270 RESPIRATORY ORGANS OF AQUATIC ANIMALS. 
(hr, fig. 149), lying near the edge of the shell, and copiously 
supplied with blood-vessels. In the Oyster, these are freely 
exposed to the surrounding element, the lobes of the mantle 
being separated along their entire length; but where the 
mantle-lobes are united along 
their margin, so as to shut-in 
the gills, there are two ori¬ 
fices, often prolonged into 
tubes (as in the Tellina, fig. 
150), through one of which 
the water is drawn-in for the purpose of respiration, whilst 
through the other it passes out, as in the Tunicata. In the 
aquatic Gasteropoda there is scarcely any part of the body to 
which we do not find the gills attached in some species or other. 
In the naked marine species, which may be called Sea-slugs, 
they form, fringes which are sometimes disposed along the sides 
of the body, as in the Tritonia and Glaucus (figs. 151, 152), 
Fig. i5 U.—Tellina. 
Fig. 152.— Glaucus. 
sometimes arranged in a circle around the end of the intestine, 
as in the Doris (fig. 153,—see also fig. 139); and are some¬ 
times covered-in, more or less 
completely, by a fold of the mantle. 
In most of the species that pos¬ 
sess shells, the gills form comb¬ 
like fringes, which are lodged in 
a cavity inclosed in the last turn 
of the spiral shell; and to this 
cavity the water is admitted, sometimes by a large opening, 
sometimes by a prolonged tube. In the Cephalopoda, we find 
Fig. 153.— Doris. 
