VASCULAR SYSTEM 281 



Urodela can only make a slight noise, but the Anura have a 

 considerable voice, which is much intensified by the vocal sacs 

 of the mouth and larynx. 



Vascular system. There is always a special anterior section 

 of the ventricle, the contis arteriosus, which is rhythmically 

 contractile and guarded by semilunar valves at its two ends. 

 Moreover there is a further resemblance to fishes in the fact 

 that the ventricle (conus division of it) gives off only one artery, 

 the ventral aorta or truncus arteriosus. It is true that this 

 vessel is always very short and that in the Anura it is actually 

 divided into two by a horizontal septum, but its homology with 

 the ventral aorta of fishes and embryos cannot be disputed. 

 In possessing this structure the Amphibia present a piscine 

 feature, and one in which they markedly differ from the Reptilia 

 and higher Vertebrata. In other features of the vascular system 

 they approach the higher forms and depart from fishes, e.g. in 

 the presence of an auricular septum, of an inferior vena cava, of 

 a branch of a posterior vascular arch to the lung sacs, in the fact 

 that in the adults of the higher forms branchial structures are 

 not present on the vascular arches, that the vascular arches 

 tend not to be connected with each other dorsally, and in the 

 differentiation in the higher forms of the right systemic arch 

 from the left. But with regard to these features it must be 

 noted that the first three are already found in Dijrnoi, and that 

 the others are only characteristic of the higher members of 

 the Amphibia. 



The heart is contained in the pericardial sac and is typically 

 five-chambered, consisting of sinus venosus, two auricles, a single 

 ventricle and a conus arteriosus (bulbus cordis). The chambers 

 are more compacted together than in fishes, the sinus venosus 

 being placed more forward on the dorsal side of the auricle. 

 The left auricle is smaller than the right and the interauricular 

 septum is incomplete in Urodeles and Gymnophiona, complete 

 in Anura. Tlie sinus venosus opens into the right auricle, and 

 the pulmonary veins into the left. All the chambers of the 

 heart are rhythmically contractile. Except at its base the cavity 

 of the ventricle is broken up by muscular strands, so as to present 

 a spongy character and its wall is without blood vessels. The 

 conus arteriosus is spirally twisted and usually possesses a longi- 

 tudinal valve, the attachment of which lies along the axis of the 



