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 , 
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 the Mollusca 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 
m' a i f v' 
Fig. H9. —Respiratory apparatus of the Oyster. 
*, one of the valves of the shell; v f , 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. 
V, %• 47. 
Fig. 148. 
Larva of 
Ephemera. 
