ORGANS OF VISION. 



395 



of pigment with three lenses partially imbedded in it. The outer covering 

 of the eye is the retina, which is mainly composed of rod-bearing cells ; 

 the rods being placed in contact with the outer surface of each of the 

 lenses. In the presence of three lenses the eye of Sagitta approaches in 

 some respects the eye of the Arthropoda. 



Arthropodan eye. A satisfactory elucidation of the phylogeny 

 of Arthropodan eyes has not yet been given. 

 All the types of eyes found 



in the group (with exception 

 of that of Peripatus)^ 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 de- 

 scent 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 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: 



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

 by a thickening of the cuticle. Such eyes are confined to the Tra- 

 cheata. 



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

 the retinal cells are placed immediately behind the lens, found 

 (Lowne) in the larvae of some Diptera (Eristalis), and also in some 

 Chilognatha. 



(6) A type of simple eye found in some Chilopoda, and in some 

 Insect larvse (Dytiscus, etc.) (fig. 282), the parts of which are entirely 

 derived from the epidermis. There is present a lens (l) formed as a 

 thickening of the cuticle, a so-called vitreous humour {gl) formed of 

 modified hypodermis cells, and a retina (r) derived from the same 



Fig. 281. Eye of an Alciopid (Neo- 

 PHANTA CELOx). (From Gegenbaur ; after 

 Greef.) 



i. cuticle ; e. continuation of cuticle in 

 front of eye; I. lens; h. vitreous humour; 

 0. optic nerve; o'. expansion of the optic 

 nerve ; h. layer of rods ; p. pigment layer. 



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

 of the Chffitopoda, 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. 



