SAUROPSIDA. 259 
parts of the body, and it follows, as a matter of necessity, that 
all these parts are supplied with a mixed fluid, consisting partly 
of impure or venous blood, and partly of pure or arterial blood. 
In the Crocodiles, in which there are two ventricles completely 
separated from each other, the 
same result is brought about 
by means of a communication 
which takes place between the 
great vessels which spring from 
the ventricles, in the immediate 
neighbourhood of the heart. 
From this brief description it 
will be seen that the peculiarity 
of the circulation in Reptiles 
consists in the fact, that the 
lungs and all parts of the body 
are supplied with mzxed blood ; 
whereas in the higher Verte- 
brates the lungs are supplied 
with pure venous blood, and 
the various tissues of the body 
with pure arterial blood. 
As regards the structure of 
the lungs, it is merely to be 
noted that there is no partition 
(diaphragm or midriff) sepa- 
rating the two cavities of the 
thorax and abdomen, and that 
the lungs, therefore, often at- 
tain a great proportionate size, 
sometimes extending through 
almost the whole length of the 
cavity of the trunk, There 
are also no air-sacs communi- 
cating with the lungs, as in the 
Birds, 
Fig. 139.—Diagram of the circulation of 
a Reptile. a Right auricle, receiving 
venous blood from the body; a’ Left 
auricle, receiving arterial blood from 
the lungs ; w Arterio-venous ventricle, 
containing mixed blood, which is driven 
by (2) the pulmonary artery to the 
lungs, and by (0) the aorta to the body. 
(The venous system is left light, the, 
arterial system is black, and the ves- 
sels containing mixed blood are cross- 
shaded.) 
Lastly, all Reptiles are essentially oviparous, some being 
ovo-viviparous. 
The egg-shell is usually parchment-like, but 
in other cases contains more or less calcareous matter. 
The class RePéilia is divided into four living and five extinct 
