146 ELEMENTARY BIOLOGY 



In all the animals that have blood, excepting only the insects, 

 the respiration of the interior cells is related to the blood. 

 That is, the cells get their oxygen from the blood, and they dis- 

 charge their carbon dioxid to the blood (see Chapter XXXIV). 



177. Respiration and blood. In all such animals we there- 

 fore apply the word respiration to the process by which the 

 air is brought from the outside to the blood and the carbon 

 dioxid is thrown out. The simplest kind of blood respiration 



FIG. 45. How the lobster breathes 



The featherlike gills of these crustaceans are protected by an extension of shell which 



incloses them almost completely. By the action of appendages connected with the 



mouth organs a constant current of water is made to pass over the gills through the 



space under the shield, moving from the back edge forward 



is found in such animals as the earthworm. In this the respi- 

 ration takes place by osmosis through the moist epidermis, or 

 skin. In some worms there are extensions of the skin surface 

 into little outgrowths, called gills. In clams and oysters there 

 are special outgrowths that multiply the breathing surface in 

 much the same way (Fig. 44). In the lobster, crab, crayfish, 

 and related animals there are special structures in which there 

 is a great deal of surface in a comparatively small space, 

 crowded together in a particular region of the body (Fig. 45). 

 When we come to animals with backbones, we find that 

 the breathing organs are connected with the food pipe, so that 

 all of them can, and many of them do, breathe through the 



