362 SYDNEY J. HrCKSON. 



Tke Eye of Spondylus. By Sydney J. Hickson, B.Sc, 

 B.A., Scholar of DowninjJ: College, Cambridge, Assist- 

 ant to the Linacre Professor, Oxford. 



Through the kindness of Mr. Balfour I was enabled to 

 study the structure of the eye of one species of this genus of 

 Lamellibranchs. The margin of the mantle of several speci- 

 mens was sent to him from the zoological station at Naples, 

 treated with osmic acid and absolute alcohol, and with abso- 

 lute alcohol alone. I was unable to determine the species to 

 which the specimens belong. 



The eyes of Pecten have been studied, as I pointed out in 

 my last paper,^ by several observers, but I have been unable 

 to find any special reference either in figures or in writing to 

 the eye of Spondylus. I hope, therefore, that this paper 

 will supply the omission. 



The eyes are situated on the edge of the mantle among 

 the fringing tentacles, and seem to be as numerous as they 

 are in Pecten, but one important difference is that the stalk 

 which is so apparent in the latter genus is here very short, 

 in fact, in some cases the eyes seem to rest on the edge of 

 the mantle itself. Being considerably smaller in size than 

 in the large Pecten mazimus I have been unable to follow 

 out all the details of the histology of the various parts, but I 

 have observed all the main points in the general anatomy of 

 the eye tolerably satisfactorily. 



The cornea is composed of the same layers as in Pecten, 

 the external epithelium of columnar cells being well marked. 

 Round the edge of the cornea this epithelium becomes much 

 thicker and densely filled with the dark brown pigment 

 described in a similar position in the eye of Pecten, thus 

 embracing the sides of the eye-cup. 



The lens is, again, composed of nucleated cells which are 

 more or less strap-shaped externally and internally become 

 polygonal ; but here the lens is large and projects into the 

 cup of the eye as it does in Pecten maximus, leaving little or 



> This Journal, Oct., 1880. 



