RESPIRATORY ORGANS OF AQUATIC ANIMALS. 



269 



stantly to flow, by mechanism adapted for the purpose. Their 



form and position in the Crab are shown at b, b', fig. 47 



Although these animals usually reside in the water, 



or only quit it occasionally, there are some species, 



known under the name of land-crabs, which have 



the power of living for some time at a distance 



from water. In order to prevent their gills from 



drying up, which would destroy their power of 



acting on the air, there is a kind of spongy 



structure in the gill-chamber, by which a fluid is 



secreted that keeps them constantly moist. 



316. In theMoLLuscA we find the gills arranged 

 in a great variety of modes. In the lowest class, 

 the TUNICATA, the respiratory membrane is merely 

 the lining of the large chamber formed by the 

 mantle (fig. 63), through which a stream of water 

 is continually made to flow by ciliary action 

 ( 319); and this surface is sometimes extended by the 

 folding or plaiting of the membrane. In most of the CON- 

 CHIFERA, however, we find four lamellae or folds of membrane 



Fig. 148. 



LARVA OF 



EPHEMERA. 



Fig. 149. RESPIRATORY APPARATUS OF THE OYSTER. 



, one of the valves of the shell ; v 1 , its hinge ; m, one of the lobes of the mantle; 

 m', a portion of the other lobe folded back; c, muscles of the shell; br, gills; 

 b, mouth ; t, tentacula, or prolonged lips; /, liver; i, intestine; a, anus- co 

 heart. 



