32 



PHYSIOLOGY 



Relation Between the Blood and the Gill Membrane. 

 — Immediately under this membrane is a network of 

 blood vessels with very thin walls, which connect the 

 gill surface with every other part of the body. The air 



Fig. 18.— .4, Gills of a crayfish exposed by cutting away the shell. (From Hertwig.) 

 B, Larval form showing external gills. (From Hertwig, after Sarasins.) 



in the water is separated from the blood by only the thin 

 gill surface and the walls of the blood vessels. By 

 osmosis, any gas in solution in the water can find its way 

 into the blood and any gas in solution in the blood can 

 find its way into the water. Relative pressure governs 

 the process. As in one-celled animals, oxygen passes into 

 the blood, for its pressure is greater in the water than 

 in the blood ; and carbon dioxide passes into the water, 

 for its pressure is greater in the blood than in the water. 

 Situation of the Moist Membrane. — If one of these 



