238 ECHINODERMA. 



this is certainly the main function of ten which lie near 

 the mouth. 



Alimentary Canal. 



The alimentary canal passes through Aristotle's lantern, 

 and the intestinal portion lies in two and a half coils around 

 the inside of the shell to which it is moored by mesenteries. 

 It contains fine gravel, sand, and some organic debris. It 

 ends near the centre of the apical disc, whence the pedi- 

 cellariae have been seen removing the faeces. 



Accompanying the first coil of the gut is a canal or 

 " siphon," which opens into the gut at both ends. Accord- 

 ing to Cuenot, a current of water traverses this tube, which 

 thus, by reason of its thin walls, carries oxygen to the cor- 



FIG. 77. Ventral half of Sea Urchin. (From CARUS, 

 after TIEDEMANN.) 



</, Aristotle's lantern in centre ; oe, oesophagus ; z, intestine ; 

 S, intestinal blood vessel ; R, radial water vessel in an ambulacra! 

 area ; A , an inter-ambulacral area ; m, muscles of the lantern. 



puscles of the body fluid. The spacious body cavity is 

 lined by ciliated epithelium and contains a " perivisceral " 

 fluid, whose corpuscles have a respiratory pigment (echino- 

 chrome). When the fluid of a perfectly fresh sea urchin is 

 emptied out, the contained corpuscles unite in plasmodia, 

 forming composite amoeboid clots (cf. Proteomyxa, &c.). 



Water Vascular System. 



The madreporic plate communicates with a membranous 

 stone canal, which runs downwards into a circular vessel 



