COMPARATIVE ANATOMY OF ORGANS. 641 
bly perfect eye, consisting of a projecting spherical lens 
covered by the skin, behind which is a vitreous body, a 
layer of pigment separating a layer of rods from the exter- 
nal part of the retina, outside of which is the expansion of 
the optic nerve. yes are also situated on the end of the 
body in some worms, and in a worm called Polyophthalmus 
each segment of the body bears a pair of eyes. 
The eyes of mollusks are, as a rule, highly organized, un- 
tilin the cuttle-fish the eye becomes nearly as highly de- 
veloped as in fishes, but still the eye of the cuttle-fish is not 
homologous with that of Vertebrates, since in the former 
the crystalline rods are turned toward the opening of the 
eye, while in Vertebrates they are turned away from the 
opening of the eye, so that, as Huxley as well as Gegen- 
baur show, the resemblance between the eye of the Ce- 
phalopods and of the Vertebrates is a superficial one. 
While, as we have seen, the eyes of the worms and the 
mollusks are situated arbitrarily, by no means invariably 
placed in the head, in the Crustaceans the eyes assume in 
general a definite position in the head, except in a schizo- 
pod crustacean (Huphausia), where there are eye-like organs 
on the thorax and abdomen. In insects there are both sim- 
ple and compound eyes occupying definitely the upper and 
front part of the head. 
The eyes of the lancelet are not homologous with those 
of the higher Vertebrates, being only minute pigment spots | 
comparable with those of the worms. In the skulled Ver- 
tebrates the eyes are of a definite number, and in all the 
types occupy a definite position in the head. 
The Har.—The simplest kind of auditory organ is to be 
found in jelly-fishes, where an organ of hearing first occurs. 
tn these animals, situated on the edge of the disk, are minute 
vesicles containing one or more concretionary bodies or 
crystals, Reasoning by exclusion, these are supposed to rep- 
resent the ear-vesicles or otocysts of worms and mollusks ; 
and the concretions or crystals, the otoliths of the same kind 
. of animals. 
The otocysts or simple ears of worms and mollusks are 
minute and usually difficult to find, especially the auditory 
