The Early Vertebrates and the Fishes 



themselves in the mud, and remain in a torpid condition, breathing 

 air by their lungs, until the rainy season comes round, perhaps 

 four or even six months later. Correlated with the special 

 method of respiration is a special type of blood system, whereby 

 part of the blood is pumped direct to the lungs, and returns 

 direct to the heart. There are two auricles, to receive the 

 blood from the lungs and the general circulation respectively, 

 but only one ventricle, in which the two streams become 

 mixed. Figs. 80 to 82 illustrate the three existing Dipnoi, and 

 the structure of the lung is shown in Fig. 83. 



It is obvious, from the distribution of the lung fishes, and also 

 from geological evidence, that the group was once very plenti- 

 fully represented, and has only been preserved from total ex- 

 tinction by peculiar circumstances. 



Regarding the position of the group, some zoologists regard 

 them as the direct ancestors of the Amphibians. Others believe 

 that the group had a common origin with the bony fishes and 

 the Amphibia in some early form of Ganoid. In any case, the 

 Dipnoi possess an extraordinary interest as showing the begin- 

 nings of adaption to a life out of the water. 



103 



