454 COMPARATIVE ANATOMY CHAP. 



gether independent systems : (1) the superficial oral, (2) the deeper 

 oral, and (3) the apical system. 



(a) The superficial oral nervous system is developed on the oral 

 side of the body, and is always more or less superficial. Its most 

 important and constant constituents are : (1) a nerve ring encircling 

 the oesophagus, and (2) radial nerves radiating from this ring, and 

 corresponding in number with the radii. This system innervates the 

 integument, the ambulacral appendages, and the intestinal canal. It 

 occurs in all Echinodermata without exception. 



(b) The deeper oral nervous system accompanies the superficial 

 system on its inner side (i.e. on the side turned towards the body 

 cavity). In the Ophiuroidea and Asteroidea it is paired in each radius, 

 i.e. its trunks or ganglia lie on the two sides of the radial nerves of 

 the superficial system. In the Echinoidea and Holothurioidea, on the 

 contrary, it is unpaired in each radius, and consists of a trunk or ridge 

 in close contact with the radial nerve of the superficial system on its 

 inner side. In the Ophiuroidea and Asteroidea a more or less complete 

 ring surrounding the oesophagus seems to be developed in this system, 

 but this is wanting in the Echinoidea and Holothurioidea. The Crinoids 

 and those Echinoids which have no masticatory apparatus have no 

 deeper oral nervous system. 



This system innervates the muscles which run in the oral side of 

 the body wall ; in the Holothurioidea it perhaps innervates the whole 

 dermo- muscular tube, but in the Echinoidea probably only the muscles 

 of the masticatory apparatus. 



(c) The apical nervous system is specially strongly developed in 

 the Crinoidea. It consists of a nerve envelope, surrounding the 

 chambered organ, and forming a centre from which five nerves run out 

 along the axial canals of the brachial skeleton, penetrating as far as 

 to the distal joints of the pinnulse (cf. Fig. 356, 8, p. 413). 



The apical nervous system is also continued into the stalk and the 

 cirri. It innervates the whole of the musculature which moves the 

 arms and the cirri. 



In the Asteroidea the apical nervous system consists of radially 

 arranged nerve trunks which meet at the centre of the disc ; there is 

 one trunk for each arm. These trunks run along the middle line of 

 the arms, immediately above the body cavity, and innervate the dorsal 

 muscles of the arms. 



In the Ophiuroidea and Echinoidea a delicate nerve trunk runs 

 within the wall of the aboral circular sinus throughout the whole 

 course of the latter ; this is the genital nerve ring. 



The Holothurioidea have no aboral system. 



A. The Superficial Oral Nervous System. 



Until comparatively recently this system was the only nervous 

 system known in Echinodermata. 



