ORGANS OF RESPIRATION. 



181 



The respiratory and circulatory organs resemble, in essential 

 points, those of the Dipnoi, and stamp the Amphibia as connecting 

 links between the aquatic animals which breathe by gills and the 

 higher Vertebrates with pulmonary respiration. In all cases there 

 are two lung sacs, either simple or provided with cellular spaces ; but 

 in addition to these there are, either in the larva or in the adult 

 animal (Perennibranchiata, fig. 58), three (or four) pairs of gills, 

 which sometimes lie in a cavity covered by a reduplication of the 

 skin and provided with an external opening, and sometimes project 

 freely on the neck as branched or tufted cutaneous appendages. The 

 respiratory movements are effected, in the absence of a thorax 

 capable of distension and contraction, by the muscles of the hyoid 

 bone and by the abdominal muscles. The unpaired air-tube (trachea), 

 which is supported 



ff 



XTS 



F IG. 623. Aortic arches of an old frog larva (from Bergmann 

 and Leuckart). Aa, the aortic arches uniting into the 

 descending aorta (Ad) ; Ap, pulmonary artery ; Kg, cepha- 

 lic arteries ; Br, gills. 



by cartilaginous 

 rods, is usually ex- 

 ceedingly short and 

 wide, like a larynx, 

 and in the Anura 

 alone is developed 

 to form a vocal 

 organ, which pro- 

 duces loud croak- 

 ing sounds and is 

 in the male sex fre- 

 quently reinforced 

 by a resonating ap- 

 paratus, consisting 

 of one or two sacs communicating with the buccal cavity. 



As long as the respiration is carried on entirely by means of gills 

 the structure of the heart and the arrangement of the principal 

 arterial trunks are the same as in Fishes. Later, when the pulmonary 

 respiration begins, the circulation becomes double and the auricle 

 becomes divided by a septum into a right and left chamber, of which 

 the right receives the veins from the body, the left those from the 

 lungs. The ventricle, on the contrary, still remains single, and 

 therefore contains mixed blood. It leads by a muscular rhythmically 

 pulsating conns arteriosus into the ascending aorta with the reduced 

 vascular arches. 



In the first period of larval life there are four pairs of vascular 

 arches, which surround the pharynx without dividing into capillaries 



