What external features illustrate larval characteristics?

Neoteny

Some salamanders exhibit neoteny, becoming sexually mature while still retaining some larval characteristics. In some species, the neotenic characteristics have become a permanent state of the species (obligate neoteny = paedogenesis; both are called paedomorphic conditions). These have lost the ability to undergo complete metamorphosis (hellbender, amphiuma, and mudpuppy). In others, the animal does not undergo complete metamorphosis in nature, but can be induced to do so in the lab.  An example of this is the Mexican axolotl (Ambystoma mexicanum), a large gilled salamander found in Mexico.  In some species of salamanders, members of a population exhibit neoteny only when they are under stressful environmental conditions (such as crowding, scarce food, low concentrations of trace elements). This is called facultative neoteny.  Tiger Salamanders (Ambvstoma tigrinum) frequently do this, but local populations of marbled salamanders show a tendency toward neoteny in Virginia.

Look at the adult mudpuppy, siren, hellbender, and amphiuma. 

1) What external features illustrate larval characteristics?