CHAPTER VIII 



ECHINODERMATA 



SCHEME OF CLASSIFICATION 



I. AsTEROiDEA — Starfish. 



II. Ophiuroidea — Brittle-Stars. 



III. EcHiNoiDEA — Sea-urchins. 



IV. HoLOTHUROiDEA — Sea-cucumbers, Trepangs. 

 V. Crinoidea — Crinoids, Encrinites. 



This phylum is one which seems to lie quite off the main track of the 

 evolution of the animal kingdom : it is of comparatively little economic 

 or medical importance and it might justifiably be omitted from such a 

 text-book as this. The group, however, contains creatures, such as Star- 

 fish and Sea-urchins, which are familiar objects of the sea-side, and it 

 is further of such interest both morphologically and physiologically as 

 to make at least an outline sketch desirable. Material for such an 

 outline is provided by the common starfish (Astertas, Fig. ii6. A), which 

 is easily obtained and easily dissected. 



The most conspicuous feature of the starfish is that which gives it 

 its name-j-its radiate symmetry — five arms projecting outwards from a 

 central portion or disc. There are two distinct surfaces, a lower or oral 

 surface with the mouth in its centre, and an upper aboral or apical surface 

 with the very minute anus (Fig. ii6. A, a) near its centre. From the 

 mouth there passes outwards along the oral surface of each arm a deep 

 groove — the ambulacra! groove — from which there project numerous 

 little cylindrical semi-transparent tube-feet (t.f), each with a round sucker 

 at its end, by the co-ordinated movement of which the starfish is able 

 to draw itself slowly along a solid surface. The region bearing the tube- 

 feet, which in other Echinoderms need not necessarily have the form of 

 a groove as it has in the starfish, is termed an ambulacrum and from this 



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