub-group of oligochaetes and consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Reproduction: Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied. The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvae, that live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites and produce a ring-like cocoon around their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.

Important of Annelides

Earthworms (Oligochaetes) support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is no longer in favor with doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.

Classification

1.            Class Polychaetes: have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors.  Most are marine animals, although a few species live in fresh water and even fewer on land.

Clitellates have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch  or, in moniligastrids, yolky eggs that provide nutrition for the embyros. The clitellates are sub-divided into:

2.      Class Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed organic materials.

3.      Class Hirudinea, whose name means "leech-shaped" and whose best known members are leeche^s. Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators They have suckers at both ends of their bodies, and use these to move rather like inchworms.

The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes. Pogonophora / Siboglinidae were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them.

 

Distinguishing features

Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent. Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt – on the other hand arthropods' cuticles are made of the more rigid α-chitin, and molt until the arthropods reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.

Summary of distinguishing features
	Annelida	Recently merged into Annelida		Closely related	Similar-looking phyla
		Echiura	Sipuncula	Nemertea	Arthropoda]	Onychophora]
External segmentation	Yes	no	no	Only in a few species	Yes, except in mites	no
Repetition of internal organs	Yes	no	no	Yes	In primitive forms	Yes
Septa between segments	In most species	no	no	No	No	No
Cuticle material	Collagen	collagen	collagen	none	α-chitin	α-chitin
Molting	Generally no;[17] but some polychaetes molt their jaws, and leeches molt their skins[25]	no[26]	no[26]	no[26]	Yes[19]	Yes
Body cavity	Coelom; but this is reduced or missing in many leeches and some small polychaetes[17]	2 coelomata, main and in proboscis	2 coelomata, main and in tentacles	Coelom only in proboscis	Hemocoel	Hemocoel

Segments of an annelid

  Prostomium

  Peristomium

O Mouth

  Growth zone

  Pygidium

O Anus

Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external chaetae  and, in some species, appendages. The frontmost section, called the prostomium contains the brain and sense organs, while the rearmost, called the pygidium or periproct contains the anus. The first section behind the prostomium, called the peristomium is regarded by some zoologists as not a true segment, but in some polychaetes the peristomium has chetae and appendages like those of other segments. The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called teloblastic growth. Some groups of annelids, including all leeches, have fixed maximum numbers of segments, while others add segments throughout their lives.

Body wall, chetae and parapodia

Annelids' cuticles are made of collagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and mucus-secreting glands in the epidermis protect their skins. Under the epidermis is the dermis, which is made of connective tissue, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of the coelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips, whose contractions make the segment shorter and fatter. Some annelids also have oblique internal muscles that connect the underside of the body to each side.

The setae ("hairs") of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The parapodia ("limbs") of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked. Chetae are made of moderately flexible β-chitin and are formed by follicles, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming microvilli, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta. Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle. Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia) and gills.

Nervous system and senses

The brain forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it. The brains of polychaetes are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the peristomium. In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.The rest of the central nervous system is generally "ladder-like", consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body. However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling the single nerve chord has no ganglia and is located in the epidermis. As in arthropods, each muscle fiber (cell) is controlled by more than one neuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement. The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body. Nuchal ("on the neck") organs are paired, ciliated structures found only in polychaetes, and are thought to be chemosensors. Some polychaetes also have various combinations of ocelli ("little eyes") that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images. The compound eyes probably evolved independently of arthropods' eyes. Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withdraw into their tubes. Some burrowing and tube-dwelling polychaetes have statocysts (tilt and balance sensors) that tell them which way is down. A few polychaete genera have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers".

Respiration

In some annelids, including earthworms, all respiration is via the skin. However, many polychaetes and some clitellates (the group to which earthworms belong) have gills associated with most segments, often as extensions of the parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.

Feeding and excretion

Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular pharynx that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two families, the Eunicidae and Phyllodocidae, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. Filter feeders use "crowns" of palps covered in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.

The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the anus on the underside of the pygidium. However, in members of the tube-dwelling family Siboglinidae the gut is blocked by a swollen lining that houses symbiotic bacteria, which can make up 15% of the worms' total weight. The bacteria convert inorganic matter – such as hydrogen sulfide and carbon dioxide from hydrothermal vents, or methane from seeps – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria. Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia. Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials as urine. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first – in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia. In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result, the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.

Reproduction and life cycle

Asexual reproduction

Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism. Some oligochaetes, such as Aulophorus furcatus, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding. However, leeches have never been seen reproducing asexually. Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed. Annelids are the most complex animals that can regenerate after such severe damage. On the other hand, leeches cannot regenerate.

