point. Instead they are continued around the connectives to 

 join the ganglia in contact with, and posterior to them. 



To me it seems probable that the separation into the two 

 groups that have developed into the classes Lamellibranchiata 

 and Gastropoda took place at an early date in the history of the 

 Mollusca, probably before a complicated head apparatus was 

 developed, and while the nervous system was of a very simple 

 nature. If this was the case, we have no reason to search for 

 pleural ganglia in lameilibranchs, for it is very probable that they 

 never had them. In fact were ganglia ever present in this region 

 in lameilibranchs, it would be more reasonable to view them as 

 new formations for special purposes than as direct descendants 

 from, and accordingly homologous with, the pleural ganglia of 

 gastropods. The gastropod and lamellibranch are so different in 

 structure and habits that we may reasonably expect important 

 differences in their nervous- systems. Gastropods and Cephalo- 

 pods possess accessory ganglia that have evidently been devel- 

 oped to perform special functions. That such centers may be 

 comparatively easily developed is indicated by the fact that the 

 circum-pallial nerves of the scallop are essentially such centers. 

 Is it not then more likely that pleural ganglia have been devel- 

 oped in the groups that need them than that lameilibranchs, 

 which, so far as we know have never been more complicated 

 than they are to-day, should have formerly possessed these 

 ganglia and have since quite uniformly lost them? 



3. 



Eyes. — gSNSE organs. 



The number and position of the eyes has been discussed in 

 connection with the structure of the mantle, on the lobes of 

 which they are borne. They have been so frequently and well 

 described by other investigators that it does not seem necessary 

 to give a detailed description here. Each eye (fig. 26) is club- 

 shaped, pigmented near its outer end, and its position and gen- 

 eral appearance indicate that it is probably a modified tentacle. 

 The extreme end is occupied by the cornea, (co.) which con- 

 sists of a single layer of transparent epithelial cells that are con- 

 tinuous with the layer of somewhat thicker cells that forms the 

 remainder of the covering of the tentacle. Near the free 

 extremity these cells are completely filled with a dark-brown or 



48 



