REPTILIA: REPTILES CHARACTERISTICS-CLASSIFICATION

Reptiles are the first vertebrates adapted completely for terrestrial life. They are advanced than amphibians. The reptiles lay eggs on land. The embryo is protected by embryonic membranes. The development in reptiles takes place outside the aquatic medium.

 reptiles

 

Reptiles are poikilothermic or ectothermic and lung breathing tetrapod vertebrates.

The reptiles arose in Carboniferous period from Labyrinthodont amphibians and they virtually dominated during the Mesozoic era. The giant dinosaurs like Plesosaurus, Pterosaurus, Tyrannosaurus, Ichthyosaurus, Brontosaurus, Gigantosaurus lived during Mesozoic era- Hence, the Mesozoic era is called the Golden age of reptiles. Most of the reptiles became extinct in late Cretaceous.

Reptiles are more common in warmer regions of the world and some are totally absent in Arctic and Antarctic regions. They generally creep on their belly (repere = creep), hence the name reptiles. At present class Reptiliaincludes turtles, tortoises, terrapins, lizards, snakes, crocodiles and sphenodon.

The study of reptiles is known as Herpetology.

REPTILES GENERAL CHARACTERISTICS

1. Reptiles are cold blooded or poikilothermous or ectothermal vertebrates i.e. the body temperature is variable according to the environmental changes.

2. The skin is dry and skin glands are absent.

3. Body is covered with horny epidermal scales or scutes. This is an adaptation for terrestrial mode of life.

4. Body is divided into four regions namely head, neck, trunk and tail.

5. Two pairs of Pentadactyle limbs are usually present. Limbs are modified into Paddles in aquatic reptiles, but are lost secondarily in snakes.

6. Tympanum, middle and internal ears are present. But in snakes, only internal ear is present.

7. Skeleton is completely ossified. Skull is with a single occipital condyle (monocondylic).

8. True sternum is present except in snakes and chelonians. Ribs are attached to sternum.

9. Vertebrae are procoelous. Vertebral column is divided into cervical, thoracic, lumbar, sacral and caudal regions.

10. The temporal region of skull bears temporal fossae and arcades. The lower jaw is complex and made up of six bones on each side.

11 . Tibia-fibula are free, the pectoral girdle has an T-shaped interclavicle.

12. Teeth are found in both the jaws (except in Chelonia). The digestive system is well developed with digestive gland.

The rectum is divided into three parts namely coprodaeum, urodaeum and proctodaeum.

13. Respiration is affected by a pair of lungs (pulmonary) cloacal respiration is found in some turtles; functional gills are never present. Lungs are asymmetrical in snakes.

14. Heart is three chambered with two auricles and partially divided ventricle (in crocodiles the ventricle is completely divided).

15. Red blood corpuscles are biconvex, oval and nucleated.-

16. Both hepatic and renal portal systems are well developed.

17. Twelve pairs of cranial nerves are present.

18. Excretion takes place by one pair of metanephric kidneys. Mesonephros is the functional kidney in the embryonic stage. Kidneys lack nephrostomes; nitrogenous excretory waste is chiefly in the form of solid uric acid (uricotelic). Urinary bladder is absent in some lizards, crocodiles and snakes.

19. All are unisexual. Males possess copulatory organs except Sphenodon.

20. Fertilization is internal; mostly oviparous (except some snakes and lizards).

21. Eggs are large megalecithal with abundant yolk; eggs are laid on land and they are protected by calcareous shell (cleidoic). As the yolk is situated towards one side of the egg, the reptilian eggs are described as telolecithal.

22. Cleavage or segmentation is meroblastic (incomplete or partial).

23. The embryo during development is protected by extra-embryonic membranes, viz; amnion, allantois, chorion and yolk sac.

24. Metamorphosis is absent i.e. the young ones resemble the adults (development is direct).

CLASSIFICATION OF REPTILES

The Living Reptiles Are Classified Into Four Orders

1. SQUAMATA

2. RHYNCOCEPHALIAH

 

3.CROCODILIAreptiles-classification

4. CHELONIA

DISTINCTIVE CHARACTERS OF SQUAMATA

1. It is the largest order of living reptiles. It includes lizards and snakes.

2. The body is covered with horny epidermal scales.

3. Temporal fossae in the skull become secondarily reduced to one pair in lizards and they are absent in snakes.

4. Vertebrae are procoelous.

5. Teeth are homodont and pleurodont.

6. Cloacal opening is transverse. Males possess a pair of eversible copulatory organs (hemipenis).

This order is divided into two sub orders.

