480 



ARTHROPODA. 



that of Peripatus) 1 present marked features of similarity, but I 

 am inclined to view this similarity as due rather to the character 

 of the exoskeleton modifying in a more or less similar way all 

 the forms of visual organs, than to the descent of all these eyes 

 from a common prototype. In none of these eyes is there 

 present a chamber filled with fluid between the lens and the 



FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur; 



after Greef.) 



i. cuticle; c. continuation of cuticle in front of eye; /. lens; //. vitreous humour; 

 o. optic nerve; 6 '. expansion of the optic nerve; b. layer of rods; /. pigment layer. 



retina, but the space in question is filled with cells. This 

 character sharply distinguishes them from such eyes as those of 

 Alciope (fig. 281). The types of eyes which are found in the 

 Arthropoda are briefly the following : 



(i) Simple eyes. In all simple eyes the corneal lens is 

 formed by a thickening of the cuticle. Such eyes are confined 

 to the Tracheata. 



There are three types of simple eyes, (a) A type in which 

 the retinal cells are placed immediately behind the lens, found 



1 The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that 

 of the Chcetopoda, but resembles much more closely the Molluscan eye. The hypo- 

 dermis and cuticle form together a highly convex cornea, within which is a large optic 

 chamber, the posterior wall of which is formed by the retina. The optic chamber 

 would appear to contain a structureless lens, but it is possible that what I regard as a 

 lens may, on fuller investigation, turn out to be only a coagulum. 



