Eyes of MoUuscs and Arthropods. 575 



The inner edge of the raantle is continued inwards to form the 

 enormou8 velnm, the free edge of which is beset with small tentacles; 

 PI. 28, fig. 2 represents a section through the mantle edge of a young 

 Pecten opercnlaris. If one imagines all of these folds inereased, and 

 a greater developmeut of the tentacles, a very good idea of the adult con- 

 dition will be obtained. Only the Ophthalmie fold is ciliated, the remain- 

 ing portions are, however, studded with seuse hair papillae. 



The distributiou of the eyes offers several interesting peculiari- 

 ties that ha ve not been sufficiently treated by previous writers. They are 

 largest and most numerous at the anterior and posterior ends of the fiat, 

 left valve ; at those poiuts also, the pigment ring of the eye is lighter 

 and less developed, while the eye stalks are short. The pigmented ring 

 is larger and blacker, and the stalks longest, in the median part of the 

 mantle, but. at the same time, the eyes are farther apart, and placed at 

 pretty regulär intervals. One of the most remarkable things, is their 

 arrangement in pairs, something in the foUowing mauner: two large 

 ones are followed by two smaller ones, all at regulär intervals ; then 

 followtwo large ones, farther apart, with a single one betweeu, and so 

 on; although the sequence may not always be the same, still the 

 paired arrangement is never disguised. I thought, at first, to recognize 

 a definite law in the succession of large and smaller ones, but was 

 finally compelled to admit that the number of eyes not only varied con- 

 siderably, but that the sequence of the groups was not at all Constant. 

 We will show, in speaking of the development, that, although the 

 number and size of the eyes may differ in individuals of the same age, 

 no formation of new eyes takes place, after the attainment of more than 

 a couple of centimetres in size. 



Another fact worthy of consideration is that^ on the posterior edge 

 of the mantle, and therefore near the rectal opening, there is a special 

 group of six or seven large eyes closely placed, and all nearly equal in 

 size to the largest eyes in the group near the mouth opening ; some of 

 the eyes in the latter group are distinguished by beiug much smaller 

 than the others. Although it is difficult to define exactly the difference 

 between the two groups, — since they often vary considerably, — 

 still they may be always recognized by the characteristics given above. 



The eyes of the rigbt mantle are much smaller, and almost uniform 

 in size ; they are situated at the summit of long stalks arrauged at nearly 

 regulär intervals. 



In no species of Pecten is the difference between the curvature of 

 the right and left valves so strongly marked , as in Pecten Jacobaeus^ 



