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 8, 0’, fig. 47, 
Although these animals usually reside in the water, 
or only quit it occasionally, there are some species, 
known under the name of /and-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 the Motxvsoa 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 fig. 148. 
mantle (fig. 63), through which a stream of water Larva or 
is continually made to flow by ciliary action ******** 
§ 319); and this surface is sometimes extended by the 
ing or plaiting of the membrane. In most of the Con- 
CHIFERA, however, we find four lamelle or folds of membrane 
Fig. 149.— REsPIRATORY APPARATUS OF THE OYSTER. 
#, one of the valves of the shell; wo’, its hinge; m, one of the lobes of the mantle; 
m’, a portion of the other lobe folded back; c, muscles of the shell; 4r, gills; 
6, mouth ; #, tentacula, or prolonged lips; f, liver; i, intestine; a, anus: co 
