VII MOLLUSOA—THE NERVOUS SYSTEM 129 



One pair of these cords run along the body laterally, these are the 

 lateral or pleurovisceral cords ; the second lie ventrally, and are the 

 pedal cords. The visceral and the pedal cords of each side unite 

 anteriorly, and when so united become connected vi'ith those on the 

 opposite side by a transverse commissure, vphich runs in front of and 

 over the oesophagus and contains ganglion cells ; this is the cerebral 

 or upper half of the oesophageal ring. The pleurovisceral cords unite 

 posteriorly above the rectum, forming a visceral loop. The pedal 

 cords are connected both inter se and with the pleurovisceral cords by 

 anastomoses, so that the whole nervous system strikingly recalls the 

 ladder nervous system of the Turbellaria and Trematoda. 



a. Chitonidse (Figs. Ill and 51, p. 40). — The scheme just given 

 is founded upon the nervous system of Chiton. The typical ganglia of 

 the central nervous system of the Mollusca are not yet, in Chiton, found 

 as distinct ganglia united by means of commissures and connectives, 

 but the ganglion cells are equally distributed along the commissures 

 and connectives, an arrangement which is probably primitive. The 

 upper oesophageal ring thus corresponds with the cerebral ganglia and 

 the commissures connecting them, and in the same way the pedal 

 cords contain the whole central portion of the pedal nervous system, 

 and the pleurovisceral cords the central portion of the visceral, pallial, 

 and branchial nervous systems. Only in one single species of Chiton 

 (C rubicundus) two distinct (cerebral) ganglia occur near each other 

 in the middle line in the upper half of the oesophageal ring. 



Looking more closely at the nervous system of the Chitonidee, ve have to 

 observe : (1) the arrangement of the cesophageal ring and the medullary cords ; (2) 

 the peripheral ganglia ; (3) the nerves of the ladder-like nervous system ; (4) the nerves 

 running from the central nervous system (cesophageal ring and medullary cords). 



1. Form and arrangement of the central nervous system. — The visceral cords 

 run back one on each side in the lateral body wall above the branchial groove ; these 

 two cords unite above the anus. The pedal cords run in the dorsal part of the 

 pedal musculature somewhat near one another, from before backward, to end without 

 uniting where the rectum commences. The oesophageal ring consists, in the first 

 place, of the semicircular portion mentioned above, which, on account of the peculiar 

 shape of the body of the Chiton, lies in the same plane as the visceral cords. Poste- 

 riorly, each limb of this semicircle divides up into the pedal and visceral cords. At 

 the point where the pedal cord rises from the ring, a cord with a thickened base 

 separates from it and runs inwards ; this, uniting below the mouth with a similar 

 cord from the other side, foi-ms the lower half of the oesophageal ring. The upper 

 and lower halves together form the closed oesophageal ring. 



2. Besides this central nervous system there are peripheral ganglia connected 

 with it by nerve cords consisting only of nerve fibres. 



(a) The buccal ganglia together form a horseshoe-shaped ganglionic mass below 

 the oesophagus, which mass is connected on each side by the cerebrobuccal connective 

 with the thickened portion of the lower oesophageal ring. The buccal ganglionic 

 mass in 0. ruMcunchis divides into two paired ganglia and one unpaired ganglion 

 joined to one another by connectives. The buccal ganglia innervate the oesophagus 

 as far as the stomach and also the oral aperture. 



(6) On each side, from the lower half of the cesophageal ring, somewhat further 

 VOL. II K 



