698 EVOLUTION OF ANIMALS Part V 



external or cloacal opening. This opening is usually called the anus, but this 

 term does not homologize the structure with higher vertebrates in which the 

 term anus always signifies the external opening of the intestine only. 



Cooperating Fluids — Blood, Tissue Fluid and Lymph. Circulating 

 blood transports substances to cells where they are needed and away from cells 

 to which they are a burden. Like other vertebrates, frogs have three body 

 fluids; the tissue fluid that is in direct contact with the cells and through which 

 all substances must pass in order to reach them; and the circulating blood and 

 related lymph in their respective vessels. All three fluids are dependent upon 

 the water content of the body, especially so in frogs. 



The blood consists of fluid plasma and cells. Its general functions are the 

 transport of oxygen and carbon dioxide, food and water, waste substances of 

 metabolism, and hormones. Although largely water, the plasma also includes 

 blood proteins, salts, and metabolic products. On account of the frog's low 

 temperature, its plasma carries more oxygen in solution than that of the warm- 

 blooded birds and mammals. 



The red cells (erythrocytes) are relatively large and each is bulged out by 

 its prominent nucleus. There are about 400,000 per cubic millimeter, most 

 abundant just before the breeding season, a relatively small number compared 

 to the four to five millions per cubic millimeter in human blood. Their small 

 surface exposure and the space taken up by the nucleus combine to reduce 

 their efficiency in carrying oxygen. In certain salamanders (Batrachoseps) 

 many red cells lose their nuclei as they mature just as mammalian red cells do, 

 but this is very rare in amphibians. Red blood cells ordinarily develop in the 

 spleen. Only when metabolism of frogs is at its height in spring do red cells 

 arise in the red marrow of bone as in mammals. The white cells (leucocytes) 

 are colorless and nucleated, about 7000 per cubic millimeter of blood. Spindle 

 cells (thrombocytes) are nearly twice as numerous as the white cells and ex- 

 tremely minute, disappearing from blood which has been shed for any length 

 of time. 



Blood Vessels and Circulation. In the frog and with few exceptions in 

 the vertebrates in general, blood circulates within a system of vessels, the 

 heart, arteries, capillaries, and veins. Lymph flowing through tubes and open 

 spaces provides fluid with a return route to the heart, an alternative to that of 

 the veins. In the frog, the characteristics and functions of the three types of 

 blood vessels are similar to those of other vertebrates. The reader is referred 

 to the discussion of these in Chapter 12 and to figures 34.17, 34.18, and 

 34.19. 



Heart. The frog's heart is a muscular pump that pushes the blood through 

 blood vessels, but does not affect it in any other way. It is enclosed in a thin 

 but strong membranous sac, the pericardium, containing just enough fluid to 

 let the heart slip easily as it beats. 



