UNIT 6 CLASSIFICATION OF ANIMALS - 111 Structure 6 Introduction Objectives 6.2 Phy luni Mollusca Monop lecophara 1'01 yplacophora Aplecophora (iastropoda tlivi~lvia Scaphopoda Cephi~lopotla 6.3 Phylum Ecliinoderniata Asteroidca Ophi~uaidea Echi~~oideu I-Iolotl~uroidea Crinoidea 6.4 Other Phyla 6.6 Tcrniinal Questions 6.7 Answers 6.1 INTRODUCTION In this unit we continue our study of coelomate invertebrates. You have learnt that coelom could be defined as a cavity lined by an epithelium of cells derived from the embryonic mesodenn. Phylum Mollusca which includes soft bodied animals with a she11 is the first phyluni that you will be studying in this unit. lY~iswill be followed.by the study of a group of spiny skinned. esclusively marine living animals. the echinoderms. Towards the end of the unit you will study the nanies of certain phyla. the minor phyla, each of which coniprise of a limited number of species and whose systematic position is not very clear. As was done in the previous unit. in this unit also we shall describe the characters of each phylum. classify the phylum upto classes giving examples and briefly describe the organisation of the group. After studying this unit you should be able to: relate the general characters of phylun~Mollusca and have a clear understanding of the major classes it comprises, point out the characteristic features of phylum Echinodermata. briefly relate their structural organisation and mention the important characters of various classes included under the phylum Echinodermata. 6.2 PHYLUM MOLLUSCA Phylum Mollusca is one of the largest phyla among the invertebrates and includes over 50,000 living species and about 35,000 fossil species. They coniprise snails, clams and squids etc. The rich fossil record of the phylum is due to the presence of a mineralised shell in many species. Although the majority of n~olluscsare aquatic. occupying both fresh and marine waters, some species live on land as well. We shall first study, the general characters of the phyluni and then study each class briefly. Characteristic Features 1. Bildermlly symmetrical. 2. Generally them is r distinct heatl irnd it musculilr foot; the tlors;tl hotly wall forms the mantle fnlds which enclese the mirntle cirvity. 3. Often there am gills and lungs for rcsl)iri~tion,formed hy moditied mirntle. 4. A hard, calcams shell secretetl Ly mirntle, ~)rotcctingthe soft kldy, is common. 5. CncInm is limited tn spaces ilrt~un~lthe hei1r-t (~)erici~rdii~lcirvity), in the gnnads and in the kidneys. 6. Circulatory system is open type in most ti~rms,with heirr-t, I,lootl vessels and sinuses. 7. Excretory organs are metirncl)hri(lii~,the sirc-lilw Iticlneys, opening ~)rosimirlly into the pericardium ;rnd distally into the milntle cilvitg. 8. Nenwus system consists of nell-tlcvclo1,etl gi~ngliil(cercl,rirl, ~~ctli~l,~)lcuritl i~ncl visceral) most of them concentratetl into ;I ring n.itR conncctivcs i~ntlcommissures. 9. Digestive system is cornplci, with chi~ri~ct~risticrasping orgiln, the ~'ild~li~; itnus emptying into the mitntle c;~vity. 10. Spiral cleavrg, usuirlly with intlirect tlevelopment, itccomoilnied by first ir trochnlthow l;tni~ancl sometimes with :I sec~~ndveliger Iilmit. Molluscs gcncmllg nicasilrc sc\,cral ccntimclcrs in lcngtli. l'liougli ~iiolluscsappcar ro be a hetcrogenous ;~sscmbl:~gcof i~ninlals.Ilicy li;ivc a basic body plan This can bc i~ndcrstoodfro111 tlic stud! of tlic gcncraliscd bod!, plan of a hypothetical ancestral mollusc (Fig 6 I) Tlic \lcntrrrl surfi~ccor tlic body is flat and 111uscularand ronns tlic locomotory org:ai tlic fool Thc 1ic;id is closc to it, at onc aid. Tlic dorsal sidc of the bod! is more or Icss o\,oid in s11;lpc and contiiins tlic internal organs. thc viscernl mass. Tlic visccral oiass is co\.crcd b!, an cpidcrmis. tlic m;tntle or ~~illlium.Tlie mantle c~lcloscsa cn\.irg. Ilic mirntle cinit!. Tlic nianllc secretes tlic overlying protective shell and the cdscs of the mantle arc mosl i~cti\c in sccrctilig tlic shell. rad~r~i <-- -t/ \ \ elI pcdal ncrvc osphradium 0 Fig. 6.1 : Organisation of a generalised mollusc. Tlic shell has three layers (Fig. 6.2)' (1) The outer, horny layer made up of eonchiolin, a modified protein. This layer is called 1)eriostr;tcum. It is protective to the underlying layers and is secreted by tlie fold of the mantle edge only. (2) The middle prismatic layer is iiiade up or prisins of calciuni carboilate dorsally packed, in a matrix of protein. This layer is also secreted by tlie glandular margin of the mantIe. (3) The innermost is the nacreous layer. This is calcareous material, and is layered down by the mantle surface continuously. This is the iridescent mother-of-pearl of many moIluscs. Pearl is fornied between mantle and shell in response to trapped foreign