CRABS AND SHRIMPS 203 



ciples as those of Insects, yet present some particulars 

 worthy of your notice. In the Crabs and Lobsters they 

 consist of numerous facets, behind each of which is a con- 

 ical or prismatic lens, the round extremity of which is 

 fitted into a transparent conical pit, corresponding to a 

 vitreous body, while the conical 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 circumspec- 

 tion than insects have; for besides the extensive convex- 

 ity 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, and 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. 



If ever you should chance to meet with the exotic 

 Crustacea of the genera Corycczus and Sapphirina, you 

 would see a form of eye of a quite remarkable and unique 

 character. It is described by Dana in the following terms: 



"A pair of simple eyes consisting of an internal pro- 

 late lens, situated at the extremity of a vermiform mass of 

 pigment, and of a large, oblate lens-shaped cornea. The 

 cornea is connected intimately with the exterior shell of 



