viii ECHINODERMATA NERVOUS SYSTEM 457 



the base of the root, and then sends off a branch which runs to the tip of the tube- 

 foot (Fig. 372, 1). 



Nerves of the radial trunks. Two pairs of nerves arise at regular intervals from 

 the radial nerve trunks, as well in the free portion of the arm as in that included in 

 the disc. These are the tentacle nerves and the peripheral nerves. 



The tentacle nerve has a short course to the base of its tentacle, and then- forms 

 a ganglion the tentacle ganglion which embraces the base of the tentacle. The 

 nerve contains along its course nerve cells, which are still more plentiful in the ten- 

 tacle ganglion. The tentacle nerve and the ganglion are, like the radial nerve trunk, 

 accompanied by an epinenral canal. From this basal tentacle ganglion, the tentacle 

 nerve, which remains in the subepithelial layer, ascends to the tip of the tentacle. 



The peripheral nerve at first runs in close contact with the tentacle nerve of the 

 same side as far as to the base of the tentacle ; it then runs further laterally, pene- 

 trates into the lateral wall of the arm, running through the skeletal mass of the 

 latter, and breaks up into branches which innervate the ventral, lateral, and dorsal 

 integument of its own side of the arm. 



In those Ophiurids, the sides of whose arms are provided with spines (i.e. in the 

 majority of cases), peripheral ganglia are formed at the bases of these spines. 



The nervous system becomes somewhat more complicated in the portion of the 

 arm which is included in the disc. Branches of the peripheral nerve ascend apically 

 at the side of the bursa which is turned to the arm, i.e. on the radial side of the 

 genital plate described on p. 361, when it meets a lateral nerve (8) running along this 

 plate. Distally this lateral nerve is continued into an aboral marginal nerve (6), the 

 branches of which innervate the periphery of the disc. Proximally, the lateral 

 nerve passes over into an interradial nerve (11), which runs along the outer side of 

 the interradial muscle. 



In some cases (Ophiothrix, Ophiocoma, Ophioglypha), the first pair of tentacle 

 nerves and their accompanying peripheral nerves are followed by a pair of nerves 

 which run towards the bursal aperture, subsequently uniting with the lateral 

 nerves. 



In 0})hioglijplia, a delicate pair of nerves which innervate the integument of the 

 ventral (oral) side of the arm runs out between each of the regularly recurring pairs 

 of tentacle and peripheral nerves. 



4. Echinoidea (Figs. 353 and 358, pp. 410 and 419). 



In all Echinoids, even in those which are provided with a 

 masticatory apparatus, the ossophageal ring remains in close proximity 

 to the mouth, on the inner side of Aristotle's lantern, and at a con- 

 siderable distance from the water vascular and blood vascular rings, 

 which appear to be in a manner lifted up by the lantern. From the 

 oesophageal ring, especially, in the radii, nerves run to the oesophagus, 

 gradually breaking up into a plexus, which is still traceable even in 

 the wall of the first coil of the intestine. From the radial nerve 

 trunks, as in the OpMuroidea, at intervals which correspond with the 

 ambulacral feet, nerves for these feet and peripheral nerves are 

 given off. In the regular Echinoidea and the Clypeastroida, the tube- 

 feet nerves and peripheral nerves arise together, but in Spcdangoida 

 they arise separately. The points of origin of these two sets of 

 nerves from the radial nerve trunk are arranged alternately, in 

 accordance with the alternate arrangement of the two rows of 



