CLASS AMPHIBIA 489 



The group of vertebrates which Necturus represents is of par- 

 ticular interest because of its transitional position between aquatic 

 and terrestrial forms. Necturus is aquatic and fishlike in its pos- 

 session and use of external gills, although it has only three arches. 

 The body is used like that of a fish in SAvimming ; that is, by lateral 

 strokes of the tail against the water. It is terrestrial in the de- 

 velopment of pectoral and pelvic girdles and limbs for crawling. 

 Also lungs are developed for aerial respiration, although not highly 

 functional. The sixth or last pair of aortic arches of the primitive 

 series gives off a pulmonary artery and still retains the connection 

 to the dorsal aorta. This portion of this arch is known as the duct 

 of Botallus. The heart has become three-chambered; there are now 

 two auricles instead of only one as in fish. The posterior cardinal 

 veins are still present, but their function is partially taken over 

 by the newly developed post cava. 



Necturus remains in a larval condition throughout its life, be- 

 comes sexually mature, and reproduces without metamorphosis. Such 

 a condition is referred to as neoteny. The retention of external gills 

 is a very marked larval feature. 



Food and Digestive System 



This animal is quite inactive and requires relatively little food. It 

 does make use of several aquatic inhabitants including crayfish, other 

 small crustaceans, snails, insect larvae, leeches, some minnows, and 

 occasionally fish eggs for food. 



The mouth is located in the anterior, terminal position and with a 

 fairly wide gape. Teeth are located on the premaxillae, vomer, and 

 palato-pterygoid bones of the upper jaw, and the dentary and splenial 

 bones of the lower jaw. The tongue is broad and only slightly mov- 

 able. The internal nares enter the mouth cavity as a slit on each 

 side between the two dorsal rows of teeth near their posterior termi- 

 nations. More posteriorly, in the lateral walls of the pharynx are the 

 two pairs of gill slits or pharyngeal clefts. Still more posteriorly 

 there is a very small inconspicuous pharjoigeal prominence with a 

 tiny slit, the glottis. The esophagus leads posteriorly from the 

 pharynx and joins the anterior or cardiac portion of the prominent 

 stomach. The stomach has the typical shape and appearance of this 

 organ in the lower vertebrate groups, possessing only the cardiac por- 

 tion anteriorly and the posterior narrowed pyloric portion. This 



