380 THE TRANSITION TO LAND 



are surrounded by a mass of transparent jelly, however, which protects 

 them to some extent in the water, but it dries up quickly on the land. 



There is one group of amphibians which retain the tail as an adult. 

 These are commonly called the salamanders or newts. Some of these 

 live almost entirely in the water and may keep their gills throughout 

 life, but they also develop lungs and use both gill and lung respiration. 

 Others live mostly on the land, but are not as well adapted to life on 

 the land as the toads and they must crawl under rocks and logs during 

 the daytime to prevent their skin from drying. 



The common mud puppy (Necturus maculosus) is a large river 

 salamander which retains its gills throughout life. Since it is found 

 in large rivers which do not dry up in the summer, this is a useful adap- 

 tation. It is a fearsome-looking creature, and many fishermen think 

 that it is deadly poisonous. Many will not touch one if they happen to 

 catch one on their line. Actually, it has teeth so tiny that it is incapable 

 of inflicting a bite that will break the skin on man. Mud puppies are 

 regularly dissected in courses in comparative anatomy and, for this 

 reason, the demand for them is very great. 



The mud eel (Siren) is another interesting salamander which keeps 

 its gills throughout life. It has a long, slender, eel-like body with tiny 

 front legs, but no hind legs. It is nearly helpless on land, but in the 

 water it can swim with undulating movements of the body with the front 

 legs folded back against the body. At rest, however, the front legs help 

 this peculiar salamander to keep upright and to walk around slowly. 

 Evidently, in the distant past a mutation or series of mutations of the 

 genes resulted in the loss of the hind limbs ; and the species survived as 

 well, or perhaps even better, without them, and this characteristic be- 

 came established in the species. Hind legs might interfere with the 

 smoothness of the body when the mud eel was trying to escape from an 

 enemy. 



The Colorado axolotl is a very interesting species which is found 

 abundantly in the mountain streams of Colorado. Like all amphibians, 

 the larvae have gills. There is no metamorphosis, however, and the 

 gills are retained throughout life. It is sometimes said that they remain 

 larvae in general body structure, but become sexually mature. In other 

 regions there is a tiger salamander (Ambystoma tigrinum) which looks 

 just like the Colorado axolotl as larvae, but they undergo metamorphosis 

 and become spotted adults without gills. When some of the Colorado 

 axolotls were taken east they soon changed color, lost their gills, and 

 crawled out of the aquarium as typical tiger salamanders. It was dis- 

 covered that absence of iodine in the water of the mountain streams 



