548 BERTKAM G. SMITH 



ative study of the cranial elements, came to the tentative con- 

 clusion that Necturus is a permanent larva. The generalization 

 formulated by Boas has recently been reiterated and expanded 

 by Versluys ('09); briefly stated, his views are as follows: 



The great resemblance of the perennibranchs to salamander 

 larvae is only a consequence of the fact that the first are also 

 larvae, but larvae which no longer come to full development as 

 their ancestors did; the metamorphosis is imperfectly undergone 

 or wholly omitted. Nevertheless these larvae become sexually 

 mature, as in neoteny. In the course of time, in adaptation to 

 their aquatic habitat, they have become degenerate in many re- 

 spects. The derotremes are salamanders that have become fixed 

 in the transitional stage or metamorphosis. Thus the aquatic 

 urodeles have a terrestrial ancestry; they are forms which have 

 reverted to an aquatic mode of life. 



The probable course of events giving rise to the perennibranchs 

 may be described as follows: While the mature salamanders 

 are constructed after the fashion of a land animal, their larvae 

 live in water and in the course of time have become more and 

 more adapted to aquatic life. They have extended their larval 

 organization and thus increased the difference, which must be 

 overcome by metamorphosis, between the larva and the grown- 

 up animal. In time some organs show arrested development 

 or degeneration of such a sort that the larvae can no longer develop 

 into land-living salamanders; they remain life-long water dwell- 

 ers. The condition is one of fixed neoteny (Boas '96) . Accord- 

 ing to this view the perennibranchs are disconnected from one 

 another and have evolved as neotenic larvae from different 

 salamanders. 



Confirmatory evidence for this view comes from the studies 

 of Emerson ('05) on Typhlomolge. In some details of its struc- 

 ture this animal shows a remarkable similarity to the larvae of 

 Spelerpes ruber. Probably Typhlomolge has been derived from 

 the neotenic larva of some salamander of the family (Plethodonti- 

 dae) to which Spelerpes belongs, adaptations to a subterranean 

 water life having been added. 



