Neoteny is the retention of larval characteristics by an organism that has reached sexual maturity. Neoteny is evident in the retention of gills, as in some salamanders, the best known example of which is the axolotl of the western United States and Mexico; it also plays an important part in the life cycle of many invertebrates. An entire order of tunicates (sea squirts), the Larvacea, is permanently larval, never reaching a typical adult form. Several times, the development of neotenic larvae may have been important in the course of evolution.