450 VERTEBRATE LIFE AND ORGANIZATION 



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Figure 23.3. Neotenic salamanders. 4, The mudpuppy, Necturus maculosus, is a 

 permanent larva. B, The tiger salamander, Anibystoi/ia tigritiuin, metamorphoses in 

 most environments, but fails to do so in certain mountain lakes. C, The axolotl, or neo- 

 tenic form of Ambystoma tigrinum. {A, Courtesy of Shedd Aquarium, Chicago; B-C, 

 courtesy of the Philadelphia Zoological Society.) 



Several groups of salamanders, including the mudpuppy {Necturus 

 maculosus, Fig. 23.3 A), have become entirely aquatic. The development 

 of the reproductive organs has been speeded up in relation to develop- 

 ment of other parts of the body. Sexual maturity is achieved in the 

 larval stage and metamorphosis is never completed. This is another 

 example of neoteny, a phenomenon encountered earlier in the lower 

 chordates. The hormone of the thyroid gland, thyroxin, is necessary for 

 metamorphosis. The failure of Necturus to metamorphose appears to 

 result from the inability of the tissues to respond to thyroxin rather than 

 from an absence of this hormone. Thyroxin is produced, for the thyroid 

 of Necturus hastens metamorphosis when transplanted to frog tadpoles. 



