The eyes of pecten, spondylus, amussium and allied lamellibranchs, with a short discussion on their evolution

The eye ofPecten has excited an unusual amount of interest for many years, and many workers have described its structure. The present author devoted a considerable period to the study of its minute histology in 1908, and was able to make clear for the first time certain fundamental features in the structure of the retina. This work was confirmed in all essential points by Kupfer (1916). Several writers have commented upon the resemblances between the eyes ofSpondylus andPecten , although with the exception of Hickson's account of the structure of theSpondylus eye and a very brief reference by Hesse, there is no paper dealing especially with this genus. There is a general impression, too, thatPecten andSpondylus stand alone amongst Lamellibranchs in the possession of the most complicated type of eye, with two layers of sensory cells in the retina, and no mention appears in the literature of such genera asAmussium ,Chlamys andPedum , which are, no doubt, very closely related topecten , although Ridewood placedAmussium in the family Mytilacea.