562 William Patten 



especially thickened over the sunken, retina! layer of the invaginated 

 eyes. The thick ciiticula over the pigmented areas ìs formed of two 

 layers ; a thin, externa! one, dense and refractive, and a thicker, less 

 refractive, inner one, which has not so completeiy !ost its vita! proper- 

 ties. In, and below the iatter layer is foiind the network of nerve 

 fibriüae constituting the retia terminaüa. It is this layer which 

 is so greatly developed over the retina! ce!ls of the in- 

 vaginated eyes, whi!e the outer andthinner oneremains 

 unchauged. Over the invaginated eyes, the retina! cuticuia is 

 composed aimost entireiy of iunumerable and distinct nerve fibres, which 

 may often be traced be!ow the cuticiüa, between the retinal cells 

 (PI. 30, fig. 42). At the basai ends of the cells, there are often con- 

 siderable Spaces in which may be seen continuations of the sanie fibres, 

 which pass through the basai membrane into the underlying tissues as 

 isolated branches, not united to form a special optic nerve. When the nerve 

 fibres bave once passed beyond the basa! membrane, it is difficult to 

 distinguish them from those of the connective tissue. 



These eyes are so difficult to macerate that I bave not obtained 

 many data in that way, but stili enough to convince me that the same 

 arrangement of nerves is fouud bere, as in Haliotis^ which will be de- 

 scribed in greater detail further ou. For example, when I was for- 

 tunate enough to isolate some of the retina! cells uninjured, almost with- 

 out exception, two or three nerve fibres were seen attached to their sides, 

 with the free ends projecting some distance beyond either end of 

 the cells. This shows that, when the latter were in position, the 

 nerves exteuded into the cuticular layer, which is merely an exag- 

 gerated form of the ordinary cuticula containing the nervous r e tia ter- 

 mi nalia found everywhere above the pigmented hypodermis. The 

 ordinary cuticula represents the sum of the products of the individua! 

 cells, and, in most cases , the outline of each celi is distinctly visible 

 upon it. This go es so far that it is often possible to separ- 

 ate the cuticula into its component elements; but espe- 

 cially is this the case on the modified, retina! portion 

 of the hypodermis, where the product of each celi has 

 attained a high stage of individuality, giving rise to the 

 rods. 



The invaginated eyes , therefore, are composed of the same ele- 

 ments as the faceted, that is, a centrai colorless celi, probably contain- 

 ing an axial nerve fibre and two nuclei, together with a cuticular rod 

 supporting a specialized part of the retia terminalia (a retinidium); 



