PHYSIOLOGY. 399 



the conditions are more complicated. In these the nutri- 

 ent fluid is also the bearer of the oxygen ; and, in order that 

 the fluid may obtain this element specialized 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. 200) the gills project 

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

 ratus for changing this 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' 

 (p. 69), and the fish and tadpole by taking water into 

 the mouth and forcing it out through the gill-slits. The 

 lungs of the higher vertebrates possess a resemblance to 

 the trachea of the insects in that air is drawn into them; 

 but here the similarity ceases, for in the vertebrates the 

 air is brought from the lungs to the working parts by the 

 intervention of the nutrient fluid (blood). 



The methods by which air is drawn into the lungs 

 vary. The frog swallows the air by aid of the muscles 

 extending across the throat between the halves of the 

 lower jaw, and that this swallowing is the only way of 

 forcing air into the lungs is shown by the fact that if the 

 mouth be kept from closing the animal will suffocate.* 

 In the Sauropsida the muscles between the ribs and those 

 forming the walls of the abdomen are concerned in the 

 inspiration and expiration of air; while in mammals the 



* The skin is a very important organ in the respiration of the 

 Batrachia (see p. 337). 



