Caecilians

Caecilians are aquatic and burrowing animals, lack limbs and look more or less like large earthworms. They are the only group of amphibians to possess dermal scales (found only on tails). There is a chemosensory organ called 'tentacle' between eye and nostril (circled in the figure). Eyes are generally reduced or covered by skin. Caecilians occur throughout the tropics, except in Madagascar and east of Wallace's line in Australia.
Salamanders

Salamanders are four-limbed amphibians resemble lizards. Salamanders lack external ears and do not vocalize, instead they rely on chemical signals for communication. Some of the members are lungless. Salamanders exhibit paedomorphosis (Gk: paedo = child, morph = form). That is, in their tadpole stage itself; they attain reproductive stage of adult. Salamanders principally occur in North America and northern Eurasia. But lungless salamanders do occur in Central and South America.
Anurans
 

Anurans are four-limbed, terrestrial juveniles and adults completely lack true tails. Based on the morphology, they are categorized into frogs and toad. Generally, frogs are smooth skinned, aquatic amphibians, whereas toads have rough and warty skin, bony ridges and prefer terrestrial conditions. Anurans hindlimbs and feet are greatly elongated. Body is more adapted to light weight and strength for jumping.

          Vocalizations are the most important communication signals for anurans. Only males of the anurans produce calls. This is used as advertisement calls (to attract the female during breeding season and to advertise the ownership of territory), aggressive calls (to defend from other males). There are other types of calls, namely reciprocal calls (soft low calls from females [either during courtship (also referred as advertisement calls) or reproductive stage]. Females do not have a vocal sac and very few can vocalize. Females sometimes give very low reciprocal call for the male advertisement calls. When producing the calls, male force the air into their lungs with positive pressure excreted by throat muscles. When these muscles contract, air is forced out of the lungs and into the buccal cavity.

          Of the three orders, anurans are most wide spread. Frogs are found in all continents except in Antarctica.