1538 



COLLEGE ZOOLOGY 



one of the chief products of oxidation. These 

 substances usually escape from the cells by 

 diffusion, are transported by the circulatory 

 system, when this is present, and cast out of 

 the body by means of an excretory system. 



In fresh-water protozoans, most excretory 

 substances diffuse directly into the surround- 

 ing medium; water, however, does not; it is 

 eliminated by the contractile vacuoles. In 

 the lower invertebrates such as sponges and 

 coelenterates, excretory substances are elimi- 

 nated by the cells in the body wall, with the 

 aid of amoeboid wandering cells as trans- 

 porting agents. Many of the flatworms are 

 provided with a complex system of tubes, 

 some of which end in peculiar cells with a 

 bunch of cilia at the end, whose flickering 

 movement has given them the name flame 

 cells. The cilia create a current in the tubes, 

 which carries the wastes out of the body 

 through excretory pores. We encounter an- 

 other type of excretory system in the earth- 

 worm, where coiled tubes termed nephridia 

 extract waste products from the blood, and 

 carry them, together with excretion-loaded 

 coelomic fluid, out of the body through ne- 

 phridiopores. The crayfish has one pair of 

 excretory organs called green glands, which 

 are probably homologous with the nephridia 

 of earthworms. The organs of excretion in 

 insects are long, slender Malpighian tubules 

 that are coiled about in the body cavity and 

 discharge excretory products into the an- 

 terior end of the hind-gut. 



A large part of the water in vertebrates is 

 excreted through the lungs by evaporation. 

 Sweat glands also excrete water but are of 

 little importance in the elimination of other 

 excretory material. The sweat glands are 

 very valuable, however, in the regulation of 

 body temperature, since, by the evaporation 

 of the water they excrete, heat is lost, which 

 balances the heat constantly produced in 

 the body. The liver excretes certain sub- 

 stances, such as the decomposition products 

 of hemoglobin, which are carried in the bile 

 into the duodenum and out of the body in 

 the feces. The feces are not chiefly excre- 



tory products of metabolism; most of their 

 contents never have been a part of the pro- 

 toplasm of the body. 



The principal excretory organs of verte- 

 brates are known as kidneys. The kidneys 

 (Fig. 390) excrete urine, which is carried by 

 two ureters to the bladder, and from the 

 bladder to the outside of the body through 

 the urethra. The kidneys of man are bean- 

 shaped tubular glands, about Wi inches 

 long. The renal arteries, which come directly 

 from the aorta, supply them with blood, and 

 the renal veins carrv the blood from the kid- 

 neys to the inferior venae cavae. In section, 

 the following regions are revealed: an outer 

 cortex, an inner striated medulla divided 

 into about 8 to 18 cone-shaped pyramids, 

 and the expanded end of the ureter known 

 as the pelvis. Blood entering the kidney 

 brings waste matter which is filtered from 

 it, collected in renal tubules, and discharged 

 into the ureter. 



One kidney contains about a million renal 

 corpuscles. The renal corpuscles and tubules 

 are somewhat similar in structure and func- 

 tion to nephridia such as those of the earth- 

 worm. Each kidney begins in the cortex as 

 a capsule surrounding a clump of capillaries, 

 the glomerulus; this is where the blood 

 plasma containing metabolic wastes filters 

 into the tubules. The capsule opens into a 

 convoluted tube that leads to the loop of 

 Henle, where useful materials including 

 some water are returned to the blood stream; 

 this opens into a collecting tube which leads 

 to the pelvis. From one to two quarts of 

 urine are excreted per day by an average 

 adult. Some of the constituents of normal 

 urine are urea, creatinine, ammonia, hip- 

 puric acid, and purine bodies. 



Abnormal kidney function is not to be 

 taken lightly. Certain salts may form kid- 

 ney stones in the pelvis of the kidney, and 

 if they are too large to pass through the 

 kidney ducts, they may cause great pain; 

 sometimes requiring surgical removal. Some 

 abnormal constituents which appear in the 

 urine arc albumen, acetone bodies, excess 



