248 GENERAL ZOOLOGY 



swim skillfully by making undulatory movements with its 

 muscular body and tail (B). 



The digestive system is much like that of other verte- 

 brates, consisting of a mouth, pharynx, oesophagus, stom- 

 ach, intestine, liver and pancreas. There is a bladder 

 opening from the ventral wall of the intestine which re- 

 ceives the urine brought by ducts (ureters) from the kid- 

 neys. The heart has three chambers, a ventricle and two 

 auricles. One auricle is filled with " impure" blood from 

 the larger veins, the other receives aerated blood from the 

 gills. The structure of the heart is such that, though both 

 kinds of blood enter the ventricle and are pumped out 

 together, the freshest blood goes to the head and visceral 

 organs, while most of that charged with waste products 

 reaches the gills. The gills are tufts of filaments through 

 which waste gases are exchanged for oxygen. Slender 

 lungs are also present and if the water becomes stagnant, 

 Necturus may go to the surface and gulp in air. 



Self-protection. The mud-puppy is protected in many 

 ways from its enemies. It is acceptable food for water 

 snakes and other predaceous animals, but usually escapes 

 through its nocturnal habits and protective coloration. 

 It may, like other cold-blooded vertebrates, change its 

 colors somewhat to suit the background on which it rests. 

 All amphibians have poison glands in the skin, and the 

 bitter secretion from these makes the mud-puppy distaste- 

 ful to many animals which might otherwise feed upon it. 

 If attacked or captured, Necturus will bite viciously, but 

 the wounds inflicted are not poisonous, as many fishermen 

 believe. 



If Necturus loses a leg or a piece of its tail, the lost parts 

 are regenerated after a time. The process of regeneration 

 and growth in salamanders has been the subject of careful 

 experimental study, and some fundamental facts have been 

 discovered. For example, if the portion of an embryonic 

 salamander eye known as the optic cup is removed and 

 grafted almost anywhere under the skin of the head, a 



