Difference between Urodela, Anura and Apoda

Amphibians are vertebrate tetrapods that include salamanders, frogs, toads and caecilians. The word amphibian, that is a Greek word, translates to dual life referring to their ability to survive both in terrestrial and aquatic environments. There are three extant orders under the class Amphibia namely, Urodela (salamanders), Anura (frogs and toads) and Apoda (caecilians). This article focuses on differentiating between the three orders.

Urodela

The order Urodela, also known as Caudata comprises the tailed amphibians, salamanders and newts. They have the least specialised bodies out of all amphibians and appear lizard-like. They have an elongated, slender and stout body with well developed musculature and a tail. The classification of the Urodela order is as follows:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Order: Urodela

The salamanders are a diverse group of organisms. Around 620 species of salamanders are known. They can survive in terrestrial, aquatic, arboreal, and burrowing environments. They prefer living in moist places on land but have to return to water for breeding. However, some species are totally aquatic in nature. The terrestrial salamanders may reach up to 1 m in length and have tails as long as the body. The aquatic salamanders are larger and may grow up to 1.8 m long.

They can respire through gills, lungs, skin and oral cavity. They develop gills at the larval stage which can either remain through their entire life or can be shed after metamorphosis. Lungs in adult salamanders are not as complex as mammals, they are simple and sac-like. They have four limbs in two pairs, the most developed among all amphibians. They lack middle ear cavities but can sense airborne vibrations.

The salamanders are predators that feed other amphibians, insects, fishes and small mammals. They possess teeth in both upper and lower jaws and even the larvae possess teeth. The terrestrial salamanders have a long tongue that is sticky in nature. They flick out their tongue to prey, and the whole event takes less than half a second.

The salamanders have a soft body with slimy skin. This acts as a line of defence to escape the predators. Secondly, they have organs on their tail that secrete poisonous substances to keep the predators away. Their fertilisation is mostly internal.

Anura

The order Anura of the class Amphibia comprises frogs and toads. They are the most diverse group of amphibians with more than 4500 species known. The frogs and toads of the order Anura have short, tailless bodies, big mouths and flat heads with long, muscular hindlimbs. The taxonomic classification is as follows:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Order: Anura

Their body structures have evolved to help them in jumping. Animals with long hind limbs and smooth, slimy bodies are referred to as frogs. On the other hand, animals with shorter limbs and rough skins are referred to as toads.

The frogs are terrestrial, aquatic as well arboreal inhabitants. The muscular hindlimbs strengthen their crouching position that propels the frog high in air. Frogs mostly feed on insects and invertebrates, smaller rodents and other frogs. The frogs lay their eggs in clumps on quiet water surfaces or submerged vegetation. The eggs are fertilised externally. The egg cracks to form a tadpole that is mostly aquatic in nature.

The tadpole then undergoes metamorphosis to form an adult frog. The adult frog loses the tail and limbs when webbed toes are developed. They develop eyelids, jaws and teeth and mucous glands on the skin. They have a visible tympanic membrane on each side of their head. The gills in the tadpole disappear in the adult stage to form primitive lungs.

The frogs save themselves from the predators by their jumping skills. Another mechanism is the presence of parotid glands near the head that produces toxic substances.

Apoda

The class amphibia comprises limbless amphibians classified in the order Apoda, also called Gymnophiona. The limbless amphibians are also referred to as caecilians. The group is known as caecilians because the word caecus in Latin means sightless or blind, which the limbless amphibians are.

The classification of limbless amphibians is as follows:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Order: Gymnophiona / Apoda

They are burrowers that live in moist soils near streams, lakes and ponds. They have cylindrical bodies with annular rings. They completely lack limbs, so that the smaller ones look like worms and the longer ones look like snakes. About 189 species of the group are known that are divided into 35 genera and 10 families.

They are found in tropical regions all over the world. They are mostly viviparous in nature and the fertilisation is internal. These amphibians mainly feed on earthworms and other soft-bodied animals. The caecilians are the only amphibians that have tentacles that are chemosensory in nature, and help in locating their quarry.

They have sharp teeth arranged in two rows that help in mastication and swallowing. They have bony skulls that help in burrowing. They have specialised skin glands that produce toxins to repel the predators. They are not poisonous. The respiration is through lungs, skin, or oral cavity. The locomotion takes place by musculature and hydrostatic motion. They also possess dermal scales. Their hearts are three-chambered.

Their eyes are covered by skin in most of them; they lack middle and outer ear cavities but can detect low frequency sounds. The caecilians lack a bone marrow, and hence are dependent on liver, kidney and spleen for blood cell production.

Urodela vs Anura vs Apoda

Urodela

Anura

Apoda

Description

The order Urodela comprises tailed salamanders and newts.

The order Anura comprises tailless toads and frogs.

The order Apoda comprises limbless amphibians.

Habitat

The salamanders live in moist, dark places.

The frogs prefer tropical environments and can thrive on both land and water.

They are mostly burrowers that live in swampy solid near lakes, streams and ponds.

Body Structure

They have an elongated, slender and stout body with well developed musculature and a tail.

They have short, tailless bodies, big mouths and flat heads with long, muscular hindlimbs.

They have cylindrical bodies with annular rings.

Limbs

They have four limbs of almost equal size.

They have four limbs, but the hindlimbs are longer.

They are limbless.

Tail

They possess tails, sometimes as long as their body.

They are tailless.

A very short tail is present or is absent.

Fertilisation

Fertilisation is internal.

Fertilisation is external.

Fertilisation is internal.

Diversity

There are about 620 known species.

They are the most diverse group of amphibians, with more than 4500 known species.

There are about 189 known species.

Feeding Habit

They feed on insects, fishes and small mammals.

They feed on invertebrates, smaller rodents and other frogs.

They feed on earthworms and other soft-bodied animals.

Locomotion

The four limbs help in the movement of salamanders.

The longer hind limbs help in the jumping of frogs.

The locomotion takes place by musculature and hydrostatic motion.

Defence

The salamanders have a slimy skin that protects them from the predators. Another mechanism is the presence of glands near their tails that secrete toxic substances.

The frogs mostly get rid of their predators by their jumping skills. Additionally, they have parotid glands near their head that secretes mucus and other toxic substances.

The limbless amphibians are fossorial, so they don’t face much predation.

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Frequently Asked Questions on Difference between Urodela, Anura and Apoda

What characteristics distinguish amphibians from other terrestrial vertebrates?

The amphibians have a complex life cycle, they undergo metamorphosis from a larva into an adult, whereas terrestrial vertebrates develop directly.

What is the distinguishing feature of amphibians belonging to the Apoda group?

In general terms, the amphibians are referred to as tetrapods, having four legs. But the Apoda group comprises animals that are limbless amphibians.

Why are amphibians considered to be a unique evolutionary group?

Amphibians are a unique group because they can thrive on both land and water. They undergo metamorphosis when they develop from a larva to an adult. Also, they are cold-blooded animals, a trait they share with fishes, invertebrates and reptiles.

Do amphibians breathe with lungs or gills?

The amphibians can breathe through their lungs, skin, gills and oral cavity. The salamanders breathe through primitive lungs, the frog tadpole breathes through gills, and the adult frog breathes through lungs.

What are the three main orders of Amphibia?

The class Amphibia comprises three living orders namely, Urodela, Anura and Apoda. The Urodela consists of salamanders and newts, the Anura consists of frogs and toads and the Apoda consists of limbless amphibians.

What are the characteristics of Anura?

The order Anura has short bodied, tailless organisms. They have strong and long hind limbs that aid in the process of jumping.

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