 

Sexual reproduction

It is thought that annelids were originally animals with two separate sexes, which released ova and sperm into the water via their nephridia. The fertilized eggs develop into trochophore larvae, which live as plankton. Later they sink to the sea-floor and metamorphose into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head); a small area round the trochophore's anus becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth. About 14% use a similar external fertilization but produce yolk-rich eggs, which reduce the time the larva needs to spend among the plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch – some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch. These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have a penis that inject sperm into the female. While most polychaetes remain of one sex all their lives, a significant percentage of species are full hermaphrodites or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent gonads, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn. Most mature clitellates (the group that includes earthworms and leeches) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and all copulate. Earthworms store their partners' sperm in spermathecae ("sperm stores") and then the clitellum produces a cocoon that collects ova from the ovaries and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae.

 

Ecological significance

Soil fertility. The burrowers loosen the soil so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating the decomposition of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily. Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers, and in some cases conserving earthworms may be essential for conserving endangered birds. Terrestrial annelids can be invasive in some situations.

Burrowing species increase the penetration of water and oxygen into the sea-floor sediment, which encourages the growth of populations of aerobic bacteria and small animals alongside their burrows. Although blood-sucking leeches do little direct harm to their victims, some transmit flagellates that can be very dangerous to their hosts. Some small tube-dwelling oligochaetes transmit myxosporean parasites that cause whirling disease in fish.

Interaction with humans. Ragworms are commercially important as bait and as food sources for aquaculture, and there have been proposals to farm them in order to reduce over-fishing of their natural populations. Some marine polychaetes' predation on molluscs causes serious losses to fishery and aquaculture operations. Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.

The use of leeches for the medically dubious practise of blood-letting have come from China around 30 AD hence medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports eg. Hirudo medicinalis is treated as an endangered species by both IUCN and CITES. More recently leeches have been used to assist in microsurgery, and their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which also prevents tumors from spreading.

Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are biomineralized with calcium salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusual proteins that bind strongly to zinc.

7.      Phylum Arthropoda

Classification of the Phylum Arthropoda

Phylum called Arthropoda (arthropods) is the largest and most successful of the animal phyla. All arthropods have segmented bodies divided into a head, jointed legs and abdomen. 



The major Classes of living arthropods are as follows with one example each: 

Class Arachnida -  spiders  

Class Crustacea -  crabs                                                                         

Class Merostoma- horseshoe crabs

Class Diplopoda -  millipedes 

Class Chilopoda -  centipede 

Class Insecta or Hexopoda - insects

             

Class trilobites are an extinct group of arthropods that lived in the seas of the world for about 380 Mya (million years ago), from the Precambrian 610 Mya 

to around the end of the Permian 230 Mya. 

Class Insecta – true insects are the most important members of the phylum Arthropoda. Typical adult insects have one pair of antennae, three pairs of legs, and three body regions: head, thorax, and abdomen. The class Insecta is divided into some 29 to 40 major orders. Insects make up more than 

half of all living things in the world. There are more than a million known species of insects  and there are many more waiting to be discovered.

Etymology:- From the Greek Arthron a joint and Pous for foot

Characteristics of the Arthropoda:-
1)Bilaterally symmetrical (in most cases).
2)Body has more than two cell layers, tissues and organs.
3)Body cavity a true coelom.
4)Most possesses a through straight gut with an anus (in most cases).
5)Body possesses 3 to 400+ pairs of jointed legs.
6)Body possesses an external skeleton (in most cases).
7)Body is divided in 2 or 3 sections.
8)Nervous system includes a brain and ganglia.
9)Possesses a respiratory system in the form of tracheae and spiracles (in most cases).
10)Possesses a open or lacunnar circulatory system with a simple heart, one or more arteries, and no veins, (in most cases).
11)Reproduction normally sexual and gonochoristic, but can be parthenogenetic.
12)Feed on everything.
13)Live everywhere.

8.      The Phylum Mollusca

Etymology:- From the Latin Molluscus meaning soft of body

Introduction

After the Arthropods the Molluscs are the most successful of the animal phyla in terms of numbers of species. There are about 110,000 species known to science most of which are marine. They occupy a vast range of habitats however both aquatic and terrestrial, from the arctic seas to small tropical streams and from valleys to mountainsides 7,000 metres high, there are a few adapted to live in deserts and some are parasitic. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid which can weighs 270 kg and measures up to 12 metres long in the body, with tentacles as much as another 50 metres in length. Many species are common and many more a beautiful. Most species secrete a shell of some sort, these shells are long lasting and have been collected by human beings for thousands of years, some of these shells, and the pearls which come from oysters, which are also molluscs may be among the earliest forms of money.

