Most obvious characteristic is the tail-like appendage which is actually an everted extension of the cloaca only found in males. These frogs are considered primitive because of some characteristics which include: 9 vertebrae (other frogs have 8 or less), type III aquatic larvae, their inability to vocalize, and their free ribs.
Ecology
Adult diet consists of invertebrates and arthropods, while tadpoles graze on algae attached to rocks. In reporduction, the males use their "tail" to internally fertilize, and because of this, less sperm is lost to the streams. To keep themselves from drifting away, the tadpoles latch onto substrates using their mouth as a suction cup.