Page:Popular Science Monthly Volume 20.djvu/471

Rh the group of isopods, respire by foliaceous or feathery appendages beneath the abdomen.

The higher crustaceans—crabs, lobsters, and shrimps—have arborescent gills inclosed in cavities on the side of the thorax. They are attached



to the bases of the legs, but the motion of the latter would here have little effect to renew the water. This is accomplished by a curious valve placed in the excurrent orifice of each chamber, which resembles in principle the screw-propeller of a steamship. The observer will find the excurrent orifice at the side of the mouth, and the incurrent



orifice between the edge of the shell and the bases of the legs. Curious combs and brushes are provided to keep the gills clean and the filaments separate.

Bivalve mollusks, as clams, mussels, scallops, etc., have gills made of foldings of the "mantle" or skin. (See Fig. 10, page 467, of the August "Monthly.") There are generally two for each side of the