On the Visual Organs in Lamellibranchiata. 469 



animai where it be of most use, and in the majority of cases this would 

 be at the oral end of the animai ; but in the Lamellibranchiata . where 

 we have probably a degenerate g-roup and the head atrophied or absent 

 and not exposed to the light , the morphological eyes have become lost 

 and other adaptive organs developed , where they would be of the 

 greatest service. 



We have therefore in the Lamellibranchiata. the obliteration 

 through disuse of true and perhaps once tolerably well developed eyes 

 and the formation of adaptive visual organs either in an early stage of 

 development or they have reached a state sufficiently high for the uses 

 of the animai. 



In a higher form even than Patella we have an interesting advanee 

 as shown by Fraisse in Haliotis. Here we have a lens present. but 

 the eye still open anteriorly; the pigmented cells, however, which form 

 the ball of the eye have no trace whatever of the refractive cuticle 

 (fig. 13) Seen in Patella and the Lamellibranchiata, the want being 

 supplied by a distinct lens. Nerves also are so differeutiated as to be 

 Seen in this form. 



I may add here that I saw no such appearance in the Lamelli- 

 branchiata. when I made transverse sections of the whole cell as 

 Fraisse represents for Patella (fig. 11). I am inclined to think that he 

 made an error in stating , that the clear protoplasm extended to the 

 surface as he represents. I had found that the cells of the Lamelli- 

 branchiata also were clear of pigment in the center , but by studying 

 very thin sections and also oblique ones , I came to the conclusion, 

 that this only existed at the lower part of the cell , when the pigment 

 became deposited on the sides of the cells, at the ends, however, the 

 whole was filled with pigment. 



End of March 1884. 



