74 INVERTEBRATE ZOOLOGY 



Tracheae are invaginations of the body wall to form a system 

 of air tubes which carry atmospheric oxygen directly to all parts 

 of the body in insects. The intimate structure of the tracheal 

 system is discussed in greater detail in Chapter XV. 



In the arachnids, an invagination of the body wall leads into 

 an organ called a book-lung, the walls of which are composed of 

 much-folded delicate membranes through which oxygen is taken 

 from the air-filled cavity of the book-lung and carbon dioxide 

 is given off. 



In some molluscs, the cavity which ordinarily bears gills 

 lacks these structures and has become secondarily modified as a 

 lung sac into which atmospheric air is periodically admitted and 

 from which the air bearing carbon dioxide is ejected. 



Excretory System. — As a result of katabolic processes, wastes 

 are formed within every living cell. The carbon dioxide men- 

 tioned above is only one of the important waste substances. 

 Water, ammonia, urea, and other nitrogenous compounds accu- 

 mulate within the living protoplasm. Many of these wastes are 

 soluble in water and tend to diffuse through the walls of the cells. 



In many of the Protozoa, contractile vacuoles facilitate the 

 collection and elimination of these wastes. Among the coe- 

 lenterates and the Porifera, direct diffusion through the cell 

 surfaces into the surrounding water suffices for the elimination 

 of these katabolic products. In the Metazoa, the first specializa- 

 tion for their elimination is the protonephridial system of flat- 

 worms, rotifers, and trochophore larvae. This system consists 

 of a series of tubules associated with flame cells. Such a system 

 may be provided with an enlarged excretory vesicle within which 

 the excretory matter accumulates before it is discharged through 

 the excretory pore. 



A great many of the coelomate animals have a metanephridial 

 system which picks up excretory wastes from the coelomic cavities 

 and from the body fluid. The unit of structure in such a system 

 is essentially a funnel-like nephrostome which communicates 

 with the outside by means of a tubule. The nephridia are paired 

 organs segmentally arranged in annelids, while in the Crustacea 

 they are considerably modified in form and are limited to one or 

 two pairs. 



A group of thin-walled Malpighian tubes communicates with 

 the intestine of insects and discharges excretory wastes from the 

 body fluid into the digestive canal. 



