58 



Comparative Animal Physiology 



related animals are low permeability of the body surface, reabsorption of 

 water during excretion, and selection and occupancy of regions of "optimal" 



humidity. 



Reptiles, Birds, and Mammals. The water problem of a land vertebrate and 

 of a land insect is, like that of marine fishes, retention of water, but without 

 the complication of an inward salt gradient, Pearse^'"'' ^V has listed many of 

 the factors in the transition from water to land. 



Reptiles are more independent of a moist environment than Amphibia 

 because of protection by horny scales against surface evaporation, and because 

 their eggs, in most groups, are cleidoic (protected) or else the animals are 

 viviparous. An intact newt loses water about as fast as a skinless lizard.**'' 

 Table 5 gives the blood and urine concentrations of a number of reptiles. 



These figures show that the blood of marine turtles is somewhat more con- 

 centrated than that of land and fresh-water turtles, and that the urine of the 

 marine forms is relatively still more concentrated. There is great individual 

 variation in concentration of tortoise blood." In no turtle, however, has the 

 urine been reported as being hypertonic to the blood. The renal corpuscles 

 of most land reptiles are poorly vascularized. ''' These corpuscles are small 

 and have connective tissue centers with capillaries on the outside only; vascu- 

 larity is better in kidneys of turtles and crocodiles than in kidneys of snakes 

 and lizards. The amount of urine excreted is small, and in snakes and lizards 

 the urine may be solid or semisolid. In the alligator the urine flow is 0.4 1.2 

 ml./kg./hr., compared with 1.5-20 ml. in Rana, and in the alligator glomerular 

 filtrate is 1.5-3.4 ml./kg./hr. compared with 2.8-40.0 ml. in the frog. ^^^ The 

 loss of water from the skin of a tortoise is estimated to be only 0.1 ml./ 100 

 cm.-/hr. ^^'^ The principal loss of water from land reptiles, then, must be by 

 way of the lungs, and desert reptiles get most of their water from food. Bux- 

 ton*''" mentions an Australian lizard which can absorb water through the skin 

 after a rain. Numerous reptiles— C/iryseiuis, Anolis, Phrynosoma, the alliga- 

 tor Cnemidophonis, and the garter snake Thmnnophis sirh(/;s— all drink water 

 (under experimental conditions) to compensate for a deficit. ^ Amphibia in 

 water deficit take up water only by skin absorption. 



Both birds and mammals have the ability to excrete a hypertonic urine. 

 Their kidneys, particularly those of mammals, have the thin loop of I lenle 

 (Fig. 20). Renal corpuscles in birds are poorly vascularized, and filtration is 

 low. The osmotic concentration of cloacal urine of the hen may be slightly 

 higher than that of the blood. ''^ Korr '^'^ found the osmotic concentration of 

 the blood of a hen to be equivalent to 0.925-0.940 per cent NaCl, and that of 

 ureteral urine to be the equivalent of 0.95-1.1 per cent NaCl, while normal 



