ORGANS OF VISION. 



471 



exist except in the case of the Arthropoda, Mollusca and Chor- 

 data. From such data as there are, combined with study of the 

 adult structure of the eye, it can be shewn that two types-of 

 development are found. In one of these the percipient elements 

 are formed from the central nervous system, in the other from 

 the epidermis. The former may be called cerebral eyes. It is 

 probable however that this distinction is not, in all cases at 

 any rate, so fundamental as might be supposed ; but that in 

 both instances the eye may have taken its origin from the 

 epidermis. In the eyes in which the retina is continuous with 

 the central nervous system, these two organs were probably 

 evolved simultaneously as differentiations of the epidermis, and 

 continue to develop together in the ontogenetic growth of the eye. 



Some of the eyes in which the retina is formed from the epi- 

 dermis have also probably arisen simultaneously with part of the 

 central nervous system, while in other instances they have arisen 

 as later formations subsequently to the complete establishment 

 of a central nervous system. 



Ccelenterata. The actual evolution of the eye is best 

 shewn in the Hydrozoa. The simplest types 

 are those found in Oceania and Lizzia 1 . In 

 Lizzia the eye is placed at the base of a 

 tentacle and consists of (fig. 276) a lens (/) 

 and a percipient bulb (oc). The lens is a 

 simple thickening of the cuticle, while the 

 percipient part of the eye is formed of three 

 kinds of elements: (i) pigment cells; (2) 

 sense cells, forming the true retinal elements, 

 and consisting of a central swelling with the 

 nucleus, a peripheral process representing a 

 hardly differentiated rod, and a central pro- 

 cess continuous with (3) ganglion cells at 

 the base of the eye. In this eye there is 

 present a commencing differentiation of a 

 ganglion as well as of a retina. 



The eye of Oceania is simpler than that of Lizzia 

 in the absence of a lens. Claus has shewn that in 



FIG. 276. EYE OF 



LlZZIA KOELLIKERI. 

 (From Lankester; after 

 Hertwig.) 



/. lens; oc. percep- 

 tive part of eye. 



1 O. and R. Hertwig. 

 1878. 



Das Ncruensystem u. Sinnesorgane d. Medusen. Leipzig, 



