Amphibia. 185 



remove impurities ? Without attempting here to explain 

 this in detail, it may be stated that, owing to the way in 

 which the arteries branch from the ventricle, and to an 

 ingenious valve arrangement, ( I ) the best blood is sent to 

 the head, (2) the next best blood (somewhat mixed) is sent 

 to the body, and (3) the most impure to the lungs and skin. 



The Lymphatic System. In skinning the frog it is very noticeable 

 that the skin is attached only occasionally, leaving a free space between 

 the skin and muscles over the greater part ot the body. In these 

 spaces is a more or less watery liquid, the lymph. Lymph is also found 

 in the body cavity around the internal organs. The lymph system is 

 part of the blood system. Lymph may be described as part of the 

 liquid of the blood that has soaked out of the regular blood tubes and 

 gotten into the lymph spaces. There are two pairs of contractile 

 ' lymph hearts, 11 one, whose pulsations may have been observed, near 

 the anus. The other pair is between the transverse processes of the 

 third and fourth vertebrae, and cannot readily be found. 



Excretion of Impurities. The lungs and skin remove 

 carbon dioxid from the blood as it circulates through them. 

 The nitrogenous waste matter is taken from the blood as 

 it flows through the kidneys. From each kidney a tube 

 called the ureter passes back to open into the cloaca on 

 its dorsal surface. 



The Colors of the Frog. The prevailing colors are green 

 and brown, though some are marked by black spots, and 

 sometimes these black spots are made more distinct by a 

 whitish border. The frog does not frequent bare ground, 

 but is usually to be found near plants, whether in water or 

 on land. Its colors are generally similar to the surround- 

 ings, so that it is not a very conspicuous object, and some% 

 times keen eyesight is needed to discover it even when in 

 plain view. By distending its lungs the frog easily floats 

 in water. When so floating only the top of the head is out 

 of water, with its three projecting points, the snout and 



