I92I.J N. Annandale & R. B. .S. SewEix: Vivipara. 239 



of the hinder portion of the cerebral ganglion. The anterior 

 cerebro-pedal commissure is long and narrow and of a white colou'- ; 

 it passes downwards external to the lateral retractor muscle of the 

 buccal mass to join the pedal ganglion. The posterior cerebro- 

 pleural commissure is broad and ribbon like, and can be seen to 

 consist of two parts, an anterior brownish-coloured portion and a 

 posterior white portion. Inferiorly it fades into a very ill-defined 

 left pleural ganglion. Below this ill-defined ganglion a short broad 

 commissure passes downwards, being joined by the cerebro-pedal 

 commissure to the pedal ganglion. On the right side of the body 

 there is no distinct cerebro-pleural commissure. "The right pleural 

 ganglion lies close against the posterior end of the cerebral 

 ganglion and is only separated off from it by an ill-defined neck. 

 The pleuro-pedal commissure is in consequence proportionately 

 long. On this side of the body the arrangement of these ganglia 

 conforms to what Moore (1903, p. 276.) terms the ' epiathroid ' 

 type of nervous system. 



From the pleuro-pedal commissures several fine nerves arise ; 

 of these one, usually the largest, can be seen to arise from the 

 commissure low down near its point of union with the cerebro-pedal 

 commissure. This is the epipodial nerve and that on the left, 

 which is usually slightly the larger, supplies the epipodium on that 

 side, while the right supplies the fold beneath the right tentacle 

 and the inner or left half of the syphon. The other nerves pass 

 upwards and outwards to the tissues at the base of tentacle. The 

 pedal ganglia are long ribbon-like structures of a brown colour, 

 which are connected above with the cerebro-pedal and pleuro- 

 pedal commissures. Each of these ribbons is composed largely 

 of ganglionic nerve- cells, and in consequence the whole length of 

 the structure must be regarded as being homologous with the more 

 compact pedal ganglia of other molluscs. Immediately beneath 

 the radular sac the two pedal ganglia are connected together 

 by a wide short pedal commissure which passes from side 

 to side below and in front of the posterior retractor muscles 

 of the buccal mass, and behind and above them the terminal 

 portion of the cephalic aorta passes downwards and backwards in 

 the middle line. From this point the two ganglia pass downwards 

 and backwards lying between the white muscle mass of the foot 

 and the soft grey tissue of the sole. At first the two cords diverge 

 somewhat, but posteriorly they again converge towards the middle 

 line. A series of three or four transverse commissures pass across 

 from side to side uniting them together at different points in their 

 length. A series of nerves arise from the pedal ganglia and 

 spread forwards and outwards. The first pedal nerve arises from 

 just below the pedal commissure and passes forwards towards the 

 anterior margin of the foot ; it sends off a branch which passes 

 inwards towards the middle line and forms an anastomosis with 

 its fellow of the opposite side. The remaining nerves pass out- 

 wards in a radiating manner and form a very elaborate anas- 

 tomosis around the margin of the foot. The nerves divide and 



