vir MOLLUSCA—THE NERVOUS SYSTEM 131 



radular organ which is situated on the floor of the buccal cavity. The two sub- 

 ladular ganglia are united by a short commissure. 



(c) Two small gastric ganglia, connected by a fine commissure, lie at the anterior 

 end of the stomacli, and are joined on each side to the anterior end of the visceral 

 cord by a long connective. 



3. The nerves of the ladder-like nervous system. — Tlie two pedal cords are con- 

 nected by anastomosing conmiissures along their whole length, but no nerves are 

 given oft' by these commissures to the pedal musculature. In Chiton rahicundus the 

 visceral and pedal cords are united by numerous connectives, which, in other 

 Chitonidte, appear either to be wanting or to be reduced to one single anterior or 

 postei'ior anastomosis. 



4. The nerves running from the central nervous system : — 



(a) Nerves of the oesophageal ring. — Numerous nerves rise from the upper or 

 cerebral portion of the n'sophageal ring to innervate the cephalic part of the mantle, 

 the snout, the upper and lower lips, the gustatory buds on the lower wall of the 

 oral cavity, and the musculature of the buccal mass. The lower portion of the 

 oesophageal ring, besides the connectives to the buccal and subradular ganglia, 

 sends oft^ from its median portion another pair of nerves, which run along the base 

 of the buccal cavity. 



(h) Nerves of the pleurovisceral cords. — Each of the pleurovisceral cords 

 gives ofi" two nerves to each gill. Besides these they send many nerves to the 

 mantle, and, })Osteriorly, nerves which enter the body cavity, probably running to 

 the kidneys and the heart. 



(c) Nerves of the pedal cords. — The pedal cords give off" on each side seven or 

 eight nerves outwards to the lateral musculature of the body, and specially numerous 

 nerves run down from it to the pedal musculatuie (inner and outer pedal nerves). 

 These pedal nerves are richly branched, and, anastomosing w'ith one another, form 

 a complete neural network in the foot. 



h. Solenogastpes. — The central nervous system of the Solenogastres 

 differs from that of the Chitonidce principally in a tendency to form 

 distinct ganglia ; the pedal and pleurovisceral cords, nevertheless, 

 still retain their outer coating of ganglion cells along their Avhole 

 length. Fig. 112 is a diagrammatic representation of the structure of 

 the nervous system of Proneomeiiia Sluiteri. The fused cerebral ganglia 

 in the middle line are very large. On both tlie pleurovisceral and 

 the pedal cords ganglionic swellings can be distinguished : (1) three 

 pairs of posterior visceral ganglia ; (2) two anterior pedal ganglia. 



The posterior visceral ganglia are connected by cords, which run 

 transversely over the rectum and correspond, to some extent at least, 

 with the loop by which the two visceral strands in Chiton are united. 



The two anterior pedal ganglia are connected by a strong trans- 

 verse commissure, which may correspond with the venti'al half of the 

 oesophageal ring of Chiton. 



Further, the pleurovisceral cords are joined with the pedal cords, 

 and the latter are also connected inter se by transverse connections 

 along their whole length. The pleurovisceral cords likewise are con- 

 nected by arched transverse commissures.^ 



^ These connectives and commissures, however, do not seem to run uninterruptedly 

 from one cord to the other. 



