332 COMPARATIVE ZOOLOGY OF CHORDATES 



and both the arterial blood from the left auricle and the 

 venous blood from the right auricle open into it, there is a 

 mixture in the ventricle which is sorted out into the two 

 channels in the truncus by the spiral septum and valves. 

 In newts, the septum between the auricles tends to break down, 

 as does the septum in the truncus. In the embryonic stages 

 of amniotes the septum between the auricles remains incom- 

 plete also, until the time of hatching or birth, in connexion 

 with functional details of the embryonic circulation. 



The hearts in the amniotes fall into two classes, neither of 

 which can be derived from the other, and which must have 

 been separately evolved from the amphibian condition. The 

 conus is reduced and incorporated in the wall of the ventricle, 

 but while in one group which may be called Sauropsidan the 

 truncus is split right down to the ventricle into three channels, 

 in the other or Theropsidan group it is split into only two 

 channels. 



The three channels in the Sauropsida are the pulmonary, 

 the right systemic, and the left systemic. The two latter cross 

 over one another so that the right systemic springs from the 

 left side of the ventricle, while the left systemic arises with the 

 pulmonary from the right side of the ventricle. In the lizards, 

 snakes, tortoises, and Sphenodon, the ventricle is still single, 

 although there is a septum which divides it incompletely. 

 The left auricle, as always, contains the arterial blood, most of 

 which goes into the right systemic arch. In the crocodile, 

 the interventricular septum is complete, but it is formed in 

 such a way that while the right systemic arch gets all the 

 arterial blood from the left auricle, the left systemic arch 

 together with the pulmonary, gets only venous blood from the 

 right auricle. There is a small foramen (of Panizza) between 

 the right and left systemic arches which allows a little inter- 

 change of blood. The condition in the bird is like that of the 

 crocodile except that the left systemic arch has been abolished 

 altogether, which is not surprising, seeing that it could only 

 distribute blood which is almost purely venous. In the bird, 

 therefore, with its four-chambered heart, there is no mixture 

 of arterial with venous blood ; all the venous blood in the 



