AMPHIBIANS:APODA-URODELA-ANURA

AMPHIBIA-CHARECTHERS-CLASSIFICATION-EVOLUTION

 
The amphibians are vertebrates which lead a dual mode of life. Most of the amphibians live partly in fresh water and partly on moist soil. Amphibians are the first and primitive tetrapods (tetra = four, poda = foot).clip_image001
EVOLUTION OF AMPHIBIANS
The earliest amphibians came into existence during the Devonian period- The extinct osteolepids were the closest relatives of Amphibia. osteolepids fish characteristics exibits striking similarities between the extinct  and the earliest fossil amphibians such as Ichthyostega, Eryops confirmed the origin of Amphibiafrom the Crossopterygian stock. The lobed fins of Osteolepids were the fore-runners of the pentadactyle limbs of Tetrapoda. The probable ancestors of amphibia are osteolepis and eusthenopteron, the rhipidistians of Devonian period.
Amphibians stand between the fishes and reptiles both in structure and function representing the transition of life from water to land. A’ number of changes had taken place in the organisation of the body of an amphibian.
The amphibians were predominent during the Carboniferous period. Hence, the Carboniferous period is regarded as “the age of Amphibia“. Most of the amphibians became extinct by the turn of Mesozoic era leaving behind a few representatives as modern Amphibia.
AMPHIBIANS-CHARACTERISTICS
Amphibians are worldwide in distribution. However, they are not found in marine habitat. Amphibians exhibit a lot of variation in their size and shape but they show a number of common characters. They are :
1. Amphibians are tetrapods which live partly in water and partly on moist soil hence called Amphibia.
2. The temperature of the body in amphibians changes according to the changes in the environment. Hence, they are called poikilothermous or ectothermal or cold blooded tetrapods.
3. The body is divisible into head, trunk and tail. But tail is absent in frogs and toads. Neck is present in urodela and absent in Anura and Apoda.
4. The skin of Amphibians is naked, glandular and moist. Scales are absent except in caecilians. The moist scale less skin serves as an organ of respiration.
5. The body of amphibians possesses two pairs of pentadactyle limbs. But they are absent in caecilians (Apoda). The limbs are of equal size in Urodela and unequal size in Anura.
6. The external nostrils communicate with the buccal cavity through a pair of internal nostrils. The development of internal nostrils paved the way for pulmonary respiration..
7. A pair of eyes with immovable upper eye lid and movable lower eye lid, is located on the head. The lower eye lid bears a transparent nictitating membrane (third eye lid). Eyes are degenerated in caecilians and in some urodeles.
8. There is no external ear in Amphibia. Tympanum is present only in frogs and toads. Columella auris is the only ear ossicle present in the middle ear, it is a modified hyomandi bular.
9. Maxillary teeth are usually present. But teeth are totally absent in toads.
10. There is a sticky protruding tongue which helps in capturing food.
11. The endoskeleton is mostly bony in nature.
12. Skull in Amphibia bears two OCCIPITALCONDYLES. Hence, it is de­scribed as Dicondylic.
13. Sternum is first developed in Amphibia. It is absent in caecilians.
14. Vertebrae in Amphibia are procoelous or amphicoelous type.
15. Ribs are usually absent. When the ribs are present in some forms, they do not reach the sternum.
16. Digestive system is well developed in Amphibia. The alimentary canal opens into cloaca at the posterior end.
17. Salivary glands are absent in the buccal cavity.
18. Respiration is effected by skin, buccopharyngeal cavity and lungs. The larvae and some aquatic adult forms respire by gills.
19. There is no distinct tracheal tube due to the absence of neck. The voice box in Amphibia is laryngo tracheal chamber. A pair of elastic, bladder like lungs is present to carry on pulmonary respiration.
20. The red blood cells are oval, biconvex and nucleated in amphibians.
21. The heart in Amphibia is 3 chambered with 2 auricles and 1 ventricle. Truncus arteriosus and sinus venosus are also associated with .the heart.
22. Both hepatic and renal portal systems are well developed.
23. The cerebellum of brain in Amphibia is poorly developed and the olfactory lobes are indistinct.
