CHAPTER 4 ANIMAL KINGDOM

1 46 BIOLOGYWhen you look around, you will observe different animals with differentstructures and forms. As over a million species of animals have beendescribed till now, the need for classification becomes all the moreimportant. The classification also helps in assigning a systematic positionto newly described OF CLASSIFICATIONI nspite of differences in structure and form of different animals, there arefundamental features common to various individuals in relation to thearrangement of cells, body symmetry, nature of coelom, patterns ofdigestive, circulatory or reproductive systems. These features are usedas the basis of ANIMAL classification and some of them are discussed of OrganisationThough all members of Animalia are multicellular, all of them do notexhibit the same pattern of organisation of cells. For example, in sponges,the cells are arranged as loose cell aggregates, , they exhibit cellularlevel of organisation.

2 Some division of labour (activities) occur amongthe cells. In coelenterates, the arrangement of cells is more complex. Herethe cells performing the same function are arranged into tissues, hence iscalled tissue level of organisation. A still higher level of organisation, ,organ level is exhibited by members of Platyhelminthes and other higherphyla where tissues are grouped together to form organs, each specialisedfor a particular function. In animals like Annelids, Arthropods, Molluscs, ANIMAL KINGDOMCHAPTER ofAnimals2015-16(19/01/2015) ANIMAL KINGDOM4747 ANIMAL KINGDOME chinoderms and Chordates, organs haveassociated to form functional systems, eachsystem concerned with a specific physiologicalfunction. This pattern is called organ systemlevel of organisation. Organ systems in differentgroups of animals exhibit various patterns ofcomplexities. For example, the digestive systemin Platyhelminthes has only a single openingto the outside of the body that serves as bothmouth and anus, and is hence calledincomplete.

3 A complete digestive system hastwo openings, mouth and anus. Similarly, thecirculatory system may be of two types:(i) open type in which the blood is pumpedout of the heart and the cells and tissues aredirectly bathed in it and(ii) closed type in which the blood is circulatedthrough a series of vessels of varying diameters(arteries, veins and capillaries). can be categorised on the basis of theirsymmetry. Sponges are mostly asymmetrical, , any plane that passes through the centredoes not divide them into equal halves. Whenany plane passing through the central axis ofthe body divides the organism into two identicalhalves, it is called radial , ctenophores and echinodermshave this kind of body plan (Figure ).Animals like annelids, arthropods, etc., wherethe body can be divided into identical left andright halves in only one plane, exhibit bilateralsymmetry (Figure ). and TriploblasticOrganisationAnimals in which the cells are arranged in twoembryonic layers, an external ectoderm andan internal endoderm, are called diploblasticanimals, , coelenterates.

4 An undifferentiatedlayer, mesoglea, is present in between theectoderm and the endoderm (Figure ).Figure germinal layers :(a) Diploblastic (b) Triploblastic(a)(b)EctodermMesogleaEndod ermMesodermFigure (b) Bilateral symmetryFigure (a) Radial symmetry2015-16(19/01/2015) CoelomPresence or absence of a cavity between the bodywall and the gut wall is very important inclassification. The body cavity, which is linedby mesoderm is called coelom. Animalspossessing coelom are called coelomates, ,annelids, molluscs, arthropods, echinoderms,hemichordates and chordates (Figure ). Insome animals, the body cavity is not lined bymesoderm, instead, the mesoderm is present asscattered pouches in between the ectoderm andendoderm. Such a body cavity is calledpseudocoelom and the animals possessing themare called pseudocoelomates, ,aschelminthes (Figure ).

5 The animals inwhich the body cavity is absent are calledacoelomates, , platyhelminthes (Figure ).Figure sectional view of :(a) Coelomate (b) Pseudocoelomate(c) AcoelomateThose animals in which the developing embryo has a third germinal layer,mesoderm, in between the ectoderm and endoderm, are calledtriploblastic animals (platyhelminthes to chordates, Figure ). some animals, the body is externally and internally divided intosegments with a serial repetition of at least some organs. For example, inearthworm, the body shows this pattern called metameric segmentationand the phenomenon is known as NotochordNotochord is a mesodermally derived rod-like structure formed on thedorsal side during embryonic development in some animals. Animals withnotochord are called chordates and those animals which do not form thisstructure are called non-chordates, , porifera to OF ANIMALSThe broad classification of Animalia based on common fundamentalfeatures as mentioned in the preceding sections is given in Figure (19/01/2015) ANIMAL KINGDOM4949 ANIMAL KINGDOMThe important characteristic features of thedifferent phyla are PoriferaMembers of this phylum are commonly knownas sponges.

