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Unit 4 Classification of Multicellular Animals-I

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Unit 4 Classification of Multicellular Animals-I UNIT 4 CLASSIFICATION OF MULTICELLULAR ANIMALS-I Structure Introduction Oblectives Metazoan branches : Mesozoa, Parazoa and Eumetazoa Parazoa - Phylum Porifera - sponges . Characteristic features Classificahon Phylum Cnidaria Characterstic features Classification of Phylum Crudaria Coral Reefs Phylum Ctenophora Characteristic featurea Claqsification Phylum Platyhelminthes . Characterist~cfeahrrts Class~fication Pseudocoelomata - Phylum Nernatoda Phylum Nematoda Characterishc features Class~ficabon Pseudocoelomata - Phylum Rotifera Charactenac features Classification Summary Terminal Questions Answers 4-1 INTRODUCTION .-. In the previous unit you have studied how the multicellular animals (Animalia) originated from the simpler protozoan protists. You have also learnt how the simpler multicellular animals further evolved, giving rise to the diversity of the animal life which came to exist on the earth. In the present unit you will study the characteristic features of some of the phyla comprising this &mal diversity namely, Porifera, Cnidaria, Ctenophora, Platyhelminthes, Nematoda, and Rotifera, and also study their further classification into classes. Ob-iectives After studying this unit you should be able to: recognise characters which differentiate one phylum described in this unit, from any other: describe the characteristic features of different classes of animals coming under each phylum described in this unit; , .O give examples of various classes of animals treated in this unit; .' give a basic idea of ge organization of the body of animals belonging to different I ' - classes of animals treated in this unit; I and explain-the mod6 of &ation of coral reefs and their significance. i ? - ,IYvrrs#ty nf hhd Life II (C 'tPPai6culiwn) 4.2 METAZOAN BRANCHES : MESOZOA, PARAZOA '\ AND EUMATAZOA We have already seen rhst the Kingdom Animalia comprises all the multicellular animals (metazoans). These are divided into three branches : I) Mesozoa, 2) Parazoa and 3) Eumetazoa. The Mesozoa comprise a tiny single phyIum (Phylum Mesozoa); the Parama comprise two phyla a tiny single phylum Plaoozoa and the rest make up the phylum Porifera comprising the sponges. Most of the metazoans come under the major branch Eumetazoa You must note that the cellular layers of Porifera (sponges) are not homologous to the germ layers of Eumetazoa. Their developmental pattern is also , quite different from that of the Eumetazoa. The Parazoa are regarded as an early evolutionary side branch of the animal kingdom, which did not give rise to any other group of animals. l DISTINCTIVE FEATURES OF PHYLUM POIUFERA AND EUMATAZOA b Phylum ori if era Eumetama 1. Animals are sessile. with Animal are mostly mobile, with irregular shape and mostly regular shape and some form of with no symmetry. symmeuy. 2. i'iasues are absent or poorly defined: Tissues are well defined. 3. There are no organs ~elldefmedorgans are present. 4. There is no mouth and Animals are provided with digestive tract. mouth and digestive tract 5. Body swface is porous. Body surface is not porous. 6. No sensory cells or nerves. Sensory cells & nerves, with better little co-ordination. co-ordination. 7 Internal cavities lined by Bod cavities are not lined choanocytes. by choanocytes. 8. Phvsiological division of Physiological division of labour is not well marked. labour is well marked. 9. Cellular layers not Cell layers homologous to homologous to germ lavers germ layers. The branch Eumetazoa, as we have seen above, consist of metazoans with organs, a mouth and digestive cavity These can be divided into two groups : Radiata and B~lateria. Radiata are charactksed by radial (or biradial) symmetry; they have tentacles and a limited number of organs. They have also a digestive cavity with mouth opening to the exterior. The Radiata include two Phyla : Cnidaria and Ctmophora. The Bilateria are bilaterally symmeuical animals and include the rest of the eumetazoans. 43 PARAZOA PHYLUM PORIFERA - SPONGES Yoin have just now iead that Barazoa comprise two phyla : a tiny phylum Placozoa and - Phylum-Porifera comprising the sponges. Of Utesetwo phyla. we will be dealing here with only one phylum namely. phylum Porifera, 'the sponges. \ ,, The word "porifera" has been derived frcrm-Lafin words 'porus' @ore) and 'fera' (bearing), meaning pore - bearing organis~s. Porifera are multicellular animals incapable of making movement as they arc.a~achedto the substratuin like a plant. Porifera present a great variety of forms and hn\c 5000 species or more described species. ,They may be cup-shaped, saucer-shapcd. mushroom-shaped. lobed, digitate, brani:he&??tiTegt&. As a general rule, the form is extremely variable even in the same-s@i-&&d is, therefore;af little use in identification They are almost alwavs attachi?a'3i&o~e5gii&b&* '&iU%cwon gf-t& fresh water spongill idae, they are I marine, and are found at alI depths., lndivichiab of the family Clionidae bore into CIpUUIutlon or ~\lltkdular sheik and stones. .AniAmh-1. 