Eres of Mollusca <and Arthropods. 5§l 



cells are smaller, but not flattened, while, wliere the two surfaces of the 

 lens meet, they are so minute as to be distinguished ouly with great 

 difficulty. By isolating the lens, this part is seeu to project as a ragged 

 membrane, composed of fibres continuous with those of the conuective 

 tissue capsule (PI. 29, fig. 23 c L). This membrane will be called the 

 suspenso ry ligament. The outer surface of the lens is covered 

 with two sets of fibres, most conveniently observed in the isolated cor- 

 nea, or ou the surface of the lens, to either of which they may remain 

 attached PL 29, fig. 37). They form a layer of streng, circular fibres, 

 concentrically arranged, and superimposed by a smaller number of ra- 

 diatiug ones extending from the periphery of the lens to the centro, to- 

 ward which they gradually diminish in size. In cross sections, the 

 circular fibres of the lens appear as a row of dots,. forming a sharp de- 

 markatiou of the outer surface of the lens (PI. 29, fig. 19 c.f. l.). 



The inner surface of the lens is sparingly supplied with branching 

 fibres, which in Pecten opercularis accumulate near the centre to form 

 a fibrous mass containing an occasionai nucleus, and connectiug the 

 lens with the septal membrane (PI. 29, Fig. 23 /. l.). This internal liga- 

 ment I have been unable to find in Pecten Jacohaeus^ where, if present 

 at all, it is much less developed ; one may, however, observe on the sur- 

 face, the branching fibres which, becoming more abundant towards the 

 exterior, finally unite with the radiating, or circular, ones to form the 

 suspensory-ligament. 



The lens of Pecfeti operndaris, being smaller than that oi Pecten 

 Jacohaeus, is more favorable for Observation, andhere one may see, what 

 probably exists in the latter genus likewise, a special accumulation of 

 circular fibres to form two contractile rings, close together on the inner 

 and outer surface of the lens (PL 29, fig. 23 a^ and ci^]. In Pecten 

 pusio^, the lens is relatively small, and the inner surface less convex 

 on account of the enormous development of the cornea (PL 29, fig. 10). 

 In Pecten tarius the high, conical lens is firmly attached by a conuec- 

 tive tissue ligament to the septum, and the pupil is proportionately small. 



Recent authors have hardly been more successful than the earlier 

 ones, in determining the true shape of the lens. Keferstein (11) be- 

 lieved it to be spherical, while Hensen (12), who in this respect 

 is less accurate than usuai, represents it as Alling the entire space in 

 front of the retina, and, although bis own observations agree with those 



1 Fig. 23 represents more correctly tbe shape of the lens than fig. 19, where 

 it is not quite deep enongh. 



