388 



('(EL EN TEH ATA. 



supposed; but that iu both instances the eye may have taken its 

 origin from the epidermis. In the eyes in which the retina is con- 

 tinuous 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 



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

 mis have also probably arisen simultane* usly with part of the central 

 nervous S3'stem, while in other instances they iiave arisen as later 

 formations subsequently to the complete establishment of a central 

 nervous system. 



Coelenterata. The actual evolution of the eye is best shewn in 

 the Hydrozoa. The simplest types are those found in Oceania and 

 Lizzia\ In Lizzia the eye is placed at the base of a tentacle 

 and consists of (fig. 276) a lens {I) 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: — (1) 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 process 

 continuous with (8) ganglion cells at the base of 

 the eye. In this eye there is present a com- 

 mencing 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. Clans has shewn that in 

 Charybdea amongst the Acraspeda a more highly 

 differentiated eye is present, with a lens formed of 

 cells like the vertebrate eye. 



MoUusca. In a large number of the odon- 

 tophorous Mollusca eyes, innervated by the supra- 

 oesophageal ganglia, ai'e present on the dorsal 

 side of the head. These eyes exhibit very various 

 degrees of complexity, but are shewn both by 

 their structure and development to be modifications of a common 

 prototype. 



The simplest type of eye is that found in the Nautilus, and 

 although the possibility of this eye being degenerated must be borne 

 in mind, it is at the same time very interesting to note (Hensen) 

 that it retains permanently the early embryonic structure of the eyes 

 of the other groups. 



It has (fig. 277 A) the form of a vesicle, with a small opening in 

 the outer wall, placing the cavity of the vesicle in free communication 



^ O. and K. Hertwig. Dax Nerrensystem n. Sinnesorgane d. Medusen. Leipzig, 

 1878. 



Fig. 276. Eye of 

 Lizzia Koellikeei. 

 ( From Lankester ; after 

 Hertwig.) 



I. lens ; oc. percep- 

 tive part of eye. 



