344 



COMPARATIVE ANATOMY. 



organs which are placed in the head, prove that these ganglia 

 are homologous with the cerebral ganglia of Vermes (and of 

 Arthropoda). The pedal ganglia may also be derived from a more 

 simple condition, for in many of the lower Mollusca we find them 

 replaced by an arrangement which corresponds to the ventral chord 

 of the Annulata. Longitudinal trunks are given off from the pedal 

 ganglia, and are distributed along the foot ; since they are con- 

 nected together by transverse chords, they are arranged in the same 

 manner as a ventral nerve-chain. 



Although there may be nothing really fresh in this arrangement 

 of the nervous system, inasmuch as the two ventral (or pedal) ganglia 

 must be regarded as a concentrated nervous mass, which is broken 

 up in lower forms, and constitutes a ventral ganglionic chain; yet the 

 greatly-developed visceral ganglia form an arrangement which is 

 nothing like so well marked in the Vermes as it is here. In the 

 Mollusca the visceral ganglia are not only of importance as forming 

 a part of the general nervous system, but they may also fuse with the 

 cerebral ganglia, owing to the gradual shortening of their commissures. 

 New, and primitively peripherally-placed parts, are thereby added on 

 to these central organs ; and it becomes a matter of doubt whether 

 or no these ganglia, which formerly belonged to the visceral nervous 

 system, should still be regarded as belonging to it. These parts 

 of the nervous system which supply the viscera (heart, branchial 

 apparatus, and generative organs) are the cause of great complica- 

 tions of the whole system ; owing to the way in which they vary in 

 number in different divisions, they make compainson very difficult, 



as indeed also do the great modifica- 

 tions in position undergone by the 

 primitive ganglia, in consequence of 

 the abbreviation or elongation of their 

 commissures. 



The nervous system of the Placo- 

 phora is one of the lowest found. A 

 nervous band formed of two chords 

 (Fig. 180, 0) surrounds the oesophagus, 

 but there arc no superior enlargements 

 on it; this is probably due to the absence 

 of eyes and tentacles. The inner of the 

 two chords is continued separately 

 below the oesophagus ; part meets its 

 fellow of the other side in the subpha- 

 ryngeal ganglion and part passes on 

 to a pedal ganglion (P). Each of these 

 bilateral ganglia gives off a thick nerve- 

 trunk, which passes backwards, and 

 which, like the ganglia themselves, 

 is connected with the trunk of the op- 

 posite side by transverse anastomoses set at regular distances; nerves 

 are given off to the foot from corresponding points. The outer chord, 



Fig. 180. 



Chiton cinereus. 



nerve-chord. P 



<pl Fallial nerves. 



system of 

 C Cerebral 



Pedal ganglia. 



p Pedal nerves. 



B Buccal 



ganglia (after 

 Jhering). 



H. v. 



