Comparative Anatomy of the Eye. 451 



This contraction is in sympathy with either the organs of sight or 

 touch. But the fish has a stimulating effect on the habits of the 

 Onchidium tending to develop an alertness and activity, such as in 

 some animals shows itself in general muscular movement of limbs, but 

 in this ( and others ) is shown in molecular changes of the body and the 

 stimulation and growth of sense. The back of the Onchidium is often 

 covered with eyes in various stages of development, and they all, even 

 when no eyes are developed, possess tubercles of various sizes, which 

 are rounded and smooth. These tubercles or papillae differ greatly in 

 size, and increase in number with the age of the individual. The smaller 

 ones show nothing under the outer cuticle except a simple cellular 

 layer, the epithelium, like all the univalve mollusks, but in the center 

 of the larger ones a cellular mass is found growing inward and down- 

 ward from the epidermis. From some masses of this kind the glands 

 for the secretion of the fluid are formed, and others become granular and 

 crystalline, refractive and more sensative to light. In still larger papillae 

 a layer of pigment closes around the differentiated cells, a few of which 

 are consolidated into a crystalline lens, and a nerve fibre can be detected 

 communicating with the interior. Thus the sensitive papillae, which in 

 most animals remain organs of touch, are in this one, converted to a con- 

 siderable extent into organs of sight. ( See fig. 173.) They thus occur in 

 all stages of development in some individuals, and are in different degrees 

 of perfection in different species ; being in some quite rudimentary, in 

 others remarkable for their perfection and utility. " In the Chitonidse, a 

 family of gasteropod mollusks in which dorsal eyes have recently been 



I FIG. 185 Perpendicular section through 

 the eye-pit of a Limpet-Patella, 



c, -r. ,. , (After Carriere.) 



1. Epithelial Cells. 



2. Retina Cells. 



3. Vitreous Body. 



This eye consists merely of differentiated 

 epithelial tissue. 



Limpet is a Gasteropod univalve mollusk. 

 Its eye-pits are placed at the root of the 

 tentacles and just outside of them. 



FIG. 185. 



discovered by Mosely, they are even more numerous " than in the 

 Onchidium. ' * Chiton itself indeed has none, but in Schizochiton there 

 are 300, and in Corephium more than ten thousand ! As in Onchidium 

 they probably arose as modifications of the organs of touch and are 

 supplied by the same nerves. They possess (1) a cornea, (2) a perfectly 

 transparent and strongly biconvex lens, and (3) the retina which 

 presents a layer of short but well defined rods. It is interesting that 

 they point towards the light and not, as in Onchidium, away from it. " 1 

 1 Lubbock Senses of Animals, 144. 



