Amphibia, Biology tutorial

Characteristics of the Class Amphibia:

The amphibians are characterized by the given features:

  • Moist, glandular skin which lacks keratinised scales of reptiles
  • Complex life cycles (tadpole/juveniles, eggs, adults through metamorhosis)
  • Non-amniotic eggs (they lack amniotic membrane which surrounds embryo)
  • Eggs lack the shell instead enclosed by many gelatinous layers
  • Gills at larval stage and lungs at adult stage. In several amphibians, skin is also significant in gas exchange
  • Two pairs of pentadactyl (five digits) limbs
  • Cold-blooded animals (they don't possess the constant body temperature but instead take on temperature of their environment)
  • No external ear.

Moist, scale-less skin of amphibians absorbs water and oxygen from surrounding atmosphere, but that also makes them susceptible to dehydration (loss of bodily fluids). Without moist situation, their skin dries out and they die. That describes why amphibians are most frequently found near marshlands, swamps, ponds, and other areas where moisture is available. Few amphibians turn into inactive when situations are unfavorable for survival. This period of inactivity is known as estivation when it happens in hot, dry weather and hibernation when it happens in response to cold temperatures. Activity resumes when favorable environment return. Thin skin of amphibians has several glands; among them is poison gland which protects certain species against predators. Poison from glands of brightly colored poison-dart frog is mainly toxic. Life cycle of most amphibians starts in water when female lays eggs which are fertilized outside of body. Eggs then hatch in larvae (called as tadpoles), which breathe through external gills. Larvae grow flat tails and feed on vegetation. In procedure known as metamorphosis, physical changes happen and external gills give way to lungs. Tadpoles also transform from plant-eating (herbivorous) to meat eaters (carnivorous) animals. Amphibians generally reach full adulthood at three to four years.

Class Amphibia is made of three orders that include Gymnophiona (caecilians), Urodela (urodeles - newts and salamanders) and Anura (anurans - frogs and toads).

Features of Order Anura (Amphibians without tail)

Anurans are the largest group of living amphibians, including approx 3,000 species. This order is composed of most common amphibians which we come across in environment. They comprise frogs and toads.

Members of this order have given characteristics:

  • lack true tail in adult stage
  • hind limbs are longer than front limbs; and by this they are well adapted for jumping, hopping and swimming
  • Dwell in aquatic environment, though some are well adapted to drier habitats
  • Larval forms are known as tadpoles, adults lack true teeth and are generally herbivorous, and grow hind limbs before front limbs.
  • External gills in larvae give way to internal gills with opercular chambers which permit water to flow over internal gills, prior to exiting through spiracle.

Toads have drier skin which is warty it allows them adapt to drier habitats in comparison to smooth skin of frogs. Frogs have longer and well-pronounced webbed feet than toads and are frequently in or near water. To the large extent, toads are ones we see hopping around our environment particularly during rains. They are more liberal to dry situation than frogs, which are frequently in water or not too far from a water source.

Characteristics of Order Urodela (Evident/visible tail)

Urodela are amphibians which have visible tail. Features of this order are as follows:

They have long true tail therefore name Urodela meaning evident/visible tail, lack tympanum(external ear drum). They contain small and underdeveloped legs adapted to walking; breathe by external gills and lungs. They are generally found in or near water and often reside in moist soil under rocks or logs.

The red-spotted Newt, dwells in ponds and streams in eastern and central United States. In its larval stage, its skin is bright reddish-orange. After achieving adulthood, its skin becomes olive with red spots.

Characteristics of Order Gymnophiona (Caecilians/Apoda e.g. Ichthyophis, Typhlonectes)

Members of order contain the given characteristics:

  • Lack legs therefore the name Apoda (dig burrow by ramming bony head through soft dirt)
  • Worm-like in shape - body is ringed/marked by rings
  • Practically blind
  • Tail absent or greatly reduced
  • Sensory tentacles on head
  • Internal fertilization

Members of this order are look like worms, blind and legless amphibians which are shaped like worms. Some caecilians dwell in moist soil which is rich in decayed plant matter. They also dwell in leaf litter. Other caecilians dwell in water all or most of the time. Burrowing movements of land-dwelling caecilians turn soil and therefore keep it in good state.

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