AMBULACRAL APPENDAGES. ALIMENTARY CANAL. 



275 



Jf- 



Sc 



all cases serve for locomotion. In addition to the ambulacral feet, 

 great tentacle-like tubes may be present as appendages of the water- 

 vascular system ; the circle of tentacles round the mouth of Holo- 

 thurians (fig. 209) is composed of such appendages. We also find 

 leaf -like appendages 

 arranged over four 

 or five-leaved rosette- 

 shaped areas, forming 

 the ambulacral gills of 

 the Spatanyidea, and 

 Clypeastridea (figs. 

 207 and 208). The 

 irregular Sea-urchins 

 all possess in addition 

 ambulacral feet upon 

 the ventral surface. 

 These are in the Cly- 

 peastridea almost mi- 

 croscopic in size ; they 

 are very numerous, 

 and are arranged in 

 branched rows or are 

 irregularly distributed 

 over the surface. 



The Echinodermatct 

 possess an alimentary 

 canal distinct from 

 the body cavity ; it 

 can be divided into 

 three parts oesopha- 

 gus, stomach, and 

 rectum. The anus is 

 placed usually at the 

 centre of the apical 



pole rarelvin an inter- FlG< 219 '~ HolotJlur ' a tu.lulca t opened longi udinaily (after 

 J M. Edwards). O, Mouth in the midst of the tentacles 



Wl 



Cl 



radius on the ventral 



side. The intestine 



may, however, end 



blindly, as for example 



in all the Opkixsrida* and Euryalidce 



pecten, Ctenodiscus, and Luidia, which have no anus. 



(T) ; D, digestive canal ; Se, stone canal ; P, Poliaa 

 vesicle ; Rg t circular vessel of the water- vascular system ; 

 Ov, ovaries; Ag, ambulacral vessel; M, 'ungitudinal 

 muscles; <?/, vessel tr the intestine; Cl, cloaca; Wl, 

 respiratory trees. 



also in the genera Aster o- 

 The mouth 



