112 CRUSTACEA. 



course, all of which I am treating, are aquatic, 

 and breathe by means of gills, which are variously 

 modified in the different groups. In the more 

 advanced forms, as the Crabs, we see them, on re- 

 moving the carapace, as two sets of angular, pointed, 

 whitish, finger-like organs, each of which is com- 

 posed of a vast multitude of thin plates, very 

 closely packed, but admitting the water between 

 them, which is kept in constant circulation by the 

 play of innumerable cilia. Each series is enclosed 

 in a chamber, which has two openings, one for the 

 admission of the surrounding water, the other for 

 its emission. 



In other Orders the gills take the appearance 

 of tufts, or fine filaments, and are placed at the 

 bases of the legs, or on the legs, or on the false 

 feet, or are altogether wanting; the skin, either 

 wholly or in part, in this last case, probably per- 

 forming the function of vivifying the blood. 



The increase of the species is maintained by 

 means of eggs, which are proportionally minute 

 and numerous (in these respects differing signally 

 from those of the Rotifera, which are few and 

 enormously large) ; the collection of eggs, termed 

 spawn, is commonly carried by the female, either 

 beneath the thorax or abdomen, until the time (or 

 nearly) when the young are hatched. These in 

 general appear under forms widely different from 

 those of the parent animals, which they attain 

 only by passing through a series of transforma- 

 tions, by the successive casting off of the outer skin. 



The same process of sloughing the skin or crust 

 is continued through life, or, at least, long after the 

 ultimate form is attained, and until the animal has 

 reached its full dimensions. This is a needful pro- 



