CRAES AND SHRIMPS. 201 



ability of any substance for food, by raising the little 

 door, and applying to the matter to be tested the sensi- 

 tive membrane of the internal orifice. 



Thus it is concluded that this lower or outer pair 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 partic- 

 ulars worthy of your notice. In the Crabs and Lob- 

 sters they consist of numerous facets, behind each of 

 which is a conical or prismatic lens, the round extrem- 

 ity of which is fitted into a transparent conical pit, 

 corresponding to a vitreous body, while the conical ex- 

 tremity 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 manner. To see this 

 structure would require anatomical skill ; but you may 

 here examine with a low power portions of the cor- 

 nea, 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 circum- 

 spection than insects have ; for besides the extensive 

 convexity and numerous facets of their eyes, these or- 

 gans are placed at the exti-emity of shelly foot-stalks, 

 which are themselves movable on hinges, capable of 

 being projected at pleasure, and of being moved in 

 different directions, and of being packed snugly away, 

 when not in active use, in certain grooves hollowed 



• Op. cit 

 9* 



