CHAPTER VIII 



ECHINODERMATA 



SCHEME OF CLASSIFICATION 



I . . \STKROIDKA Starfish. 



II. OpHirRomKA Brittle-stars. 



III. !:< ii i\< IIDEA Sea-urchins. 



IV. nm.oTurROiDEA Sea-cucumbers, Trepangs. 

 Y. CKINOIDEA Crinoids, Encrinites. 



Tins 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 -hook 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 (Asterias, Fig. 116, A), which 



ily obtained and easily dissected. 



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



its name its radiate symmetry five arms projecting outwards from a 



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



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



with the very minute anus (Fig. 116, A, a) near its centre. From the 



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



ambulacral groove from which there project numerous 



lindrical semi-transparent tube-feet (/./), each with a round sucker 



end, by the coordinated movement of which the starfish is able 



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



'iich in other F.chinoderms 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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