CRUSTACEA. 267 



polished glassy lens, the other of an immense num- 

 ber of lenses set in one frame, each having distinct 

 vision, but immoveable, the whole representing a 

 portion of a sphere, whose surface is cut into minute 

 facets. 



The Crustacea are the great aquatic division of 

 this host, so organized as to be capable of respir- 

 ing water ; and though some species, as the Land 

 Crabs, (Gecarcinus,) which spend a great deal of 

 time, and travel to long distances, on land, breathe 

 air ; they are provided with a sort of vessel in which 

 the sea-water can be carried, so as to allow the 

 gills to be constantly bathed in it. Comparatively 

 few inhabit fresh waters ; for the most part they 

 dwell in the ocean, where they increase and mul- 

 tiply to an astonishing extent. The majority of 

 them have two pairs of antennae ; three pairs of 

 jaws ; three pairs of foot-jaws, organs of singular 

 structure, uniting the form and functions of jaws 

 and feet, the last two pairs in some cases assuming 

 the office of feet, in others serving the purposes of 

 mastication; and ten proper feet, terminating in a 

 single nail : thus there are sometimes fourteen feet.* 



In the Lobster, the Crab, and many others, the 

 first pair of true feet take the form of broad and 

 thick claws, the pincers of which,, like a finger and 

 thumb, are opposed to each other, and constitute, 



* The fact is, that, properly speaking, the typical number of feet is 

 seven pairs ; but the anterior pairs are formed into foot-jaws in certain 

 forms, in which case there are but ten feet ; hence tlnse are called Deca- 

 poda. 



N 2 



