392 
THE OCEAN WORLD. 
The Molluscous Gasteropoda have the organs of respiration arranged 
for aerial respiration, as well as for respiration under water. 
This physiological arrangement involves important differences i» 
internal organization in these molluscs, and renders it convenient to 
divide them into two secondary groups ; namely, Pulmonary Gastero- 
poda, which breathe in the air, and hy a species of lung, and Non- 
pulmonary Gasteropocls, which breathe in the water, by means of 
branchiae, or gills. 
