A REPRESENTATIVE VERTEBRATE. FROG 



339 



the lungs and skin is oxygenated and then 

 carried back to the heart. The passage of 

 blood through the capillaries can easily be 

 observed in the web of the frog's foot or in 

 the tail of the tadpole. 



The lymphatic system of the frog includes 

 many lymph vessels of various sizes that 

 form networks co-extensive with blood ves- 

 sels but are difficult to see. The frogs and 

 toads, unlike other vertebrates, have several 

 lymph spaces between the skin and the 

 body. Four lymph hearts, two near the third 

 vertebra and two near the end of the verte- 

 bral column, force the lymph by pulsations 

 into the internal jugular and a branch of the 

 renal portal veins. The watery lymph which 

 is colorless contains leucocytes and various 

 constituents of blood plasma. 



Relations of the hearts of 

 vertebrates to respiration 



In the fish (Fig. 222) the heart consists 

 of a single muscular ventricle and a single 

 thin-walled atrium. Blood enters the atrium 

 from the body and, when the atrium con- 

 tracts, passes into the ventricle; it is pre- 

 vented by valves from returning to the 

 atrium. The ventricle forces it into arteries 

 leading to the gills; here it is oxygenated 

 and carried directly to the body tissues be- 

 fore again returning to the atrium. Valves 

 prevent it from flowing back into the ven- 

 tricle. 



In Amphibia (Fig. 220) there are two 

 atria. Nonoxygenated venous blood from 

 the body enters the right atrium and oxy- 

 genated blood from the lungs flows into the 

 left atrium. Both atria contract and force 

 their contents into the single ventricle. The 

 ventricle forces the blood through the conus 

 and truncus arteriosus, into the carotid, 

 aortic (systemic), and pulmocutaneous 

 arches. 



In most reptiles there are two atria, and 

 the ventricle is partly divided into two 

 chambers (Fig. 222). Nonoxygenated ven- 

 ous blood from the body entering the right 

 atrium is thus kept more or less separated 



from the oxygenated blood that flows into 

 the left ventricle from the lungs. When the 

 ventricle contracts, the nonoxygenated blood 

 is forced through the pulmonary arteries to 

 the lungs, and through the left aorta into 

 the dorsal aorta; and the oxygenated blood 

 is forced through the right aortic arch which 

 merges into the dorsal aorta. Thus the 

 dorsal aorta contains a mixture of both non- 

 oxygenated and oxygenated blood. 



In birds and mammals (Fig. 222), the 

 ventricle is completely separated into two 

 chambers, forming a four-chambered heart; 

 and thus the nonoxygenated and oxygenated 

 blood are kept entirely separate, hence pro- 

 viding an efficient pulmonary circulation. 



Excretory system and 

 disposal of wastes 



A certain amount of substance resulting 

 from the breaking down of living matter is 

 excreted by the skin, lungs, liver, and in- 

 testinal walls; but most of it is taken from 

 the blood in the kidney. From the kidney, 

 it passes through the urinary* (mesoneph- 

 ric) duct, and then into the cloaca. It 

 may be voided at once through the cloacal 

 opening or stored in the bladder temporarily 

 (Figs. 223 and 224). The kidneys lie dorsal 

 to the peritoneum in the subvertebral lymph 

 space. They are composed of connective 

 tissue containing a large number of urinifer- 

 ous tubules, each of which begins in a renal 

 corpuscle. The renal corpuscle consists of a 

 coiled mass of thin-walled blood vessels, the 

 glomerulus, and an enclosing, thin, double- 

 walled cup called Bowman's capsule. It acts 

 as a selective filter which removes organic 

 wastes (especially urea), excess inorganic 

 salts, and water from the body. The liquid 

 waste collected in the kidney is called urine. 

 It is carried by the uriniferous tubules to a 

 collecting tubule and thence into the uri- 

 nary duct. Ciliated funnels called nephro- 

 stomes occur in the ventral portion; these 



* Sometimes incorrectly called a ureter. The true 

 ureter is found only in the reptiles, birds, and mam- 

 mals. 