The molluscs include a number of familiar animals, including snails, oysters, clams, octopuses and squids. Many species have a calcareous shell. Molluscs are bilaterally symmetrical, although some have secondarily lost the symmetry in part; they have well-developed excretory, digestive, respiratory and circulatory systems. In all molluscs except the cephalopods, the circulatory system is open. All molluscs are characterized by having three main body regions: a head-foot, which is the sensory and locomotor part of the body; a visceral mass, containing most of the organ systems; and a mantle, which covers the visceral mass and secretes the shell, if one is present. Most molluscs also have radula, a rasping structure covered with chitinous teeth, which is used in feeding.

Molluscs are probably closest phylogenetically to the annelids. Several lines of evidence support this view. Both annelids and molluscs have very similar developmental patterns and a similar type of ciliated larva, called a trochophore larva. Unlike annelids, molluscs are not segmented, although one very primitive genus, Neopilina, shows some signs of segmentation. Although molluscs are not descended from annelids, it seems probable that the two groups evolved from a common ancestor.

Characteristics of Mollusca:-
1) Bilaterally symmetrical.
2) Body has more than two cell layers, tissues and organs.
3) Body without cavity.
4) Body possesses a through gut with mouth and anus.
5) Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules.
6) Has a nervous system with a circum-oesophagal ring, ganglia and paired nerve chords.
7) Has an open circulatory system with a heart and an aorta.
8) Has gaseous exchange organs called ctenidial gills.
9) Has a pair of kidneys.
10) Reproduction normally sexual and gonochoristic.
11) Feed a wide range of material.
12) Live in most environments. 

13) Rasping organ called a radula; present in all groups except bivalves and Aplacophora

14) Muscular foot used for locomotion and other tasks

15) A sheath of tissue called a mantle that covers the body and can secrete the shell (if there is one)

16) A mantle cavity that houses the gills or lungs

A calcium shell present in most molluscs, e.g., squid; while others have completely lost it e.g., slugs, nudibranchs and octopus.

Classification of molluscs

Mollusks can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These classes are distinguished by, among other criteria, the presence and types of shells they possess. Members of Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure. Members of Class Monoplacophora have a single shell that encloses the body. Members of Class Polyplacophora are better known as "chitons;" these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side. Members of Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels. Members of Class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling while members of Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.

Four classes of living molluscs:

Class Bivalvia, Class Polyplacophora, Class Gastropoda and  Class Cephalopoda.

Terminologies:-

ctenidium : a respiratory system, in the form of a comb, in some molluscs

captacula :the foot of a Scaphalopod, modified into tentacles for capturing prey

nephridium :a tubular excretory organ in some invertebrates

Classes in Phylum Mollusca

1.      Class Aplacophora

Class Aplacophora ("bearing no plates") includes worm-like animals primarily found in benthic marine habitats. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).

2.      Class Monoplacophora

Members of class Monoplacophora ("bearing one plate") posses a single, cap-like shell that encloses the body. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. Today, scientists have identified nearly two dozen extant species.

3.      Class Polyplacophora

The chitons are characterized by an elliptical body with a shell composed of eight plates.

Animals in the class Polyplacophora ("bearing many plates") are commonly known as "chitons" and bear an armor-like, eight-plated dorsal shell. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Chitons live worldwide, in cold water, warm water, and the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods.

4.      Class Bivalvia

This class is characterized by a hinged shell with right and left halves which covers the visceral mass. In bivalves the foot extends out between the shells and is used for locomotion. The bivalves lack a radula. Familiar bivalves include mussels, clams, and oysters. Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces. The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking "teeth" on each of the valves, forms the hinge.

5.      Class Gastropoda

In this group the visceral mass is contained in a spirally coiled shell (which may be secondarily reduced or absent), there is a distinct head with one or two pairs of tentacles, and the foot is large and flat. This group includes the snails and slugs. Animals in class Gastropoda ("stomach foot") include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral (like a spiral staircase), commonly seen in marine conches.

The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling . Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia (singluar: ctenidium) as well as a pair of nephridia (singular: nephridium).

