CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 243 



otocystic nerves to make direct connection with the cerebral 

 ganglia, but they do not join the ganglia at their nearest points. 

 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 de- 

 veloped, 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 lamellibranchs, for it is very probable that they never 

 had them. In fact were ganglia ever present in this region in 

 lamellibranchs, 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 gastro- 

 pods. The gastropod and lamellibranch are so different in struc- 

 ture and habits that we may reasonably expect important differ- 

 ences in their nervous systems. Gastropods and cephalopods 

 possess accessory ganglia that have evidently been developed to 

 perform special functions. That such centers may be compara- 

 tively 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 developed in 

 the groups that need them than that lamellibranchs, 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? 



ANATOMICAL DRAWINGS. 



It sometimes happens that in making a series of drawings in- 

 tended to illustrate different organs of the same animal, consid- 

 erable labor can be saved by using a combination of photograph 

 and ink. The figures of the present paper illustrate this saving 

 much better than is usually the case. To draw the margin of 

 the mantle, with its large number of sense organs, requires both 

 time and patience, and were it necessary to draw it for each of 

 the figures where it necessarily occurs, one would be tempted to 

 abandon it altogether. 



