348 



COMPARATIVE AKATOMY. 



connected witli the cerebral as witli tlie secondary pedal ganglia. 

 It gives off the pallial nerves ; these undergo degeneration in pro- 

 portion to the extent to which other nerves are given off from the 

 same commissural ganglia. 



In some of the Prosobranchiata some of these nerves are remark- 

 able on account of the course that they take. They are present 

 in Haliotis; where they pass off from the common pedal ganglionic 

 mass (the pallio-pedal ganglia). In other cases they are given off 

 from the commissural ganglia (Fig. 184, Go). A nerve is given off on 



the right side which passes above the visceral 

 mass to a ganglion, which supplies the body- 

 wall (supraintestiiial ganglion) (.s;^^). From the 

 left commissural ganglion a nerve passes below 

 the viscera to a subiutestinal ganglion (.sA), 

 which, like the former, is connected by a com- 

 missure with an abdominal ganglion {A). 



The two nerves, therefore, which are given 

 off from the commissural ganglia cross over 

 one another ; this peculiarity— namely, that 

 the right nerve passes to the left, and the left 

 to the right side — makes it difficult to explain 

 the genesis of these nerves. It is probably 

 due to changes in position, which have not 

 affected the internal parts only, for the ganglia 

 on these nerves give off branches to the body- 

 wall. Although a large number of the Proso- 

 branchiata are distinguished by this crossing 

 of the nerves (Chiastoneura), it does not ob- 

 tain in another division, in which the commis- 

 sure to the abdominal or visceral ganglion 

 takes a straight course backwards (Ortho- 

 ueura), except when the ganglion appears to 

 be fused with the right commissural ganglion 

 (Nerita). The commissural are generally separated from the pedal 

 ganglia, and, in the Heteropoda, are widely so (Carinaria), and in 

 this case the commissures are proportionately elongated. The 

 same thing happens also to the cerebro-pedal commissures in the 

 Heteropoda. 



There is a commissure between the pedal ganglia in the Tecti- 

 branchiata, which are consequently pushed more to the sides of the 

 body. The primitive visceral ganglia are also placed at the sides, or 

 between the pedal and cerebral ganglia (Umbrella, Gasteropteron) ; 

 there maybe commissui'al ganglia present, just as there are typically 

 in the Prosobranchiata, which send off connecting chords to one or to 

 a pair of ganglia which supply the gills ; the ganglia appear to 

 coi'respond to the above-mentioned abdominal ganglion (Aplysia, 

 Accra) . 



The pedal ganglia are still more widely separated from one 

 another in the Nudibranchiata, and are thereby approximated to 



Fig. 181. Nervous system 

 of Paludina vivipara. 

 C Cerebral, P Pedal, 

 Co Commissural gan- 

 glia. B Buccal ganglia. 

 A Abdominal ganglion. 

 sp Supra, sb Subintes- 

 tinal ganglion. %i Pedal 

 uoryes. o Otocyst (after 

 H. V. Jheriug). 



