242 RESPIRATION AND REPRODUCTION OF CRUSTACEA. 



and although provided with gills, some among them are speedily 

 suffocated by submersion in water. Indeed, their respiration 

 is so active, that the small quantity of oxygen dissolved in the 



water cannot suffice for 

 their wants ; but in the 

 air they find this gas 

 in abundance ; and an 

 arrangement similar to 

 that which we have 

 met with in certain 

 Fish ( 538), permits 

 them to remain out of 



FIG. 469. LAND CRAB. 



the water without their 



gills becoming so dry, as to be unfit to perform their functions. 

 Sometimes there is, at the bottom of the respiratory cavity, a 

 sort of basin, destined as a reservoir for the water necessary to 

 supply the requisite moisture to the gills. In other instances, we 

 find in the lower arch of this cavity a spongy membrane, which 

 seems to serve the same purpose. Most of these Land-Crabs 

 commonly inhabit moist woods, and hide themselves in holes 

 which they dig in the soil ; but the localities which they prefer 

 vary according to their species. Some dwell in low marshy 

 lands, near the sea ; others in w coded hills far from the shore ; 

 and these last at certain periods quit their habitual dwellings 

 for the sea. The Woodlice also are terrestrial Crustacea, 

 whose respiration is effected by means of leaf-like plates, which 

 are situated under the abdomen, and which, in other animals 

 formed nearly on the same plan, fulfil the functions of gills. 



780. All Crustacea are oviparous. The female is generally 

 distinguished from the male by the more enlarged form of the 

 abdomen ; and after having laid the eggs, she carries them for 

 some time, suspended under that part of her body, or even 

 inclosed in a sort of pouch, formed by the appendages to the 

 legs. Sometimes the little ones swim about in this pouch, and 

 remain there until they have undergone their first moult. 

 The young do not in general undergo true metamorphoses; 

 but sometimes they acquire, with advancing age, a larger 



