458 COMPARATIVE ANATOMY CHAP. 



ambulacral plates of an ambulacrum, and with the alternate arrangement 

 of the ambulacral feet at the two sides of the water vascular trunk. 



Each tube-foot nerve, with its peripheral nerve, passes out together 

 with the tube-foot canal of the water vascular system, through the 

 ambulacral pore, on to the surface of the test. The nerve of the 

 tube-foot is then continued in the epithelial layer to the tip of the 

 foot, without forming a ganglion. The peripheral nerve, however, 

 enters an integumental nerve layer which covers the whole body and 

 its appendages. 



The network of nerves or layer of nerve fibres, both in the intes- 

 tine and in the external integument, lies, in regular Echinoids (Cidar- 

 oida, Diadematoida) and in the Clypeastroida, deep in the epithelium, 

 but is subepithelial in the Spatangoida. 



5. Holothurioidea. 



The superficial oral nervous system is here subepithelial and 

 agrees in all respects with that of the Echinoidea. The nerve ring 

 which surrounds the mouth gives off the nerves to the oral tentacles 

 and the intestinal canal. The latter innervate also the skin round the 

 mouth, and ramify richly in the connective tissue layer of the intestine, 

 especially in its anterior portion. The radial nerve trunks give off 

 lateral branches to the tube-feet or ambulacral papillae, and "peri- 

 pheral nerves " to the integument. The latter break up into a 

 subepithelial network of fibres. 



In the Synaptidce, each radial nerve trunk, soon after it rises from 

 the oesophageal ring, gives off a pair of nerves to the auditory 

 vesicles. 



B. The Deeper Oral Nervous System. 



1. Asteroidea (Fig. 354, p. 411). 



In close contact with the inner side of each radial nerve ridge 

 (which is here epithelial) a subepithelial longitudinal band of nerve 

 cells and nerve fibres run along each side. A similar band accom- 

 panies the oesophageal ring, at least in its interradial regions. From 

 the radial bands of the deeper nervous system, lateral nerve bands 

 arise at regular intervals which correspond with the ambulacral feet. 

 These lateral bands ascend on the outer side of the radial pseudo- 

 haemal canals, and soon break up into fibres, which probably innervate 

 the muscles of the ambulacral skeleton. Nerves which emerge inter- 

 radially from the deeper nerve ring seem to innervate the interradial 

 muscles of the oral skeleton. 



2. Ophiuroidea (Fig. 355, p. 412). 



Here also, two lateral bands, consisting of nerve cells and 

 longitudinal fibres, lie in close contact with the inner side of each 