6 They are generally marine and mostlyasymmetrical animals (Figure ). These areprimitive multicellular animals and have cellularlevel of organisation. Sponges have a watertransport or canal system. Water enters throughminute pores (ostia) in the body wall into a centralcavity, spongocoel, from where it goes outthrough the osculum. This pathway of watertransport is helpful in food gathering, respiratoryexchange and removal of waste. Choanocytesor collar cells line the spongocoel and the is intracellular. The body is supportedby a skeleton made up of spicules or sponginfibres. Sexes are not separate (hermaphrodite), , eggs and sperms are produced by the sameindividual. Sponges reproduce asexually byfragmentation and sexually by formation ofgametes. Fertilisation is internal and developmentis indirect having a larval stage which ismorphologically distinct from the ofOrganisationSymmetryBody Cavityor CoelomPhylumCellular levelTissue/Organ/Organ systemRadialBilateralWith false coelom(pseudocoelomates)With true coelom(coelomates)PoriferaWithout body cavity(acoelomates)Coelenterata(Cnidaria )CtenophoraPlatyhelminthesAschelminthesA nnelidaArthropodaMollusca*EchinodermataH emichordataChordataAnimalia(multicellula r)*Echinodermata exhibits radial or bilateral symmetry depending on the Broad classification of KINGDOM Animalia based on common fundamental features(a)(b)(c)Figure of Porifera : (a) Sycon(b) Euspongia (c) Spongilla2015-16(19/01/2015)50 BIOLOGY cnidoblasts or cnidocytes (which contain the stinging capsules ornematocytes) present on the tentacles and the body.

7 Cnidoblasts are usedfor anchorage, defense and for the capture of prey (Figure ). Cnidariansexhibit tissue level of organisation and are diploblastic. They have a centralgastro-vascular cavity with a single opening, mouth on is extracellular and intracellular. Some of the cnidarians, ,corals have a skeleton composed of calcium carbonate. Cnidarians exhibittwo basic body forms called polyp and medusa (Figure ). The formeris a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, thelatter is umbrella-shaped and free-swimming like Aurelia or jelly cnidarians which exist in both forms exhibit alternation ofgeneration (Metagenesis), , polyps produce medusae asexually andmedusae form the polyps sexually ( , Obelia).Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone),Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).Figure view ofCnidoblastFigure of Coelenterata indicating outline of their body form :(a) Aurelia (Medusa) (b) Adamsia (Polyp)(b)(a)Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia(Bath sponge).

8 Coelenterata (Cnidaria)They are aquatic, mostly marine, sessile or free-swimming, radiallysymmetrical animals (Figure ). The name cnidaria is derived from the2015-16(19/01/2015) ANIMAL KINGDOM5151 ANIMAL CtenophoraCtenophores, commonly known as sea walnuts or comb jelliesare exclusively marine, radially symmetrical, diploblasticorganisms with tissue level of organisation. The body bearseight external rows of ciliated comb plates, which help inlocomotion (Figure ). Digestion is both extracellular andintracellular. Bioluminescence (the property of a livingorganism to emit light) is well-marked in ctenophores. Sexesare not separate. Reproduction takes place only by sexualmeans. Fertilisation is external with indirect : Pleurobrachia and PlatyhelminthesThey have dorso-ventrally flattened body, hence are calledflatworms (Figure ). These are mostly endoparasites foundin animals including human beings.

9 Flatworms are bilaterallysymmetrical, triploblastic and acoelomate animals with organlevel of organisation. Hooks and suckers are present in theparasitic forms. Some of them absorb nutrients from the hostdirectly through their body surface. Specialised cells calledflame cells help in osmoregulation and excretion. Sexes are notseparate. Fertilisation is internal and development is throughmany larval stages. Some members like Planaria possess highregeneration : Taenia (Tapeworm), Fasciola (Liver fluke).Figure ofCtenophora(Pleurobrachia)(a)(b)Figure Examples of Platyhelminthes : (a) Tape worm (b) Liver fluke2015-16(19/01/2015) AschelminthesThe body of the aschelminthes is circular incross-section, hence, the name roundworms(Figure ). They may be freeliving, aquaticand terrestrial or parasitic in plants and have organ-system level of bodyorganisation. They are bilaterally symmetrical,triploblastic and pseudocoelomate canal is complete with a well-developed muscular pharynx.

10 An excretorytube removes body wastes from the body cavitythrough the excretory pore. Sexes are separate(dioecious), , males and females are females are longer than males. Fertilisationis internal and development may be direct (theyoung ones resemble the adult) or : Ascaris (Round Worm), Wuchereria(Filaria worm), Ancylostoma (Hookworm). AnnelidaThey may be aquatic (marine and fresh water) orterrestrial; free-living, and sometimes exhibit organ-system level of bodyorganisation and bilateral symmetry. They aretriploblastic, metamerically segmented andcoelomate animals. Their body surface isdistinctly marked out into segments ormetameres and, hence, the phylum nameAnnelida (Latin, annulus : little ring) (Figure ).They possess longitudinal and circular muscleswhich help in locomotion. Aquatic annelids likeNereis possess lateral appendages, parapodia,which help in swimming.