30 PHYSIOLOGY 



facture of new protoplasm. As carbon dioxide is 

 formed in this process its pressure is always greater in 

 the body of the animal than in the surrounding water. 

 It therefore passes out of the animal. 



In the next higher group of animals the tissues are 

 still simple enough to allow the gases in the water that 



Fio. 16.— Ameba. (From Hertwig, after Leidy.) cv, contractile vacuole ; n, 

 nucleus ; N, food-vacuoles. 



bathes them to pass through. Their manner of breath- 

 ing therefore does not differ from that of one-celled 

 forms. 



Higher Animals. — As animals become more highly 

 developed the number of cells in the body increases, 

 division of labor occurs and similar cells become grouped 

 into tissues, each with its own special work. As the 

 whole body is then no longer adapted to the absorption 



