Anurans, urodeles and caecilians have moist and permeable skin

 Figure 1. The three orders of 
 Amphibians: caecilians (1), 
 urodeles (2) and anurans (3). 
 Photos A. Catenazzi.

The word Amphibia contains amphi=double and bios=life, 
referring to the aquatic and terrestrial life stages that 
characterize many amphibians (although not all).

Amphibians are vertrabrates tetrapods, with moist and scaleless 
skin. There are three distinct lineages: anurans, salamanders and 
caecilians (Fig. 1). The class of Amphibia is a monophyletic 
lineage. The most important derived traits of all extant 
amphibians are:
- moist, permeable skin that allows cutaneous gas exchange. 
Mucous glands keep the skin moist. Poison glands

- papilla amphibiorum: a sensory area in the wall of the 
sacculus of inner ear, sensitive to sound frequencies below 
1000 Hz (papilla basilaris is sensitive to frequencies above 
1000 Hz)

- operculum-plectrum complex: bones involved in 
transmitting sounds to the inner ear. The plectrum (columella) 
is derived from the hyoid arch; columella and plectrum are 
fused in anurans, caecilians and in some salamanders. The 
operculum develops in association with the fenestra ovalis 
in the inner ear

- green rods: retinal cells unique to amphibians

- pedicellate teeth: the crown and base or pedicel of teeth 
composed of dentine, separated by a zone of uncalcified 
dentine made of fibrous connective tissue.
Exceptions: the genera Siren, Phyllobates, Ceratophrys and 
some spp. in other genera

- structure of levator bulbi muscle: this is a thin muscle in 
the floor of the orbit innervated by the 5th cranial nerve, that 
causes the eye to bulge outward and to enlarge the buccal 
cavity. This muscle is present in anurans and urodeles and in a 
modified form in caecilians.