HOW ANIMALS BREATHE. 



455 



the group of isopods, respire by foliaceous or feathery appendages be- 

 neath the abdomen. 



The higher crustaceans crabs, lobsters, and shrimps have arbores- 

 cent gills inclosed in cavities on the side of the thorax. They are at- 



Fio. 8. Right Valve of a Common Clam. (After Morse.) 



tached 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 curi- 

 ous valve placed in the excurrent orifice of each chamber, which re- 

 sembles in principle the screw-propeller of a steamship. The observer 

 will find the excurrent orifice at the side of the mouth, and the incur- 



Fig. 9. Doris, or Sea-Snail. (After Carpenter.) 



rent 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 



ABC 

 Fig. 10. A, the snail crawling upon the surface of the mad; B, the same slightly huried; C, the 

 same nearly buried; the siphon, *, is seen curved upward. (After Morse.) 