6.      Class Cephalopoda

Members of this group have a prominent head with complex eyes and eight to ten (or more) tentacles surrounding the mouth. The shell may be internal or external. This group includes the squids and octopi. Class Cephalopoda ("head foot" animals) includes octopi, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion ("jet" propulsion) . Cephalopods, such as squids and octopi, also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway. Suckers are present on the tentacles in octopi and squid. Ctenidia are enclosed in a large mantle cavity serviced by blood vessels, each with its own associated heart. The mantle has siphonophores that facilitate exchange of water. Cephalopods ("head foot") include this octopus, which ejects a stream of water from a funnel in its body to propel itself through the water.  A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, then the female lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages.

7.      Class Scaphopoda

Members of class Scaphopoda ("boat feet") are known colloquially as "tusk shells" or "tooth shells," as evident when examining Dentalium, one of the few remaining scaphopod genera. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open . The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals.

 Other Mollusca Characteristic features

Have Body Plan

 Head foot- cephalopods

Contains sensory organs and muscles,

Visceral mass: contains digestive, reproductive, circulatory organs,

 •Mantle: skin of the dorsal body wallsecretes the shell (if there is one), cavity contains gills or lungs, Feeding and Digestion: There are both free living and parasitic forms, Most species use a tongue like organ called a radulawhen feeding

Circulatory System-open circulatory system (in most classes).  Have heart and blood sinuses, Heartcoelom.

Reproduction:-monoecious and dioecious species exist,•usually internal fertilization,•indirect development with the presence of a trochophore larva (link to annelids), and in most cases also a veligerlarva. Trochophore,

Torsion-twisting of the visceral mass through a 80°rotation•1st90°rotation usually occurs at the veligerstage •the 2nd90°rotation usually takes longer and occurs later•after torsion, the anus and mantle cavity end up over the head•this poses a serious fouling problem and many gastropods have lost their right gills, kidneys and heart auricles

8.      Phylum Echinodermata

Echinodermata are so named owing to their spiny skin (from the Greek "echinos" meaning "spiny" and "dermos" meaning "skin"). This phylum is a collection of about 7,000 described living species. Echinodermata are exclusively marine organisms. Sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms . To date, no freshwater or terrestrial echinoderms are known.

Characteristic features

Free living exclusively marine forms.

Adults are radially symmetrical while larvae are bilaterally symmetrical.

Body is represented by a central disc covered by ossicles with spines called pedicellaria.

Disc may bear extensions called arms.

Digestive system is complete.

A unique ambulacral or water vascular system is present.

Tube feet are present for locomotion and respiration. Tube feet are extended and retracted by variation in hydraulic pressure of the fluid in them and contraction of their muscles.

Nervous system has a central nerve ring with five radiating nerves.

Reproduction is sexual. Sexes are separate. Development is indirect.

Show very high power of regeneration.

Tube feet are for feeding as well.

 

Morphology and Anatomy

Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry . The endoskeleton is developed by epidermal cells and may possess pigment cells that give vivid colors to these animals, as well as cells laden with toxins. Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part pyloric stomach with a pyloric duct leading to the intestine and rectum, with the anus located in the center of the aboral body surface. In many species, the large cardiac stomach can be everted and digest food outside the body. Gonads are present in each arm. In echinoderms such as sea stars, every arm bears two rows of tube feet on the oral side which help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. The more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, and, in some cases, complete regeneration from a single limb.

Water Vascular System

Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm . Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet. These tube feet can expand or contract based on the volume of water (hydrostatic pressure) present in the system of that arm.

The madreporite is a light-colored, calcerous opening used to filter water into the water vascular system of echinoderms. Acting as a pressure-equalizing valve, it is visible as a small red or yellow button-like structure (similar to a small wart) on the aboral surface of the central disk of a sea star. Close up, it is visibly structured, resembling a "madrepore" colony. From this, it derives its name. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals (there are five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.

 

 

 

 

Other Body Systems

The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals.

Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to the madreporite.

Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.

The phylum Echinodamata  is divided into five classes.

Class	Nature of the disc	Nature of the arm	Examples
Asteroidea	Compressed along the oro-aboral axis	Five, continuous with the disc	Sea star, Asteria (starfish)
Ophiuroidea	Compressed along the oro-aboral axis	Five, long and slender demarkated from the absent	Ophiothrix, brittle star
Echinoidea	Globular or flat	Absent	Echinus (sea urchin), sand dollar
Holothuroidea	Elongated cylindrical	Modified into tentacles	Holothuria(sea cucumber)
Crinoidea	Rediced. Attached to the substratum	Ten, long and branched	Antedon (sea lily), feather star