AMBULACRAL APPENDAGES. ALIMENTARY CANAL. 



275 



M. 



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 ffolo- t 

 tlmrians (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 Spatangidea 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- 

 j)eastridea ; almost mi- 

 croscopic in size ; they 

 are very numerous, 

 and are arranged in 

 branched rows or are 

 irregularly distributed 

 over the surface. 



The Echinodermata 

 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 



Wl 



&^ 



Cl 



centre of the apical 

 pole, rarely in an inter- 

 radius on the ventral 

 side. The intestine 

 may, however, end 

 blindly, as for example 

 in all the Ophiuridce and EuryaHln-', also in the genera Aster o- 

 Ctenodiscus, and Liddia, which have no anus. The mouth 



FIG. 219. Holothuria tululwa, opened longitudinally (after 

 M. Edwards). O, Month in the midst of the tentacles 

 (T) ; D. digestive canal ; Sc, stone canal ; P, Polian 

 vesicle ; Kg, circular vessel of the water-vascular system ; 

 Ov, ovaries ; Ay, ambulacral vessel ; J/, longitudinal 

 muscles; Gf, vessel to the intestine; Cl, cloaca; If'/, 

 respiratory trees. 



