10 SYDNEY J. HICKSON. 



part of the retina is raised in the form of a tahle above 

 the level of its sides. This elevation of the central part 

 of the retina may be also seen in P. jacohcsus, though it 

 is not nearly so well marked. The folds which occur in 

 P. maximus cause the rods to appear to be given off in a 

 pinniform manner at the sides of the retina, and before 

 I found the intermediate condition in P. jacob(BUS I had 

 some difficulty in determining the true relationship between 

 the retinas of P. maximus and P. opercularis. (Compare 

 a, 5, c, fig. 11). 



In addition to those just mentioned there are other minor 

 points in which the eyes of these species differ from one 

 another, such as in the shape of the cells composing the 

 lens and in the distribution of the retinal nerve, &c., but 

 they are comparatively slight. 



General considerations. — Having thus described, in some 

 detail the anatomy of the various parts which compose 

 the eyes of Pecten, I shall, before leaving the subject, point 

 out some of their interesting morphological peculiarities. 

 It is, in itself, a remarkable thing to find a large and 

 variable number of eyes situated on an area at some con- 

 siderable distance from any central nerve-ganglion -, and, 

 when it is remembered that the class and even family (with 

 one other exception, e. g. Spondylus) to which the genus 

 belongs, possess no organs of vision at all in the adult con- 

 dition, it is altogether surprising that they should be of such 

 extraordinary complexity as they have proved to be. The 

 high structural development that this eye h^ts attained is, 

 however, not so remarkable as the fact that in many ways 

 it differs from the ordinary Invertebrate eye, and resembles 

 that of the Vertebrata. 



In the first place, the lens is built up of a large number of 

 distinct nucleated cells, which undergo a flafening at its 

 circumference very similar to that found in the eye of the 

 Vertebrata. Whether the lens is developed from the cells 

 of the epiblast, as in the Vertebrata, or from the mesoblast, 

 must at present be left unsettled, but it wdll probably be 

 found, when the development of the eye is studied, that in 

 this respect also it resembles the eyes of the Vertebrata. 

 The tapetum, a structure which is of considerable im- 

 portance to animals which are nocturnal or aquatic in habit, 

 has hitherto been described only in the Vertebrata. That 

 Pecten possesses a tapetum as highly developed as any 

 found amongst the Vertebrata is anatomically a point of 

 considerable interest; but it also indicates to a certain 

 extent the physiological capability of the eye. 



