xm 



PHYLUM CHORDATA 



347 



cricoid and the arytenoids. The trachea bifurcates posteriorly to 

 form two bronchi, right and left, one passing to each lung. The 

 lungs of the Lacertilia and Ophidia 

 are of the simple sac-like character 

 already described in the case of the 

 Lizard. In some the lung is incom- 

 pletely divided internally into two 

 portions an anterior respiratory 

 part with sacculated walls, and a 

 posterior part with smooth, not 

 highly vascular, walls, having mainly 

 the function of a reservoir. The 

 only additional complication to be 

 specially noted is the presence in the 

 Chamseleons (Fig. 1011) of a number 

 of diverticula or air-sacs which are 

 capable of being inflated, causing 

 an increase in the bulk of the 

 animal which doubtless has an 

 effect on assailants. In the snake- 

 like Lizards the right lung is larger 

 than the left, and in the Amphis- 

 baenians the latter is entirely 

 aborted. In the Snakes a similar 

 reduction or abortion of the left 

 lung is observable. In the Cro- 

 codilia and Chelonia the lungs, are 

 of a more complex character, being 

 divided internally by septa into a 

 number of chambers. 



Organs of Circulation. In the 

 heart (Fig. 1012) the sinus venosus 

 is always present, though not, except 

 in Sphenodon, distinguishable ex- 

 ternally ; its aperture of commu- 

 nication with the right auricle is 

 guarded by two valves. There are, 

 as in the Amphibia, always two 

 quite distinct auricles, the right 

 receiving the venous blood from 

 the body, the left the oxygenated 

 blood brought from the lungs by 

 the pulmonary veins. But a vital 

 point of difference between the 

 heart of the Keptile and that of 

 the Amphibian is that in the former the ventricle is always more 

 or less completely divided into right and left portions. In all 



11 



^u 



FIG. 1011. Lungs of Chamaeleon. 

 T. trachea. (From Wiedersheim's Com- 

 parative Anatomy.) 



