Fig. 281. Eye of an Alciopid (Neophanta Celox). (From Gegenbaur; after Greef.)
i. cuticle; c. continuation of cuticle in front of eye; l. lens; h. vitreous humour; o. optic nerve; o´. expansion of the optic nerve; b. layer of rods; p. pigment layer.
All the types of eyes found in the group (with exception of that of Peripatus)[188] 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 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 Tracheata.
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 larvæ of some Diptera (Eristalis), and also in some Chilognatha.
(b) A type of simple eye found in some Chilopoda, and in some Insect larvæ (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 source. The outer ends of the retinal cells terminate in rods, and their inner ends are continuous with nerve-fibres.
Fig. 282. Section through the simple eye of a young Dytiscus larva. (From Gegenbaur; after Grenacher.)
l. corneal lens; g. vitreous humour; r. retina; o. optic nerve; h. hypodermis.
(c) A type of simple eye found in the Arachnida, and apparently some Chilopoda, and forming the simple eyes of most Insects, which differs from type (a) in the cells of the retina forming a distinct layer beneath the hypodermis; the latter only obviously giving rise to the vitreous humour.