558 William Patten 



the eye of Pecten. Au incomplete knowledge of the anatomy and 

 development of these eyes has deterred ali my predecessors from mak- 

 ing any attempt to solve tliis apparently hopeless problem. They bave 

 contented themselves with the statement that we must look to the de- 

 velopment for the solution. I thought so too, and after a careful, and, 

 I believe, in the main successful study of the development, v^as much 

 disappointed to find that the solution did not appear any easier than 

 hefore. Instead of developing from invaginations, as I had expected 

 would be the case, they were formed from knob-like papillae. I left 

 the matter for a while, and turned my attention tovrards other genera 

 of Lamellibranchiata in hopes of finding some clew to the subject. 

 After studying the eyes of Arca^ I came to the conclusion that the 

 solution could be found there, ifanywhere. Although the following 

 explanation may not rest upon sufficient evidence, stili, as far as it 

 goes, it is good. Development has shovyrn that, in the young Pecten, the 

 branchial side of the Ophthalmie fold is eovered V7ith a number of 

 small pigmented pits exactly similar to the invaginated eyes of 

 Arca. There are also small pigmented papillae consisting of a few 

 ommatidia, which in the larvae are exceptionally large and well deve- 

 loped, but which in the later stages degenerate or disappear. These 

 facts indicate that the Ophthalmie fold, in the ancestors of the present 

 Pecten, was provided with many invaginated eyes, together with in- 

 numerable, well developed and isolated ommatidia. a condition which 

 could best be compared with that in Arca of to-day. A most remark- 

 able agreement is to be found between the paired arrangement of the 

 eyes of Pecten and the faceted ones of Arca. In neither genera is the 

 sequence of the pairs Constant; usually two large eyes are separated 

 by one or two small ones. We bave seen that the largest eyes of Arca 

 are arranged along a pigmented line on the summit of the Ophthalmie 

 fold, and that on the branchial side of this fold were many smaller 

 eyes in various stages of development. Now in the larvae of Pecten 

 a similar condition prevails. There is a pigmeut furrow at the base of 

 the Ophthalmie fold from which the large permanent eyes develop, while 

 on the branchial side of the fold are numerous small and transitional 

 invaginate eyes. We may suppose that, in the ancestral Pectens, there 

 were many highly developed, invaginate eyes along the pigmented 

 furrow. There is in ali Mollusca a tendency for such cup-like eyes 

 to form closedsacs. but in ali cases, exeept Pecten, the posterior wall 

 becomes the most highly developed, and gives rise, finally, to the 

 retina. If we suppose that in Pecten the vesiele, formed by the closure 



