Eyes of Molluscs and Arthropods. 613 



and retina situateci below the epithelium, But tliey are not so easily 

 overlooked in C. eJuIe, where one side of the eye is covered with browu 

 pigment, not solublc in alcohol. 



If we Start with the siipposition that all eyes are modifieations of 

 hypodermie cells. against which there can be, I tliink. no very serious 

 objection. then it is natural to expect that, the farther the organ has 

 become removed from the seat of its origiu, the older it is. and the 

 more chang-es it has undergoue. But we also und, that those organs, 

 which are phylogenetically the oldest , or which have passed through 

 the greatest number of intermediate changes, are either highly de- 

 veloped; or were so once. In the present instauce. the changes 

 necessary to evolve such au organ from simple hypodermie cells must 

 have been radicai aud numerous, and it is not easy to imagine auy 

 plausible method by which such a process could have taken place. But, 

 in spite of the fact that it is so highly differentiated as regards 

 what must have been its original condition, it is still an ex- 

 tremely simple organ, whose functional power must be of a very low 

 Order. I consider, therefore, that duriug the long and complicated series 

 of changes necessary for the evolutiou of such an organ, it at one time 

 probably reached a much higher structural, as well as functional con- 

 dition, and that the present, very simple organ is due to degeneration. 



Cardium tuberculatum. 



In this species, the pigmented band at the summit of the tentacles 

 is absent, and the brilliant lustre, so characteristic of C. edule, is al- 

 most entirely wanting. But, with a careful examination, one easily 

 sees two rows of large, ova-like cells, closely packed, and extend- 

 ing from the tip of the tentacle, where they unite in the median 

 spherical mass above the retina, nearly to its base. These cells coutain 

 diffuse coloring matter which gives them their reddish tinge. The same 

 form and general arrangement of the tentacles obtain here as in C. 

 edule^ but cilia or scuse hairs, I have not been able to detect. 



The tentacles of C. tuberculatum are very sensitive to the amount 

 and inteusity of light, while, on the other band, Irritation by contact, 

 and shocks or movements of the water caused by a sharp tap upou the 

 glass in which they are contained, apparently produce no effect upon 

 them. This is just the opposite to what is found in Pecten, where the 

 tentacles are extremely sensitive to tactile Impression, or to coarse 



