MAMMALS. 21 



blood of the right half of the heart mixes with the 

 richly oxygenated blood of the left half, the extent 

 to which this mixing takes place being proportional 

 to the size of the ^o.— -\\ //-—L.ari. 



aperture. In Keptiles, 



therefore, the blood "-^'IH/ i r 



supplied to the Itmgs '^^ ' '"^ 



is not so poor in " "-■*-— -^— J|"^ _.y[ 7.YK. 



oxygen as it might be, 

 nor, on the other hand, 

 is the blood supplied 

 to the other parts of _ 



the body completely p^^_ n.-Dkgram of a Frog-s Heart. (The 

 OXVSenated. As eon- ventricle K. Ib quite tindivliled : compare Figs. 

 •' ° ff iT • /T \ 9 and 10.) Other letters as in Fig. 10. 



sequences oi this : (1) 



respiration is feebler, and (2) the development of heat 

 less than in Mammals and Birds {i.e. reptiles are 

 cold-blooded), and (3) the chemical changes .taking 

 place in the body (the metabolism) go on more 

 slowly than in warm-blooded animals, and we can 

 understand why reptUes execute fewer movements in 

 a given space of time. 



In Amphibians (e.g. frog) the two ventricles are 

 similarly connected, but the opening is stiU larger 

 than in Keptiles, and the partition-waU may even be 

 altogether absent. It follows, therefore, that the 

 mixing of the two kinds of blood is still more com- 

 plete, and that Amphibians, too, are cold-blooded. 



The vertebrate sub-kingdom embraces the classes of 

 I. Mammals ; II. Birds ; III. Reptiles ; IV. Amphibians ; 

 V. Fishes. 



CLASS I. : MAMMALIA (SUCKLERS). 



Warm-blooded Vertebrates (p. 16), usually covered 

 with hair, and bringing forth living young, that suck 

 for some time after birth. The female is provided 



