440 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Insects are more or less warm-blooded and require 

 blood cooling, yet they are covered with a dry, hard, 

 skeletal coat, which cuts off entirely any evaporation. 

 The function of blood cooling is undoubtedly performed 

 by the tracheas, which are admirably adapted for this 

 purpose. 



Crustaceans are similarly covered and have no air 

 tubes, for they live in water, but they are again cold- 

 blooded. 



Mollusca, again, go to the other extreme. The skin 

 of these is soft, mucous, and very active, and doubtless 

 performs many functions and supplements many other 

 organs. 



In Echinoderms, again, we find the hard shell and 

 inactive surface. 



In Ccelenterates we return again to the soft and 

 active condition, which is found also in most Protozoa. 



SECTION V. 

 The Liver and its Ficnction. 



We have now treated of the eliminative organs con- 

 nected with anabolism (secretory) and those connected 

 with katabolism (excretory) ; but there are still some 

 whose functions are of a mixed character, and therefore 

 put off to the last. Chief among these is the liver. 



i. The Organ: Position and Structure. — The 

 liver, together with the stomach and the spleen, fill up 

 the concavity of the diaphragm, the liver being chiefly 

 on the right and the stomach and spleen on the left. 

 Its color, shape, and size are well known, and need not 

 detain us, as they have no special relation to function. 

 In structure it differs from other glands in not being en- 

 tirely or chiefly an aggregate of excretory tubes. On 



