660 William Patten 



other Mollusc? The innumerable eyes of Chiton and Onchidiuni imdoubt- 

 edly owe their origin to the same laws governing* the development of 

 the eyes oi Area and Pecten. The material, upon which Semper made 

 his observations concerning the eyes of OncMdium , was imdoubtedly 

 in a very poor state of preservatiou , as may be easily seen from the 

 figures. The most doubtful of all his Statements is. that the in v erte d 

 retinal layer is penetrated by the optic nerve. It is very probable that. 

 in Chiton and Onchidimn, the whole surfaee of the epithelium is filled 

 v^ith isolatedommatidia, from which, in the younger specimens, the eyes 

 are being coustantly formed by the ag-gregation of ommatidia into 

 groups, which subsequently become invaginated. 



The following Suggestion, although it does not strike at the root of 

 the real difficulties, may possibly direct attention towards a phase of the 

 .question which has not, heretofore. been considered. For iustance, in 

 ' almost every Mollusca I have examined , the epithelium was found to 

 consist, together with other Clements, of colorless gland-like cells and 

 pigmeuted ones. In certain instances, we notice a tendency of the pig- 

 ment cells to form a circle arouud each colorless one, giving rise to om- 

 imatidia irregularly scattered overlarge areas. Now there 

 ' is, moreover, a tendency for the ommatidia to collect into larger or smaller 

 / groups ; but this tendency acts at one place uearly as well as at auother, 

 1 so that innumerable groups of ommatidia will be formed at the same 

 1 time over wide areas. The development of these groups into simple 

 Ì grooves andpits, and finally into fully developed eyes, is a very simple 

 Iprocess, and may easily be observed. But what is the motive power? 

 We must necessarily assume, that, up to a certain stage, the formati on 

 of ommatidia and their collectiou into groups was of some advantage to 

 the animai (see chapter V]. If, now, we assume that the conditions for 

 the growth of these organs were exceptionally favorable. a powerful Im- 

 petus would be given which would carry them to a degree of develop- 

 ment far exceeding the uecessity of the animai. After a time many of 

 the Organs thus made would degenerate and disappear. This degene- 

 ration of many young eyes of Pecten, and of the faceted eyes iu full 

 growu Areas, may be considered as Steps towards the reduction in 

 number of the uumerous eyes thus uunecessarily produced. There is, 

 however, still another factor of great importauce, which I shall discuss 

 more fully in another place, and that is the chauge of functions under- 

 gone by the ommatidia. Two well developed eyes are , under most 

 circumstances, enough for any animai. But, if it could be showu that 

 the eyes originated from organs having other functions, so that au iu- 



