210 COMPARATIVE ANATOMY. 



little inside the calcareous ring ; anteriorly it is limited by the oral 

 integument (Fig. 113, n). As it is larger than any of the five nerves 

 (it) which it gives off, which is not the case in the nervous ring of 

 the Astero'i'da and Echinoi'da, it has more distinctly the significance 

 of a central organ ; in this point some analogies may be made out 

 between it and the ganglionic oesophageal ring of other animals. 

 But it is clear that it has no real homology whatever with any ring 

 of this kind in consequence of the origin of the oesophageal ring in 

 the Echinoderina>, already described when we were treating of the 

 Astero'i'da. The peripheral nerve-trunks pass out by openings in 

 the five larger pieces of the calcareous ring, and extend, becoming 

 broader as they go along the outer side of the bands of longitudinal 

 muscles to the hinder end of the body ; near the cloaca they again 

 diminish in breadth ; they give off fine branches along their course. 

 Each radial nerve-trunk may be divided into two layers, which are 

 separated from one another by a layer of connective tissue. A 

 vessel accompanies the radial nerves ; this is separated by a wall 

 of partition from the ambulacral vessels, which lie still more to 

 the interior. The oral ring gives off tentacular nerves, in addition 

 to these radial trunks. 



Sensory Organs. 

 § 170. 



Definite portions of the integument have, in this group also, a 

 special significance as tactile parts. The tentacles, as well as the 

 sucker connected with the water-vascular system, may be reckoned 

 as tactile organs, and the former become greatly developed, and 

 so of greater importance, when the ambulacral system is reduced, as 

 it is in the Holothuroi'da (Apoclia). 



Five pairs of vesicles, which lie on the roots of the radial trunks 

 in the Synaptidaa, are said to be auditory organs, but their sensory 

 function is as doubtful as is that of the so-called eye specks in the 

 same genus. 



Visual organs are exactly known in the Asterida only ; in all 

 other Echinoderma mere collections of pigment are regarded as eyes 

 or " eye-spots." The eyes of the sea-stars are placed at the tip of 

 each arm, which is ordinarily bent up, and so turned towards the 

 light ; they occupy a pad-like elevation of the end of the ambulacral 

 groove, the epithelial layer of which is formed of long cylindrical 

 cells, and is very thick at this point. The rod-shaped cells contain 

 pigment. The eyes lie on separate points of the " optic-pad." A 

 funnel-like cavity covered by the cuticle has its walls bounded by 

 rod-like cells, which are inclined from the periphery to the 

 funnel ; in this way their ends form the wall of the funnel. A 

 transparent body projects from the pigmented part of the cells into 

 the cavity of the funnel, and so fills up the greater part of its lumen. 



