GENERAL ZOOLOGY 



Body surface , 



Multiple 

 flames 



Malpighian 

 tubules 



External gills 



Mouth 



Anus 



Single terminal flame 



Nephridium 

 Nephridiostome 



Fig. 17.5. Schematic diagram representing a composite of many excretory mechanisms in 

 invertebrate animals. 



Excretory Systems. As a re.sult of metabolic processes, the cells of all 

 animals are constantly producing waste substances. One of the chief by- 

 products of cellular oxidation is carbon dioxide, and the metabolism of pro- 

 teins and amino acids results in the formation of various nitrogenous wastes 

 as bv-products. Carbon dioxide and nitrogenous wastes are generally of 

 no further use to the animal, and continued accumulation of these sub- 

 stances would poison the body. Therefore, one of the problems of animal 

 existence is the elimination of these wastes. As we have seen, important 

 functions of the circulatory system involve the transport of excreta from 

 the tissues to specialized sites of elimination. Carbon dioxide is commonly 

 lost to the external environment in the respiratory process, but the removal 

 of nitrogenous wastes poses special problems which have been solved in 

 different animal groups in diverse ways. Like the respiratory mechanisms, 

 these excretory devices are markedly influenced by environmental factors, 

 conditioned by the evolutionary history of the animal and its level of 

 organization. Various excretory mechanisms of invertebrates are shown 

 schematically in Figure 17.5. 



Microscopic and moderately small animals, such as Protozoa, Porifera, 

 Coelenterata, and Ctenophora, have no specialized excretory organs. In 

 these forms the area of body surface exposed to the external environment is 

 very extensive, and excreta are eliminated by diffusion. In some protozoans 

 we have seen how contractile vacuoles may be involved in the excretory 

 processes, although their primary function is to maintain the water balance 

 of the organism. In some of the simpler metazoans, such as the radiate 

 phyla, aggregations of cells have been described in which crystalline deposits 

 of nitrogenous substances are stored. These cells may perform an excretory 

 function in isolating complex nitrogenous wastes, but it has been demon- 

 strated that the principal excretory product in all these animals is ammonia, 

 which is highly soluble and easily eliminated by diffusion from small bodies. 



Most bilateral forms have developed organs of one type or another which 

 are classically interpreted as excretory. Among these are protonephridia, or 



518 



