270 



RESPIRATORY ORGANS OF AQUATIC ANIMALS. 



(br, 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), 



50. TELLINA. 



Fig. 151. TRITONIA. 



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 

 Fig. 15S.-DOHU. of the ,3^ shell . and to this 



cavity the water is admitted, sometimes by a large opening, 

 sometimes by a prolonged tube. In the CEPHALOPODA, we find 



