320 THE MAINTENANCE OF THE INDIVIDUAL 



Types of Excretory Devices 



Contractile Vacuoles. Protozoa are usually characterized by 

 some sort of contractile vacuole which serves to eliminate such sub- 

 stances as carbon dioxide, surplus water, and perhaps some non- 

 volatile nitrogenous substances. In addition to contractile vacuoles, 

 protozoa may store and later eliminate more solid wastes by the 

 formation of granules or crystals within vacuoles in the body. 



Intracellular Excretion. In some of the simplest metazoa a 

 so-called intracellular excretion takes place. This involves the inges- 

 tion of particles of waste products by certain ameboid cells which 

 leave the body and disintegrate, freeing the excretory matter within 

 their protoplasm. Associated with this process is the excretion of 

 other wastes from the surface of the body, as is characteristic of some 

 of the sponges. In addition, certain cells may store waste products 

 or there may be localized areas for excretion. 



Other Excretory Devices. In some of the coelenterates the 

 first evidence of true excretory organs appears in the form of pores 

 connected with the alimentary tract through the canal system {e.g., 

 Hydra and Discomedusae). Although other types exist they are 

 unimportant for our purposes and may be omitted. 



Among slightly higher forms than sponges and coelenterates the 

 waste products are carried to the outside through a complicated 

 system of connecting tubules in which are located occasional ciliated 

 cells, whose function appears to be to keep the fluids in motion. The 

 blind ends of these tubules are capped by minute ciliated cells of the 

 protonephridial excretory system called flame cells. These lie in the 

 parenchyma and by their movement initiate the flow of liquid and 

 soluble waste products which they have secreted through the wall. 

 The waving of the tuft of cilia in each cell is responsible for the intro- 

 duction of the term flame cell. In some cases it is believed that the 

 cells of these convoluted tubules may also reabsorb food material 

 from the passing "wastes" as well as contribute excreta to the stream. 



Reaching the higher segmented worms like the earthworm, the 

 excretory apparatus is composed of a system of paired nephridia for 

 each somite. Such nephridial systems are really a series of separate 

 units, each of which is composed of a ciliated funnel, or nephrostome, 

 and a duct that passes through the posteriad septum to empty to the 

 outside. A portion of the canal is usually glandular or secretory in 

 function and serves to discharge waste products into the tubule and 



