Morphological J'ariation and Its Causes in A. tigrinum 25 



specimen is due to a combination of causes. In the first place the 

 specimen was taken in September, as a rather slowly grown, slen- 

 der larva. It was fed in the laboratory until it metamorphosed 

 in mid-winter, during which time it became a long-tailed larva. 

 After metamiorphosis the adult was kept continuously in water 

 for nearly two years. It was a restless, active swimmer, and the 

 tail underwent a gradual expansion until the second autumn, 

 when the animal became sexualh- mature. Following this an- 

 other rapid increase occurred in ' the tail, both in breadth and 

 length. It was at this time that the tail became sharp-pointed and 

 thin at the tip, this being a modification and exaggeration of the 

 change which I have described as occurring in all males upon the 

 assumption of sexual maturity. As stated, this tail tip simulated 

 almost exactly the tail of a rapidly grown, free-swimming larva. 

 I have seen only two others that were at all like it. The char- 

 acter, though a trifling one, is interesting as an example of the 

 very considerable number of unique and definite characters which 

 may be produced in this species by special combinations of life 

 habit and external conditions. 



Of other features of the tail I will mention a few merely. The 

 thickness, laterally, is as variable as any other dimension. Of all 

 features, it varies the most during the life of the adult. It varies 

 immensely with the condition of the animals, and much more in 

 females than in males. In the female the heavy dorsum of the 

 tail seems to be as much a store of nutriment, to be accumulated 

 and drawn upon, as is the corpus adiposum. The older animals, 

 however, are apt to have the thickness of the tail absorbed with- 

 out being able to restore it save under very unusual conditions. 

 Young animals of both sexes are always thick-tailed, sometimes 

 to a surprising degree, and the dorsum of the tail is frequently 

 as thick as the ventral portion ; the cross-section thus shows an 

 even oval, which is never the case in old adults. Hardly any vari- 

 ation modifies the appearance of the animal more than this trans- 

 formation, which is purely a matter of age. In the males, sexual 

 maturity again complicates this change. Not only is the tail 

 lengthened at the tip and greatly thinned at the base, but it is also 

 much lowered at the base, at a point just over the cloacal lips ; it 



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