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W. J. DAKIN. 



it is from these lobes that the nerves arise which innervate 

 the eyes. It is interesting, therefore, to observe how the 

 development of the eyes has affected the ganglion, for in P. 

 jacobaeus and P. maximus, where the number of eyes on 

 the left mantle-lobe exceeds that on the right, the left lateral 

 lobe of the visceral ganglion is considerably larger than the 

 right, especially in the former species, whereas in P. oper- 

 cularis, where the number of eyes is more equal on both 

 sides, the left lobe is but slightly larger than the right. 

 Probably the presence of both lateral lobes is due in the first 

 instance to the great development of pallial structures. 



Text-fig. 1. 



-Pall.N. 





1. 



The retina of Pecten is of the inverted type, that is (like 

 the vertebrate eye), the recipient bodies, the rods, are directed 

 towards the tapetum, and away from the source of light 

 (text-fig. 2). In addition to this feature we have a com- 

 plexity only paralleled in a few cases in the invertebrata 

 (and even then without the inversion), for there are two 

 series of recipient cells. Inversion occurs in the Platyhelmia, 

 though the eyes are much simpler than the Pecten eye. In 

 the Lamellibranchiata the eyes are either absent or much 

 more simple as a rule than the eye of Pecten, but we have as 

 a matter of fact the two eyes most like the one we are con- 

 sidering in this group, namely, the pallial eyes of Spondylus, 

 which are practically the same as Pecten, and the eyes 



