64 VERTEBRATE ZOOLOGY 



The body of the animal may be divided into three regions,— 

 the head, the trunk, and the tail. The neck region, which is want- 

 ing in fishes and is so characteristic of land vertebrates, is just 

 beginning to make its appearance in amphibians. A distinct neck 

 is wanting, but a cervical vertebra is present, with which the skull 

 articulates. 



The Head. This body division is bluntly triangular in shape 

 and is very flat and broad. The mouth is rather small and the lips 

 are prominent. At the sides of the mouth note how the upper lip 

 dovetails into a fold of the lower lip. The two nostrils are at the 

 anterior end of the head and widely separated from each other ; 

 each can be tightly closed by a fold of skin. The eyes are small 

 and do not project. No eyelids are present ; the skin passes with- 

 out interruption over the eyes, but is transparent in front of them. 

 The ear, as in fishes, is entirely within the skull, no portion of it 

 appearing on the surface. Integumental sense buds, such as occur 

 on the head and in the lateral line of fishes, are present on the head 

 and body of Necturus, but will hardly be seen. 



The posterior part of the head is marked by a ventral trans- 

 verse fold of the skin and also by three pairs of gills. These gills 

 are not homologous to the gills of fishes but are projections of 

 the outer integument; note carefully their character and shape. 

 Between the first and second and the second and third gills on 

 each side is a gill cleft, a slitlike opening into the pharynx. Probe 

 these clefts. Necturus has a twofold method of respiration, by 

 means of lungs and through the entire outer surface of the body 

 and the gills. Inasmuch as the animal never leaves the water, the 

 latter method is the more important one, the function of the lungs 

 probably being mainly hydrostatic and similar to that of the air 

 bladder in fishes. 



The trunk and the tail pass gradually into each other. They are 

 without external segmentation, but the muscle segments can usu- 

 ally be detected through the skin. The trunk bears the append- 

 ages. A short distance back of the hinder appendages in the ventral 

 surface is the large anus, the opening of the cloaca. Note its shape ; 

 its lips are often swollen in consequence of the absorption of water 

 by the cloacal glands. 



