THE RESPIRATORY ORGANS OF AMPHIBIA. 187 



But, as the pulmonary vessels fill, the pressure on the two 

 sides o£ the septum becomes equalized, and the systemic 

 passages, which offer the next least resistance, fill with blood, 

 which is now mixed, as it comes from the middle of the 

 ventricle. Next, the septum, being driven over to the left 

 side, prevents any more blood from going into the pulmo- 

 cutaneoiis passage. At the end of the systole, the blood 

 driven out by the ventricle is almost wholly that of the left 

 auricle ; and, by this time, the resistance in the systemic is as 

 great as that in the carotid passages. Hence the latter fill, 

 and send arterialized blood to the head. 



The organs of respiration of the Amx>liibia, in the adult 

 state, are either external branchiee, combined with lungs, as 

 in the perennibranchiate TJrodela ; or lungs only, as iu the 

 other Urodela, the Batrachia, the G]jmnophiona, and, pro- 

 bably, the majority of the Labyrinthodonta. 



In the perennibranchiate Urodela, the branchial arches 

 (or some of them) are separated by open clefts (the number 

 of which varies from four to two), throughout life, and 

 three, branched, gills are continued by single stems into the 

 integument, at the dorsal ends of the branchial arches. An 

 opercular fold of the integument, in front of the gill-clefts, 

 attains a considerable size in Siredon (Fig. 58), but does not 

 cover the gills. The branchial arches themselves bear no 

 branchial filaments. Other Urodela are devoid of external 

 gills, but (as is the case in Menopoma and Amphiuma) 

 present one or two small gill- clefts on each side of the 

 neck, and are thence called Derotremata. The rest of the 

 Urodela, and all the Batrachia and Gymnopliiona, are devoid 

 of both external gills and gill-clefts, in the adult state. 



In all the Amphibia, a glottis, placed on the ventral wall 

 of the oesophagus, opens into a short laryngo-tracheal cham- 

 ber with which two pulmonary sacs are connected, either 

 directly, or by the intermediation of bronchi (as in the 

 Aglossa), or by a trachea (as in the Gymnophiona). The 

 walls of the pulmonary sacs are more or less sacculated. 

 In most Amphibia the lungs are equal in size ; but in the 



