COMPARATIVE PHYSIOLOGY. 327 



This oxygen is found either in the air or dissolved in the 

 water in which the animal lives. In the Ccelenterates, flat- 

 worms, and many other forms, the general surface of the 

 body is sufficient for the absorption of the oxygen, but where 

 the animal is larger and needs more oxygen special provi- 

 sions are needed. 



A very simple condition, physiologically, is found in the 

 insects, where air-tubes (tracheae) extend inwards from the 

 outside, their fine branches reaching to every part of the 

 body. Air is drawn into these tubes by an enlargement of 

 the body by suitable muscles, and then, when the oxygen 

 is absorbed, contraction forces out the remainder. This 

 breathing process can be seen by watching the abdomen of 

 a grasshopper or a wasp. In many Crustacea, molluscs, 

 worms, vertebrates, the conditions are more complicated. 

 In these the nutrient fluid is also the bearer of the oxygen; 

 and, in order that the fluid may obtain this element special- 

 ized portions are developed, where the circulatory fluid may 

 come into close relationship with the water (gills) or the air 

 (lungs). In some (see the figure of Doris, p. 276) the gills 

 project freely into the water, and there is no special appa- 

 ratus for changing the fluid. In other forms the gills are 

 protected by enclosure in a branchial chamber, and then the 

 water containing the oxygen must be brought here. In the 

 oyster and clam this is effected by numerous minute hair- 

 like structures (cilia) which by their constant motion draw 

 water over the gills. The squid gets its supply by enlarging 

 and contracting its mantle-cavity, the crayfish by pumping 

 water over the gills by means of its "gill-bailer," and the 

 fish and tadpole by taking water into the mouth and forcing 

 it out through the gill-slits. The lungs of the higher ver- 

 tebrates possess a resemblance to the tracheae of the insects 

 in that air is drawn into them; but here the similarity 



