618 William Patten 



iially into a clear, nerveless, viscid fluid, whicli more than fills the 

 remaining space and consequently projects somewliat over the pupil of 

 the eye. This outer portion, often very irreg-ularly shaped, is always 

 harder than the inner part, or vitreous body (fig-. 58). I have a series 

 of sections, in which the siirface of the vitreous body has become hard- 

 ened into a lens shaped portion, situated over the opening of the eye, 

 and almost entirely outside of it ßg. 58 /). This lens is conspicuous 

 on account of the difference between its index of refraction and that of 

 the vitreous body, andespecially on account of the fact that its periphery 

 alone stains deeply in haematoxylin. The vitreous body, the lens, and 

 the retinidial layer, at the edge of the optic cup mergeinto each other, 

 and by means of a graduai series of changes pass into the cuticula of 

 the hypodermic cells surrounding the optic cup. The entire gelatinous 

 mass , filling the cavity of the latter, may be reduced to a single, but 

 highly modified, cuticular layer, in which the inner Stratum, filled witb 

 the nervous fibres, coustitutes the retinidial cuticula, while the 

 vitreous body . and the irregulär outer thickeuing, or lens, represent 

 the corneal cuticula, homologous with the delicate corneal mem- 

 brane in the compound eyes of Arca, as well as the corneal membrane 

 of the invaginate forms. The retinidial layer is homologous with 

 the similarly cali ed layer in the invaginated eyes of Arca, or the 

 retinidia taken collectively of the faceted eyes. 



It is evident, therefore, that thecolorless cells mustbe considered as 

 essential Clements of the retina, both morphologically and physiologi- 

 cally, even though, in the present instance, they play a relatively sub- 

 ordinate part; or rather it would be better to say that the pigmented 

 cells, with their retinidia, have gained an unusual ascendency. This, 

 however, is necessarily the case, owing to the small specialization of 

 the retina, — the hypodermic characteristics of which are but slightly 

 modified, — together with the equally poor functional development 

 of the organ. In the phylogenetic development, accompanìed by the 

 specialization of certain cells in different directìons and the acquisition 

 of higher functional powers , it will be found that the colorless cells, 

 with their retinidia, continue to gain the ascendency , until they con- 

 stitute the essential and least variable Clements, while the pigmented 

 cells become more and more subordinate to secondary functions. 



On the upper side ofeach tentacle oi Haliotis, is a dark pig- 

 mented band,which, in the large head tentacle, is sunkeu into a furrow, 

 in many places so deep that its lips may nearly dose to form a tube. 

 The floor of the furrow consists of thick, columnar cells filled with a 



