2 THE NECTURUS 



The main justification for describing Necturus as primi- 

 tive lies in some similarities between its structure and that 

 of the fish. The body outline is fishlike, with myocommata 

 showing as zigzag lines separating the myotomes. The 

 head musculature is simple and unspecialized while the 

 limb musculature is more complicated, as would be expected 

 in an animal with the tetrapod type of limb. This suggests 

 some very interesting history back of the limb musculature 

 that has never been discovered. The skull is quite special- 

 ized, having lost several bones including the maxillae. The 

 feet have lost one toe. 



The circulation shows the influence of lungs and air 

 breathing. As in all amphibia, the heart is three chambered, 

 but other parts of the circulating system remain a mixture, 

 containing characteristics both of water and air type. 



The arterial arches are reduced from the primitive num- 

 ber six to three. The sixth becomes the pulmonary artery, 

 the fourth becomes the systemic, and the third with parts 

 of the anterior arches become the vessels of the neck and 

 head. The posterior cardinals lose their importance and 

 become small, their place being taken by the new postcaval. 

 The iliacs acquire a new connection with the renal portals, 

 while the lateral veins join and connect with the hepatic 

 portal, thus making several paths for the outlet of the 

 blood from the hind limbs. 



CLASSIFICATION OF THE AMPHIBIA 



For the convenience in placing Necturus in its position 

 in the animal kingdom, this classification is given. 



Phylum Chordata 



Subphylum Vertebrata. 

 Superclass Tetrapoda 

 Class Amphibia 



