398 W. W. SWINGLE 



peculiar group of aquatic amphibians known as perennibran- 

 chiates (Necturus, Proteus, Typhlomolge, etc.) are permanent 

 larval forms capable of reproducing the species. The group is 

 supposed to represent a sort of retrograde evolution from an 

 originally terrestrial life to permanent aquatic existence by 

 suppression of metamorphosis. The evidence for this view is 

 suggestive; briefly stated it is somewhat as follows: 



1. It is .generally conceded that the perennibranchiates do 

 not form a natural group, but are to be regarded as a hetero- 

 geneous assembly; various genera are undoubtedly represented in 

 the group. These animals probably became neotenic at a 

 phylogenetically old stage and are hence the oldest and not the 

 youngest members of the present-day urodeles. 



2. Various anatomical features of the group, such as the 

 pentadactyloid limb, presence of lungs, suppression of internal 

 gills, and connection of the pelvic girdle with the vertebral 

 column, point to a terrestrial existence somewhere in the history 

 of the group. 



3. Perhaps the most suggestive line of evidence for the view 

 that the perennibranchiates are permanent larval forms is the 

 occurrence of neoteny and paedogenesis as aberrations of develop- 

 ment in semiterrestrial species of urodeles. For instance, it has 

 long been known that the larvae of certain European salamanders 

 fail to undergo metamorphosis and occasionally attain the size 

 of 80 mm., whereas the normal size at transformation is 40 mm. 

 Larvae of Triton have been reported 80 to 90 mm. long with 

 functional gills and sexual organs fully developed. DeFilippi 

 ('61) found in one locality in Lombardy sexually mature larvae. 

 According to him, such gill breathing, sexually mature specimens 

 occur constantly in a small lake in the province of Ossola in the 

 Italian Alps. Many other cases have been reported, and the 

 classical example of the axolotl is well known. It will be recalled 

 that this creature was classified by systematists as a distinct 

 species of perennibranchiata until Dumeril described its meta- 

 morphosis into Amblystoma tigrinum. 



4. Lastly, it has been repeatedly stated in the literature that 

 one of the perennibranchiates, Typhlomolge, is hereditarily 