particles when they get covered with nacreous layer. cilialed epilheliu~n calcareous shell per~ostracalgrowlh sccretlnb dorsal cpilhelium Fig. 6.2 : A section through the mantle edge of a gastrnped snell. In molluscs, the respiratory structures are usually gills or ctenidia (Sing. Ctenidiu~n) (Fig. 6.1). The ctenidia are located one on each side of the mantle cavity. Each gill consists of a long, flattened axis projecting 'from the anterior wall of the mantle cavity (Fig. 6.3). The axis contains the muscles. nenres and blood vessels. Triangular gill filaments are attached to the sides of the broad surface of the asis as ia a comb. When the filanlents are found on only one side of the asis the ctenidiuni in known as mono~wctin;~te;\vhen they are found on bod1 sides. it is hipectinate. Water enters the lower pan of the mande cavity from the posterior end. travels upwards, flows back posteriorly to lea1.e the cavity. Cilia are located on the gills in the lateral. frontal and abfrontal regions (Fig. 6.4) Fig. 6.3 :Frontal section through iI gill. Fig. 6.4: Transverse section through ;I gill. The axis harbours two blood vessels: an ;~fferentvessel that carries blood to the gills and an efferent vessel that collects blood from the gills. The flow of the blood is fro111 the afferent vessel through the gill filaments to the efferent vessel. The blood tllils nlns countercurrent to the water flowing from frontal to the abfrontal margin nlasinlisi~lgthe oxygen uptake by the blood. Many molluscs are herbivores, feeding on algae and plants. Tile mouth leads into a cuticle-lined buccal cavity. On the floor of the buccal cavity. there is n stnlcture called odontophore. Odontophore is an elongated, muscular and canilagenous stnlcture that bears a membranous belt called radula (Fig 6.5). The radula which lies in a radular sac has transverse rows of teeth. The radula and the odontophore together do the function of scraping and collecting the food. Due to the repeated scrapi~lgactivity there is loss of radular teeth whichpare continuously secreted at the posterior end of the radula. At least one pair of salivary glands open into the buccal cavity of ~llolluscs.The buccal cavity leads into oesophagus followed by stomach (Fig 6.1) The anterior regioil of the ,stomach is lined with chitin except for a short, ciliated and ridged region called the s~rtingregion. The hepatopancreas constitute the digestive glands; their ducts open into stomach. The stomach is followed by a long, coiled intestine. The posterior region of intestine functions as a rectum concentrating the faecal pellets. The faecal materials expelled through the anus located at the posterior margin of the mantle cavity are swept away by the exhalent water current (Fig. 6.1). dont to bore . salivary gland . "d"'4". : : . p , . ,., ,, . ..' ' .. .. ., . Fig. 6.5 : The inner view of mulluscan radular apparatus. The coelomic cavity is highly reduced and is confined mainly to the pericardium surrounding the heart dorsally and a portion of the intestine ventrally. The heart of molluscs is three chambered with an unpaired anterior.ventricle and a pair of posterior auricles (Fig. 6.1). From the ventricle'an aorta arises and it branches, supplying blood to the blood sinuses. From these sinuses blood returns to the heart via the excretory organs and gills. In cephalopods (squids and octopuses) the blood is enclosed in vessels, and the sinuses are absent. The blood contains the respiratory pigment haemocyanin, and amoebocytes. Excretion in molluscs is carried out by one or two nephridia or kidneys. Each kidney is a mdanephric tubule with the nephrostome opening into the coelom at one end and the nephridiopore opening.into the mantle cavity at the other end. In many molluscs the nephridium is a blind sac. The urine is discharged through'qe nephridiopore into the mantle cavity (6.1). The nervous system of molluscs consists of a nerve ring around the oesophagus from which two pairs of nerve cords extend posteriorly (Fig. 6.1). Of these two pairs, one pair, the pedal nerve cords innervate the foot and its muscles. The other pair, the visceral pair innervates the mantle cavjty and visceral organs. Transverse connections occur between each pair of nerve cords. The sense organs of molluscs include tentacles, a pair of eyes, balancing organs-the statocysts and the chemoreceptors the ospharadia (Fig. 6.1). Molluscs may be hermaphrodites or the sexes may bb separate. The mature gametes may be released into water in which case fertilisation will be external. In forms where fertilisation is internal, sperm transfer may occur directly or the sperm will be packaged into structures called spermatophores and transferred. Cleavage is spiral and a free swimming trochophore larva (Fig 6.6) develops from the gastrula.