598 ANIMAL PHYSIOLOGY. 
rudiments of our eyes. Among invertebrates, eyes may in gen- 
eral be divided into two classes: 1. The compound or faceted 
eyes, the structure of which may be gathered from the accom- 
panying figures. It will be noted that in such the retina is con- 
vex, and is made up of large compound nerve-rods (retinule), 
separated from one another by pigment-sheaths, The picture 
Fie. 439. 
Fie. 440. 
Fic. 439.—Diagrammatic representation of compound eye in an Arthropod (after Claus). C, 
cornea; K, crystalline lens; P, pigment; FR, nerve-rods of retina; Fd, layer of fibers; 
Gz, layer of ganglion cells ; Rf, retinal fibers; Fk, crossing of fibers. 
Fic. 440.—Three facets with retinule from compound eye of cockchafer (after Grenacher). 
Pigment has been dissolved away from two of the facets. J’, corneal facet; K,c stalline 
se ‘i i pigment-sheath ; P’, chief pigment-cell; P”, pigment-cells of second order; 
, retinule. 
formed by such eyes must represent a sort of mosaic, and be 
rather deficient in definition and brightness. It will be noticed 
that in such eyes, both the cornea and crystalline lens of verte- 
brates are represented in multiple form. This form of eye is 
found in crustaceans and some insects. 2. The simple eye pre- 
vails among annelids, insects, arachnids, mollusks, and verte- 
brates. A more advanced form of such a visual organ is found 
in the cuttle-fish. It may be seen (Fig. 442) that such an eye 
corresponds fairly well with the eye of a vertebrate. 
The eye of the fish is characterized by flatness of the cornea; 
