EVOLUTION OF MUD-FISHES. 117 



have only two blind nose-pits on the surface of the head, 

 these now became connected with the mouth-cavity by an 

 open passage. A canal formed on each side, leading directly 

 from the nose-pit into the mouth-cavity, and thus even 

 while the mouth-opening was closed the necessary atmo- 

 spheric air could be introduced into the lungs. While, 

 moreover, in all true Fishes the heart consists simply of two 

 compartments, an auricle, which receives the venous blood 

 from the veins of the 'body, and a ventricle, which forces 

 this blood through an arterial expansion into the gills, the 

 auricle, owing to the formation of an incomplete partition 

 wall, is now divided into a right and a left half, 

 The right auricle alone now received the venous blood of 

 the body, while the left auricle received the pulmonic 

 venous blood passing from the lungs and the gills to the 

 heart. The simple blood-circulation of the true Fishes thus 

 became the so-called double circulation of the higher Ver- 

 tebrates ; and this development resulted, in accordance with 

 the laws of correlation, in further progress in the structure 

 of other organs. 



The vertebrate class, which thus first adapted itself to 

 the habit of breathing air, and which originated from a 

 branch of the Selachii, are called Mud-fishes (Dipneusta), 

 or Double-breathers, because, like the lowest Amphibia, 

 they retain the earlier mode of breathing through the gills, 

 in addition to the newly acquired lung-respiration. This 

 class must have been represented by numerous and diverse 

 genera during the Palaeolithic Epoch (during the Devonian, 

 Carboniferous, and Permian Periods). As, however, the 

 skeleton is soft and cartilaginous, like that of the Selachii, 

 they naturally left no fossil remains. The hard teeth of 



