258 PHYLUM ECHINODERMATA. 





radial prolongations, and lies as in all other classes in the ecto- 

 derm, but the ectoderm containing it is separated from the 

 surface and forms the lining of an epineural canal (Fig. 180, 

 cv.ep.) as in Ophiuroids and Echinoids. The circumoral part 

 (Fig. 179, 7) lies immediately round the mouth in the inner 

 wall of the circumoral part of the epineural canal (not shown in 

 the figure), and the radial nerves extend along the whole length 

 of the radius almost as far as the anus. This system gives off 

 branches to the tentacles, tube-feet, and skin, and, from its 

 circumoral part, to the gut. In the skin there is a subepithelial 

 nervous plexus in the dermis. 



The deep oral system is obscurely double (Fig. 180, nh). It 

 lies in the outer wall of the radial perihaemal canal, to the 

 epithelium of which it has the same relation as has the ecto- 

 neural system to the ectoderm. It is so closely applied to the 

 radial cords of the ectoneural system, that it was not till the 

 publication of Herouard's important work on the group that 

 the two were distinguished. It extends along the whole length 

 of the radius and is without any circumoral part. 



Sense organs. Integumentary sense organs are of course 

 present, but there does not appear to be an organ for the per- 

 ception of light. Otocysts are present in the Synaptidae and 

 Elasipodidae. In the Synaptidae there are five pairs of them 

 placed on the radial nerves at the point where these pass beyond 

 the calcareous ring. In the Elasipodidae they are more 

 numerous (from 14 to more than 100) and they occur along 

 the course of some or all of the radial nerve trunks. They 

 have numerous small otoliths and a ciliated lining. In the 

 Synaptidae the otoliths are vesicular cells with a fluid con- 

 tents and collapse and disappear when the animal is placed 

 in spirit. 



The alimentary canal. The mouth, though really terminal, 

 may in consequence of the curvature of the axis appear to be on 

 the dorsal (Psolus, Theelia, Psolidium, Colochirus) or ventral 

 (many Aspidochirotae and Elasipodidae) surface (see p. 267). 

 It is without any armature of teeth or papillae and is placed in 

 the midst of the tentacular circlet. It leads into the oesophagus 

 which a little behind the water- vascular ring is continued, often 

 without any marked line of demarcation, into the stomach. 

 The stomach is short and tubular and is continued, again often 