4.31 Characteristic features As the name of the group indicates, the surface'of the body presents a large number of minute pores called ostia (sing ostium) Through these ostia, water current enters the bod" cavity These pores actually lead into a system of channels which. after permeating almost the whole bodv, open to the exterior by one or more large exhalent 'apenings called oscula (sing. osculum). This system of spaces connecting Lhe inhalent pores or ostia with the exhalent oscula. is the canal system. The canal system may be simple as in the asconoid type or more complex as in syconoid and leuconoid types (Fig. 4.1). Through the canal system a continual streak of water is maintained bv Lhe action of the flagella. Thus the water enters the canal system through ostia and passes ou~rhrough the oscula - Fig. 4.1 : Three types of sponge structure. The degree of comple'xity from simple asconoid to complex leuconoid type bas involved mainly the water-canal aod skeletal systems, accompanied by outfolding and branching of the collar cell layer. Tbe' leuconoid type is considered tbe major plan for sponges, for it petmits greater size and more efficient water circulation. The sponge bodv is covered extemallv by an epithelial layer called Pinacoderm. This tayer is made up of flattened cells called pinacocytes (Fig. 4.2). Some of the pinacocytes are modified into porocytes by the formation of innacellular canal opening out lhrough the ostium (Fig. 4.1). Some are modlfied as contractile myocytes mged around oscula. Collencytc Pinacocyte Archaeocytc , I / I Spicules . , Choanocytc 4 Fig. 4.2 : Small section tbrougb sponge wall, showing four types o? sponge cells. Pinacoeytes are protective and contractile; choanocytes create wate_r~ur.rmtJand engulf fwd particles; ashaeotytes have a vqriety -of fmc$hns; cpllencyfer %vcretecollagen, \ I, . The flagellated Chambers of the canal system are lined by ovoid, collar cells or choanocytes (Fig 4 31, resembling in structure'the choanoflagellate protozoans These cells cany a flagellum surrounded by a cpnuaaile collar projecting lnto the sponge czvity. These cells are responsible for producing water current of the canal system. and for food capture Smaller food pdcles are engulfed by the choanocytes whereas the larger particles are passed on to archaeocytes or amoebocyies (Fig. 4 3). which are the principal sites of digestion Excurrent canal - Water flow Fig. 4.3 : Food trapping b! sponge cells A, Cutaway, section of canals showing cellular structure and direction of water flow. B. Two choanocytes and, C, structure of the lo2ar. Small red arrows indicate movement of food:payticles. Betneen the epithelial layer and-the choa~~oc!lcla!er. 1s n n~esoh!al layer consisting of ,@latirious protein mauix. which contains amoeboid cells (archaeocytes) and skeletal :bkmcnis. Archaeocytes are amoeboid cells modified to carry ou~different functions :he> engulf food particles and are hence digestive in function. as \be have ahead" seen Sclerocytes (Fig. 4.4) are spec~alisedfor secretion of spicules of the skeleton: spongocytes secrete spongin fibres of the skeleton: collencytes and lophocytes secrete collagen. Fig. 4 4 : A. Types of spicules found in sponges. There is amazing diversity, complexity. and beauty of form among the many types of spicules. B, Section through a calcareous sponge. The skeleton supporting the sponge body is mainly composed of spicules of various sizes cla=#kb ~~uftlc~~ular and shapes. The spicules (Fig. 4.4) may be siliceous. or of calciunl carbonate. The ~nima~s-I skeleton also consists of collagen and spongin. Thc spicules may have one, three, four or six rays. The structural variation of the spicules is of coilsiderable ilnportailce in classification. Spernls arise from choanocytes: oocytes arise either from choanocytes or archaeocytes. In addition. internal buds or ge~llnlulesare fonned from archaeocytes. You have to clearly understand. in coiu~ectionwith histology of sponges, that the layers of the sponge body, namely, pinacodernl, mesohyal and choanocyte layer are not coinparable to ectodernl, mesoder~lland endoden11 of higher animals. 4.3.2 Classification Now that we have studied the cliaiacteristic features of Porifem. We shall discuss here tlle classification of Porifera, giving examples of different classes. Phylum porifera is divided illto four classes : Calcarea, Hexactinellida. Deinospoilgiae and Sclerospongiae. 1. Class Citlci~reit(Citlcisl)ongiac): Thesc itre chitritcterisetl by the presence of sl)icules of citlcium citrhonitte. Sl)icules itre
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