THE HEART AND CIRCULATORY SYSTEM 107 



because of this that the condition of the heart and the circulation 

 to the lung of the Dipnoi (lungfishes) is of especial interest. 



(c) The heart and aortic arches of Dipnoi 



In all three living genera the auricle is divided completely, the 

 left one receives blood direct from the lungs in the pulmonary 

 veins. The presence of oxygenated blood on the left side of the 

 heart is general throughout all tetrapods. The two auricles com- 

 municate with the ventricles by apertures which are protected 

 by 'valves' which are not homologous with the auriculo-ventri- 

 cular valves of other vertebrates (see p. 112). The ventricle is 

 almost completely divided and the separation of oxygenated and 

 de-oxygenated blood is maintained throughout the conus by 

 a complex and twisted valve. This valve develops from four 

 longitudinal ridges and more or less completely divides the 

 conus into two sections. During its passage up the conus, the 

 blood rotates in a clockwise direction through 270°. Hence 

 blood from the right auricle comes to lie in the dorsal section of 

 the conus at the base of the ventral aorta and blood from the left 

 auricle occupies the ventral section. In Lepidosiren these two 

 parts of the ventral aorta are joined respectively to the 5th and 

 6th arches and the 3rd and 4th arches (fig. 30d). Thus de- 

 oxygenated blood from the right auricle is directed to the 5th 

 and 6th arches which supply the lungs. The oxygenated blood 

 from the left auricle passes anteriorly in the ventral part of the 

 aorta to be distributed in the 3rd and 4th aortic arches, to the 

 head and general body circulation. The degree of separation 

 maintained in the conus between these two lots of blood is not 

 so great in Protopterus and is least in Neoceratodus, which is said 

 to be unable to breathe air for any length of time. 



It is of interest to notice that the heart itself receives a special 

 blood supply which, in Lepidosiren, is derived from the 4th 

 aortic arch which may therefore be presumed to contain oxy- 

 genated blood. The hearts of the majority of fishes contain only 

 de-oxygenated blood and hence the need for a supply of oxy- 

 genated blood is particularly important. In most of them it is 

 derived from a special hyoidean circulation which has its origin 



