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. Eyes 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- 

 til in 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 (Eupliausia], 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 Ear. The simplest kind of auditory organ is to be 

 found in jelly-fishes, where an organ of hearing first occurs. 

 In 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, theotoliths 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 



