22 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



4. The bowfin or river dogfish (Amia). The body is clothed in flexible 

 scales, similar to those of the perch. There are large scales on the head. The 

 operculum and branchiostegal membranes contain bones. The nostrils have two 

 openings, the anterior one borne on the tip of the process located on the anterior 

 end of the head, the posterior opening situated in front of the eye. There is no 

 spiracle. There is a very long dorsal fin and a nearly homocercal tail. The 

 fish strongly resembles the ordinary teleost type. 



I. EXTERNAL ANATOMY OF NECTURUS 



Necturus is a salamander; it belongs to the class Amphibia, order Urodela. 

 It is an example of the lowest land vertebrates those which left the water, 

 acquired lungs and the air-breathing habit, and developed legs in place of the 

 paired fins. Obtain a specimen and note the following points. 



1. Body and skin. The body is typically vertebrate in form. It is divisible 

 into head, trunk, and tail, as in fishes. A neck is not present. The skin is 

 naked and very slimy, without scales or other hardened parts such as are com- 

 monly found in vertebrates. 



2. Head. The head is broad and flat and has a terminal mouth provided 

 with lips. It bears the usual three pairs of sense organs. The nostrils or external 

 nares are a pair of widely separated openings just back of the margin of the upper 

 lip. By probing into the nostrils determine that they communicate with the 

 mouth cavity by means of openings known as the internal nares. This arrange- 

 ment permits air to enter the mouth cavity through the nostrils, and differs 

 greatly from the condition found in the majority of fishes where the olfactory 

 sacs end blindly and have no connection with the mouth cavity. In some fishes, 

 however, as the skate, there is an external groove, the oronasal groove, extending 

 from each olfactory sac to the mouth cavity; and by the fusion of the borders 

 of this groove a closed passage from the nostrils to the mouth cavity is produced 

 (see K, pp. 206-8). The small eyes without eyelids are situated on the sides of 

 the head. The ears, as in fishes, are internal only. 



From each side of the posterior margin of the head spring three gills, each con- 

 sisting of a fringe of filaments dependent from a dorsal process. They are 

 external gills and do not correspond to the gills of fishes which are internal. 

 Between the first and second, and the second and third gills are the gill slits 

 which open, as in fishes, into the cavity of the pharynx. The animal, however, does 

 not pass water through the gill slits but respires by means of the external gills 

 which are kept in constant motion, through the general surface of the body, and 

 to some extent by means of its lungs. 



3. Appendages. The trunk bears the two pairs of appendages. These 

 correspond to the paired fins of fishes but have evolved into typical walking 

 limbs. Each consists of the following parts: upper arm, forearm, wrist, and hand, 



