[Biology Class Notes] on Amphibia Pdf

In the animal kingdom, class Amphibia refers to all amphibians- which are animals capable of surviving both on land and in water. These are vertebrates that are usually not very large in size. This term is derived from the Greek term ‘amphibious’ which essentially implies ‘double life’. This points to the fact that amphibians can live both in aquatic and terrestrial environments. However, within amphibians, it is possible to find animals that live completely on land or completely in water. 

Amphibians are a crucial link that determines all life forms that began from the oceans. Around 8000 species of amphibians have been documented to date. Their species came into existence back in the Middle Mississippian Epoch. While this has not been completely proven correct, it is believed by scientists that what we today know as amphibians were in fact the first animals that made the shift from sea to land. 

Amphibians are found everywhere in the world except in the antarctic region. They exist in the Arctic regions as well, and also in arid conditions. They are very useful in the production of crops since most varieties of amphibians usually attack and feed on insects that damage crops. 

Classification of Amphibian

Living Members of Amphibians are Grouped Under three Orders

Order 1. Gymnophiona

• Limbless blind, elongated and worm-like

• The tail is short or absent 

• Limb-girdle is absent

• Example- Caecilians

Order 2. Urodela, it is Divided into Five Suborders

Cryptobranchoidea

• Most primitive, permanently aquatic

• Adult without eyelid

• External fertilization occurs

• Example- Cryptobranchus

Ambystomoidea

• Living on land

• Adult with eyelid

• Internal fertilization

• Example- Ambystoma

Salamandridae

Provider

• Cartilaginous skull

• Example – Proteus

Meanies

Order 3. Anura

• Amphibian without tail

•  Adult without gills

• Internal fertilization

• Example- Bufo 

Scientific Classification

Kingdom – Animalia

Phylum – Chordata

Clade- Batrachomorpha

Class- Amphibia

Characteristics of Class Amphibia

General Characters 

• Amphibians are those animals that can live on land as well as on water.

• Amphibians  are dual-mode of life 

• Amphibians are the first vertebrates who made the transition from aquatic to the terrestrial mode of life.

Habit and Habitat

•  Amphibians occur in freshwater and moist water.

•  There are no marine forms.

• Some frogs live on trees. Example- Hyla (tree frog).

• TemperatureIt is (poikilothermic) and most forms hibernate in winter.

• Some of them are active in the dry summer.

Body Form

• Their body varies from broad to narrow, long to short and cylindrical.

• It is divided into four parts which include head, neck, trunk, and tail

• The neck is absent in some of the amphibians.

Appendages

• There are two pairs of pentadactyl limbs, each with 4 – 5  pure digits.

• In some amphibians limbs are absent (Apoda).

• Hindlimbs are larger than four limbs present in frogs and toads for leaping.

• These are near the same size as in salamanders and newts.

• Digits are without claws, nails or hoofs, and often have webs.

• SkinIt is smooth and moist and rich in multicellular mucus and has poisonous glands.

• It is highly vascular and is respiratory in most speciesScales are mostly absent ( except Apoda).

• Endoskeleton It is largely bony.

• Skull is flat and cacodylic i.e has two occipital condyles.

• Notochord does not persist.

• The first vertebra is specialized to provide some movement to the head.

Digestive System 

• The mouth is large and armed with teeth in the upper or both jaws.

• Teeth are acrodont.

• Amphibians (frogs and toads) are the first vertebrates to have a true tongue.

• A true tongue is a soft mucus coated and attached at the front end in frogs and toads.

• The alimentary canal leads into the cloaca.

Respiratory System 

• Respiration takes place by 

• Lungs(Pulmonary respiration)

• Living of buccopharyngeal cavity Skin (Cutaneous respiration) 

• Gills (Branchial respiration)

 

Circulatory System

• The heart is 3- Chambered with two auricles and one ventricle.

• This is an advancement over the 2-chambered fish heart.

• The left and right auricles receive blood from the lungs and rest of the body respectively.The ventricle pumps out mixed blood.

• Sinus venosus and truncus arteriosus are present.

• The renal portal system is well developed.

• RBC cells are biconvex, oval, and nucleated.

Cranial Nerves and Sense Organs

• There are 10 pairs of cranial nerves.

• There is no external ear.

• In addition to the internal ear, a single auditory ossicle is present with the middle ear. 

• Tympanum covers the middle ear

• Lateral line sense organs are present in larva and in aquatic forms.

Reproductive System

• Gonoducts lead to colada.

• Sexes are separate from sexual dimorphism.

• Males lack a copulatory organ.

Fertilization and Development

• Fertilization is mostly external (It is internal in salamander).

• They are oviparous.

• Life history often includes an aquatic larva(tadpole).

• Amphibians have evolved from lung breathing, lobe-finned, bony fishes.

• Their paired fins had a fleshy lobe that gave rise to limbs.

• This ancestral species is presently represented by the living fossil Latimeria.