318 



THE RISE OF ANIMAL LIFE 



more carbon dioxide is given off tlirougli 

 tlie skin tfian tfirough the lungs. The frog 

 has tlius made the transition to land, but, as 

 indicated by its respiratory machinery, the 

 adaptation to its new mode of life is far 

 from perfect. 



Excretory system. The excretory system 

 of the frog is essentially the same as in in- 

 vertebrates, such as the earthworm or lob- 

 ster, as far as the individual units are 

 concerned. It is made up of a great many 

 nephridia massed together into a pair of or- 

 gans, the kidneys. Urinary wastes, urea, 

 salts, and so forth, are withdrawn from the 

 blood as they pass through the kidney. The 

 blood coming forward from the posterior 

 parts of the body passes to the kidney and as 

 it does the vessels break up into tiny masses 

 (glomeruli) in tlie renal corpuscles (Fig. 

 13-38). As the blood passes tlirough the 

 glomeruli the urinary products are removed 

 in a manner similar to that in man (p. 

 525). They pass down a long coiled tubule 

 and finally reach a larger duct, the uro- 

 genital duct, in the male (Wolffian duct). 

 The corresponding duct in the female car- 

 ries urine only. The urine is deposited in 

 the urinary bladder which in turn opens 

 into the cloaca. Urinary wastes and feces, 

 as well as the genital products all pass to 

 the outside through a single opening, the 

 anus. 



Reproductive system. The male: The sex 

 organs of the male are the yellowish testes 

 located ventral and anterior to the kidneys 

 ( Fig. 13-38 ) . They hang in a sheet-like bit 

 of tissue, the mesorchium, through which 

 tiny tubules, the vasa eflFerentia, pass on 

 their way from the testes to the kidneys. 

 Upon entering the kidney, the vasa efferen- 

 tia connect with the uriniferous tubules 

 which are connected to the renal corpus- 

 cles. Therefore, the tubules carry both 

 sperms from the testis and urine from the 

 renal corpuscles. The two products flow to 

 the lateral edge of the kidney where they 

 are poured into a larger tube, the urogeni- 

 tal duct, which eventually deposits its 



sperm load into the sperm sac and its urine 

 into the bladder. In reviewing this peculiar 

 situation it might be said that the testis has 

 more or less "taken over" a portion of the 

 original urinary system in order that the 

 sperm cells might be carried conveniently 

 to the outside of the body. This may be 

 true, because lower forms such as the cyclo- 

 stomes have no ducts to convey their prod- 

 ucts to the outside of the body, whereas 

 higher forms such as man have separate 

 ducts for removing urine and sex cells from 

 the body. 



The female: Between breeding seasons, 

 the ovaries are tiny, wrinkled organs lying 

 in the same position on the kidneys as the 

 testes do in the male. Sometime in the 

 summer months when the food is abundant 

 the residual eggs lying in the walls of the 

 ovaries begin to grow; and continue at a 

 rapid pace until the ovaries are tremendous 

 in size, almost filling the body cavity. The 

 eggs develop in tiny pockets in the wall of 

 the hollow ovary (Fig. 13-39) and when 

 the breeding season approaches the mature 

 eggs burst out in the body cavity. Here they 

 are swept along by the united effort of cilia 

 which line nearly all the walls. Their goal is 

 the ostium, the tiny anterior opening of the 

 long coiled oviduct. All of the cilia beat in 

 such a manner as to direct the eggs to the 

 ostium alone. Once inside the opening, the 

 eggs make their way single file through 

 the long oviduct which is also lined with 

 cilia. During their passage they accumulate 

 a jelly-like substance on their exteriors which 

 swells rapidly the moment the eggs become 

 immersed in water. 



The oviducts are much longer and more 

 convoluted during the breeding season 

 than between seasons. Near their posterior 

 end just before they join the cloaca they en- 

 large into thin-walled sacs, the uteri. Here 

 the eggs are stored until amplexus occurs, 

 at which time they are laid. Male frogs of 

 some species possess rudimentary oviducts, 

 just as male mammals possess rudimentary 

 mammary glands. There is a time in the 