24. There are 10 pairs of cranial nerves in amphibia as in the case of fishes.
25. Lateral line sense organs are well developed in the larval stages of Amphibians and in adults of aquatic urodeles and Anurans (xen-opus).
26. The excretory organs are a pair of mesonephric kidneys. Nephros-tomes are associated with the kidneys in Amphibia.
27. Adult amphibians excrete urea. Hence they are known as ureotelic animals.
28. Amphibians are generally oviparous. Eggs are Mesolecithal (eggs with moderate amount of yolk) and Telolecithal (yolk towards vegetal hemisphere)
29. Cleavage is holoblastic and unequal.
30. Fertilization is generally external as there is no copulatory organ but it is internal in caecilians, as there is a copulatory organ.
31. Development is usually indirect i.e The life history includes larval stage. The larva undergoes metamorphosis to become adult”.
32. Parental care is seen in many amphibians.
Romer described amphibians as defeated group, as they are not able to come to land completely.
Distinctive features of Apoda or Gymnophiana
(Gr. A = no + podus = foot)
(Gr. Gymnos = naked, + ophioneos = serpent like)
1. They are limbless and tailless amphibians commonly known as caecilians or blind worms or naked snakes.
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2. Body of caecilians is elongated and worm-like. The skin-is wrinkled
With small cycloid scales embedded in the wrinkles or grooves. They are adapted for burrowing life.
3. Eyes are degenerated and covered by the skin.
4. The head bears a pair of sensory tentacles one on either side in between the nostril and eye.
5. Tympanum, limbs and girdles are absent. Notochord is persistent.
6. Vertebrae are amphicoelous. Ribs are present, but they do not reach the sternum.
7. The systemic and pulmonary trunks of one side are connected with one another by a blood vessel called ductus botalli.
8. Lungs and kidneys are asymmetrical.
9. The cloacal wall of male is eversible and serves as copulatory organ. Presence of copulatory organ is a unique feature in Apodans. Fertilization is internal.
10. Some females lay eggs in strings in the soil and coils around the strings of eggs till they hatch. Thus, the female caecilian exhibits parental care.
Three genera of caecilians are found in Kerala and Karnataka regions of South India. They are
1. Ichthyophis
2. Uraeotyphlus
3. Gegenophis
Distinctive features of Urodela or Caudata
(Gr. ura = Tail + delos = Visible) (L. cauda = tail)
1. These are scale less amphibians with a tail in the adult stage. Urodela
Comprises of salamanders and newts.
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2. Urodeles are discontinuous in distribution. They are mostly found in North America which is recognised as the head quarter of Urodela.
3. Urodeles have two pairs of short weak limbs which are more or less equal in size.
4. Eyes are small and degenerated in most of the urodeles.
5. Persistant external gills are found in some of the adult urodeles.
6. Teeth are found on both the jaws and vertebrae are amphicoelus type.
7. The larva of Amblystoma tigrinum (tiger salamander) is called Axolotl. The Axolotl larva remains in larval stage for a prolonged period and attains sexual maturity. This process is known as neoteny or paedogenesis.
8. The larva is aquatic and it won’t come out of water. So, it is called servant of water.
Eg-.Necturus, Proteus,Siren, Salamander, Amblystoma, Tylototriton
Distinctive features of Anura or Sailentia
(Gr. An = without + oura = tail) * (L. Saliens = leaping) (L.e = out + Cauda = tail)
1. These are scaie less amphibians without a tail and neck in adult stage. Anura is represented by frogs and toads which are cosmo­politan in distribution.
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2. Body is divisible into head and trunk.
3. Vertebrae are less in number (5-10) and are procoelus oramphicoeius elogated urostyle is present.
4. Fore limbs are short while hind limbs are long.
5. Tympanum is present. Well developed eyes with monocular vision, are found both in frogs and toads.
6. Gills are absent in the adult stage, but they are present in larval stage.
7. Fertilization is external and takes place in water.
8. The larval stage of Salientia is known as a Tadpole.
Eg: 1. Rana hexadactyla 2. Bufo melanosticus 3. Rhacophorus.
 
 

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