Eyes of Molluscs and Arthropods. 011 



The arg-eutea (PI. 31, fig-. 1 12 ag.) is similar to that of Pecien, and 

 consists of connectivo tissue cells. the bodies of whieh are flatteued iuto 

 memhraues, composed of minute refractive S(iuares much smaller than 

 those of Pecfen. In sectious, it appears to consist of fibres with minute, 

 refractive varicosities , and might easily be mistaken for a layer of 

 nerve fibres. The best method to observe this »Plättchen« structure of 

 the tapetum is by pressing- the ends of the tentacles almost flat under 

 a cover glass ; one then sees beneath the hypodermis a layer of what 

 appears to be fine refractive granules , the truc form of which can ouly 

 be disting-uishcd with the highest powers. The cells of the tapetum 

 are not so completely metamorphosed as to lose their nuclei ; as is the 

 case in the adult Pecfen, the nuclei are quite numerous, and may readi- 

 ly be detected as minute, compressed bodies, conspicuous by the in- 

 tensity with which they are stained in haematoxylin. The argentea en- 

 velops the whole eye, but is thickest ou the sides next the pigment and 

 toward the base of the tentacle; on the remaiuing two sides, it is reduced 

 to a thin, hyaline membrane through which the nerve fibres pass to the 

 retina. It is thus evident that the light must come from the summit of 

 the tentacle. and indeed from the iuvaginated portion away from the 

 pigmented side ; the ends of the retinal cells are therefore parallel to the 

 rays of light, as we should expect. Whether the inner ends of the cells 

 are really provided with rods. similar to those oi Pecten^ for example, 

 I cannot say. Perhaps the fact that it was difficult to obtain clear pic- 

 tures of just those parts where the rods should be , may be considered 

 as favorable to the supposition that they are true retinophorae. There 

 is a striking resemblance between these retinal Clements and those of 

 Pecten, and, until evidence shall be produced to the contrary, I propose 

 to consider them as homologous , and consequently shall designate the 

 large cells [rf.) as retinophorae, and the small ones [g.c.) as ner- 

 vous cells, homologous with the inner ganglionic ones of Pecten. 

 Whether there is a central nerve fibre in the former, or not, I was un- 

 able to determine ; this is a point of considerable importance for , if it 

 i« so . we should then have good reasou for supposing that the cells in 

 question are formed by the fusion of two cells, just as they are in 

 so many other cases. The cellular body (/; , which we shall speak of as 

 a lens, — althongh it is probable that it combines in itself another 

 functiou besides that of concentrating the light, — is composed of 

 large characteristic cells, which, however, are not confined to this 

 region alone, but extend theuce in a double row, nearly half the length 

 of the tentacle; they then break up iuto irregularly scattered cellS; 



41* 