Sub-Order 1. Lacertilia

1. It includes lizards and these are the most successful reptiles.

2. In majority, both fore and hind limbs are present, bearing clawed digits (Ophiosaurus and Barkudia are limbless).

3. Eye lids are movable. Tympanum is present.

4. A T- shaped interclavicle is present in the pectoral girdle.

5. Quadrate is immovable.

6. Ductus caroticus is present. (A small blood vessel connect­ing systemic and carotid arches of one side).

calotes-reptiles

Eg: Varanus, Calotes, Hemidactylus (wall lizard), Draco (flying lizard), Chamaeleon, Heloderma.

draco-flying lizard

draco-flying lizard1

 

chamelion

Sub-Order 2 : Ophidia

1 . Snakes belong to this sub order.

2. Body is covered with scales. Skin is moulted several times. There are 3000 species of snakes in this world out of which 300 species of snakes are poisonous. Snakes are present allover the world except in Newzealand, Ireland.

3. Both supra and infra temporal fossae are lost secondarily.

4. Appendages and girdles are absent. But in pythons and Boa,

rudiments of pelvic girdle and hind limbs present.

5. Limbs, sternum, tympanum and urinary bladder are absent.

6. The rami or mandibles are united by a ligament.

7. Teeth are pleurodonttype. (Fangs are modified maxillary teeth)

8. Snakes cannot hear air borne sounds and they can receive the sound vibrations through skin.

9. Eye lids are immovable and tongue is bifid and protrusible.

10. Lungs, kidneys and gonads are asymmetrical (left side organs are reduced).

11. Ribs are elongated and present on all pre caudal vertebrae. The movement in snakes is aided by the ventral ends of ribs and scales. Snakes may be said to crawl on the tips of ribs.

12. Pineal body is absent in the brain.

13. Majority of snakes are oviparous while few are viviparous (sea snakes and vipers).

14. Males are provided with double penis called hemipenis
15. Jacobson organs act as olfactory organs.

II. DISTINCTIVE CHARACTERS OF RHYNCOCEPHALIAH

(L. Rhynco – snout; Gr. Kephale – head)

1. The only living representative of this order is Sphenodon or Hatteria. It is found only in New Zealand. It is also known as Tautara lizard.

sphenodon

2. It looks like a lizard. The skin is scaly and scales on the mid-dorsal line form a crest. The skin is olive green.

3. Skull is diapsid and vertebrae are amphicoelous.

4. Pro atlas is present between skull and atlas as in crocodiles.

5. There is a well developed penial or parietal eye on the head and it is sensitive to light.

6. Sternum and abdominal ribs are also present; ribs are single headed and they bear uncinate process.

7. Copulatory organ is absent. Urinary bladder present.

8. Cloacal opening is transverse.

9. Sphenodon is considered as a living fossil, as it has been living since long time without any change.

10.It is an insectivorous, carnivorous and nocturnal reptile.

III. DISTINCTIVE CHARACTERS OF CROCODILIA

1. It includes Crocodiles, and alligators.

crocodilus

2. These are the largest living fresh water reptiles.

3. Tail is long and laterally compressed, which helps in swimming.

4. Body is covered by bony plates (scutes) which are overlapped by horny epidermal plates.

5. Teeth are thecodont and jaws are elongated.

6. The skull is of typical diapsid type (with two temporal fossae); there is a false palate in the skull.

7. Thoracic ribs possess uncinate processes as in birds and sphenodon.

8. Vertebrae are procoelous. Pro-altas is present.

9. Fore limbs and hind limbs are short. Fore limb bears five fingers and hind limb four toes.

10. Heart is four chambered as the ventricle is completely divided into two by a septum. There is an aperture at the point of contact of two systemic arches. It is called foramen of panizza.

11. The thoracic cavity is separated from the abdominal cavity by a membranous transverse partition, called the diaphragm.

12. Urinary bladder is absent. Cloacal aperture is longitudinal.

13. Single copulatory organ is present.

Eg : Crocodilus, Alligator,

IV. DISTINCTIVE CHARACTERS OF CHELONIA

1. They are primitive living reptiles. It includes marine turtles, terrestrial tortoises and fresh waterterrapins.

2. Limbs are paddle-like in turtles. Some times they bear claws.

3. Trunk is enclosed in a bony shell composed of dorsal carapace and a ventral plastron and the body can be retracted inside the shell.

4. Temporal fossae are absent in the skull. So, the skull is called anapsid.

5. The jaws are devoid of teeth (edentulous).

6. Single nasal opening is present.

7. The pulmonary and systemic trunks on each side are con­nected by ductus botalli.

8. Cloacal aperture is longitudinal. Copulatory organ is single. In Males, the tail is some what longer.

9. All are oviparous.

10. All are herbivorous except some marine turtles.

trionyx-turtle-tortoise

Eg : Dermochelis, Chelone mydas (Green turtle) Testudo (Ter­restrial) Trionyx (fresh water turtle).

1 comment:

  1. Hi can I ask for permission to use one of your labelled images for Calotes?

    ReplyDelete