1378 THE STORY OF THE UNIVERSE 



of antennae are the proper organs of smell, as the 

 upper and inner are of hearing. 



The eyes, though constructed on the same general 

 principles as those of insects, yet present some par- 

 ticulars worthy of your notice. In the crabs and 

 lobsters they consist of numerous facets, behind each 

 of which is a conical or prismatic lens, the round ex- 

 tremity of which is fitte'd into a transparent conical 

 pit, corresponding to a vitreous body, while the coni- 

 cal extremity of these lenses is received into a kind 

 of cup, formed by the filaments of the optic nerve. 

 Each of these filaments, together with its cup, is 

 surrounded by pigment matter in a sheath-like man- 

 ner. To see this structure would require anatomical 

 skill ; but you may here examine with a low power 

 portions of the cornea, or glassy exterior, of the eye 

 of a crab and of a lobster. In the former, you see 

 that the facets into which the cornea is divided are 

 hexagonal, like those of most insects, but in the latter 

 they are square. 



But Crustacea have a far greater faculty of cir- 

 cumspection than insects have; for besides the ex- 

 tensive convexity and numerous facets of their eyes, 

 these organs are placed at the extremity of shelly 

 foot-stalks, which are themselves movable on hinges, 

 capable of being projected at pleasure, of being 

 moved in different directions, and of being packed 

 snugly away, when not in active use, in certain 

 grooves hollowed out expressly for them in the front 

 margin of the shell. 



