THE EYE OF SPONDYLUS. 363 



no space between it and the retina, such as is found in 

 P. opercularis and, in a smaller degree, in P. Jacohceus. 



The retina (c) stretches across the eye-cup as a broad 

 band, leaving a considerable space behind for the tapetum and 

 pigment, and but little space in front between it and the lens. 

 The membrana limitans is not bent up in the middle as it is 

 in Pecten maximus^ but exhibits slight sinuation as it does 

 in P. opercularis. Thus, in this genus we have combined 

 the forward position of the retina with but slight sinuation 

 of the membrana, a condition I have-not found in any species 

 of Pecten. 



-^'t^m^m^^^'"'j'''''"'^ 



Fig. 1.— Diagram of the eye ot S[JOudylus. a, cornea ; 0, ieus ; c, retina;- 

 d, tapetum ; e, pigment ; f, retinal nerve ; tj, complimentary nerve ; 

 h, epitlielial cells filled with pigment ; k, tentacles. 



The distribution of the nerves is the same as it is in 

 Pecten, both the retinal (/) and complimentary {g) branches 

 being easily traced. 



There is in this genus, too, a well-marked tapetum {d), 

 composed of the same histological elements which I described 

 in Pecten. There is nothing to be said about the pigment 

 (e) further than that it is of the same brick-red colour as 

 in Pecten, is apparently of the same granular composition, 

 and occupies a similar position behind the tapetum. 



As regards the morphological value of this eye, it is im- 

 possible at present to say anything definite, but it seems 

 probable that the shortness of the stalk, the position of the 

 retina, and the slight development of a vitreous cavity point 

 to the fact that it is not as highly developed as the eye of 

 Pecten. It probably presents a more complicated condition 



