270 ZOOLOGY SEC*. 



which are valvular, are closed, and the air is forced through the 

 glottis into the lungs. The skin also is an important respiratory 

 organ. 



Circulatory Organs. The pericardium (Fig. 940, pcd.) is not 

 situated in front of the general ccelome, as in Fishes, but lies in 

 the coelomic cavity between the gullet above and the epicoracoids 

 below ; it consists, as usual, of a visceral layer closely adherent to 

 the heart, and a loose parietal layer, the two being continuous at 

 the bases of the great vessels and separated by a small quantity 

 of pericardial fluid. 



The heart consists of a sinus venosus (Figs. 940 and 944, s. v.), right 

 and left auricles (r. au. y I. au.), a ventricle (v., vt.), and a conus 



ayst.tr 



FIG. 942. Rana temper aria. The heart from the ventral aspect with the cavities laid open, 

 a, a', bristle in left carotid trunk ; au. v. v. auriculo-ventricular valves ; b. V, bristle in left 

 systemic trunk ; c, c', bristle in left pulmo-cutaneous trunk ; car. a. carotid artery ; car. gl. 

 carotid labyrinth ; c. art. conus arteriosus ; car. tr. carotid trunk ; I. au. left auricle ; Ig. a. 

 lingual artery ; 1. v. longitudinal valve ; put. cu. tr. pulmo-cutaneous trunk ; pul. v. aperture 

 of pulmonary veins ; r. au. right auricle ; s. au. ap. sinu-auricular aperture ; spt. aur. 

 septum auricularum ; v, v', valves ; vt. ventricle. 



arteriosus (c. art.). The sinus venosus opens into the right auricle, 

 the pulmonary veins into the left : a striking advance on the 

 Dipnoi is seen in the greatly increased size of the left auricle and 

 its separation by a complete partition, the septum auricularum 

 (Fig. 942, spt. aur.), from the right. The two auricles open by a 

 common auriculo-ventricular aperture, guarded by a pair of valves 

 (au. v. v.), into the single ventricle. The latter has a transversely 



