THE EYE OF PECTEN. \) 



retina and spreads out. The distribution of the comple- 

 mentary branch is diagrammatically represented in fig. In; 

 it seems to divide into a number of branches which enve- 

 lope the eye-cup, and probably send filaments to the cornea, 

 lens, tapetum, and epithelium. 



Comparison of the eyes of the three species, P. maximus, 

 P. jacoh(Sus, and P. opercularis. — The eye of P. maximus 

 is undoubtedly the most highly developed, the eye of P. 

 opercularis is the simplest, whilst P. jacohmus, although 

 more like P. opercularis than P. maximus, shows many 

 points in which it is intermediate between the two. 



The lens in P. opercularis is separated from the retina by 

 a considerable space (PL II, fig. 10), and consequently 

 the chamber containing the humour is relatively large. In 

 P . jacoh{&us the lens is larger than in P. opercularis, and 

 the chamber consequently smaller ; and in P. maximus the 

 lens is very large, and nearly touches the retina, the cham- 

 ber of the eye being sometimes very small. A gradation is 

 thus observed in the character of this part of the eye in the 

 three species. In P. maximus but a small space is filled 

 with humour, in P. jacoh(Bus a much larger space is filled 

 with it, and in P. opercularis there is a larger space still. 



Again, when the retinas of the three species are compared, 

 a similar gradation is found. The retina of P. opercularis is 

 comparatively thin, and the concavity and convexities of its 

 anterior surface slight. In P. jacohceus the retina is de- 

 cidedly thicker, and the anterior surface is more convex at 

 its sides than in P. opercularis ; moreover, it maybe noticed 

 that the delicate membrane which separates the anterior 

 from the posterior limbs of the rods has become bent up in 

 the regions corresponding with the anterior convexities of 

 the retina. In P. maximus all these variations become 

 much exaggerated. The retina is much thi'.ker than in 

 either of the other species ; and the side convexities of its 

 anterior surface are much bolder (PI. II, fig. 11, '«, h, c). 

 The anterior concavity does not undergo much variation. 



The shape of the membrane separating the anterior and 

 posterior limbsof the rods is greatly altered. In P. opercularis 

 this membrane is observed, in section, to stretch from side 

 to side without any well-marked curves ; in P. jacobaus two 

 well-marked curves, corresponding with the anterior con- 

 vexities of the retina, are observed ; but in P. maximus these 

 curves are converted into two distinct folds, which run up 

 into the substance of the retina. The membrane between 

 the folds does not sink again as low as it is at the com- 

 mencement of the folds, and consequently the central 



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