86 



accumulates in the more dorsal intrapallial lacunte and 

 in the spaces round the adductors. The lacunoe in the 

 contracted foot itself are only potential, and the circulation 

 there must be largely confined to that taking place in the 

 visceral artery and in the venous lacunae round the intes- 

 tine. This explanation of the condition of turgescence 

 assumes that the quantity of blood contained m the body is 

 sufficient to produce the distension of the parts in question. 

 Fleischman has shown that this is the case for Anodonta, 

 and the assumption may not unreasonably be made that 

 it is the case also in Cardvum. 



The Nervous System. 



The central nervous system in Cardium is constituted 

 by the two separate, paired, cerebral ganglia, each of which 

 represents the fusion of originally distinct cerebral and 

 pleural ganglia; the single median pedal ganglion, formed 

 by the fusion of originally lateral and paired pedal ganglia, 

 and the single median parieto- splanchnic ganglion also 

 formed from originally separate, right and left, ganglionic 

 masses. There are two pairs of connectives, the cerebro- 

 visceral connectives joining the cerebral and parieto- 

 splanchnic ganglia, and the cerebro-pedal which connect 

 the cerebral and pedal centres. The whole nervous system 

 is bilaterally symmetrical. 



The cerebral ganglion of each side (fig. 3, PI. I., Ga.c.) 

 lies quite superficially in the loose connective tissue 

 between the bases of the labial palps, the anterior dorsal 

 wall of the viscero-pedal mass, and the anterior adductor ; 

 and is easily exposed by removing the palps close to then- 

 attachments, and parting slightly the adductor from the 

 adjoining body-wall. It is oval in shape and unpigmented. 

 The right and left ganglia are joined together by the 

 long cerebral commissure {Com.) which runs in the base of 



