Class: Reptilia | Additional Study Material for NEET PDF Download

Introduction

• These were the first class of organisms to adapt to life on land. They are believed to have evolved from the amphibians millions of years ago. There are about 10000 different species of reptiles on earth. 

Class: Reptilia

• They are cold-blooded animals belonging to the phylum Chordata of Animal kingdom.
• The skull of the reptiles is modified that gives the reptiles an efficient and powerful jaw action. The modification also makes the skull light.

Characteristics of Class Reptilia

– Reptiles were originated during carboniferous period of palaeozoic era. Mesozoic era is Golden age of Reptile.

– Study of reptiles is known as ‘‘Herpetology’’.

– Members of this class are the first successful terrestrial animals.

– First reptiles are called Stem reptiles or Cotylosaurs.

– These are Cold blooded, Poikilothermal animals. Their body temperature varies according to climate.

– Mostly, terrestrial, but some are aquatic.

– Body is divided into head, neck, trunk and tail.

– Skin is dry, cornified, rough, nonglandular (Femoral gland in male lizard).

– Pentadactyle limbs. Each digit has curved claws.

– Some lizards and snakes do not have limbs e.g. Ophiosaurus lizard is a limbless lizard.

– Exoskeleton is made up of horny epidermal scales or bony scales or bony plates.

– A complete alimentary canal is found in these animals, which opens into cloaca.

– Teeth are acrodont, thecodont, pleurodont type.

–    Tounge is protrusible.

– Respiration is by lungs, but members of order Chelonia can respire through their cloaca, it is known as ‘‘Cloacal respiration’’

– Heart is incompletely four chambered, 2 complete auricles and two incomplete ventricles. Right and left both systemic arches are present.

– Ventricle of animals of order Crocodilia is completely divided into two, i.e. heart is four chambered in Crocodiles.

• External ear aperture is absent.

• Tympanum  represents external ear.

–   Tympanum is absent in snakes.          

– Sinus venosus is underdeveloped and trunkus arterious is absent. RBC are oval and nucleated.

– Bony endoskeleton.

– Skull is monocondylic

– True well developed sternum is present.

– Centrum are procoelous type. Sternal ribs are found in reptiles.

– A chevron bone is found in caudal vertebrae of these animals.

– One pair of Metanephric kidneys help in excretion.

– Members are uricotelic for water conservation.

– Brain is well developed 12 - pairs of cranial nerves are present.

– Lateral line system is absent.

–  At the roof of buccal cavity Jacobson's organ (olfactory) is present.

– Ureters, genital ducts and alimentary canal open into a single cloacal aperture.

– Unisexual animals. Fertilization is internal.

One or two penis (Hemipenis) is found in male as copulatory organ.

– Mostly oviparous, but some are viviparous also. Large amount of yolk in their eggs. i.e. eggs are Megalecithal & Telolecithal, Cleidoic & leathery eggs are present.

–  All the four embryonic membranes amnion, chorion and allantois present in the embryo. Yolksac is also attached with embryo. So this class is grouped under Amniota group.

– Cleavage is Discoidal, Meroblastic.

– Development direct i.e. larva stage is absent.

– Parental care is often marked.

– Class Reptilia is classified on the basis of presence or absence of temporal fossae in the temporal region of skull and their number.

Classification of Reptilia

Class reptilia is divided into following Major 2 sub-classes :-

Subclass – Anapsida

Order – Chelonia

– Temporal fossae are absent in the temporal region of the skull i.e. roof of skull is complete.

– Limbs are strong.

– Body is broad and oval.

– They are terrestrial, marine and fresh water animals.

– This order includes live and extinct both the animals.

• Whole body is covered by firm bony shell.

–    Exoskeleton of dorsal region of body is called carapace and skeleton of ventral region of body is called plastron.

– Horny, teethless, beak like jaws are found.

– Sternum is absent.

Bony plate present on the body are called osteoderms.

– Clawed digits. Web or membrane of found in between the digit for swimming.

– Scales are found on neck, limbs and tail.

All these three organs can be pushed into the carapace.

– Thoracic vertebrae and ribs are attached with carapace.

– Cloacal aperture is vertical and it helps in respiration. (cloacal respiration)

– Single copulatory organ is found in male animal.

– Animals are oviparous.

–    Testudo  -   Land tortoise

–    Trionyx  -    Fresh water terrapins (edible)

–    Chelone   -   Marine (tortoises) turtles

Order - Cotylosauria

e.g. Seymouria : Connecting link between

Sub-class - Diapsida

– One pair superior and one pair inferior temporal fossae are found in the temporal region of skull.

Sub-class Diapsida is divided into two super orders.

(1)  Lepidosauria

(2)  Archosauria

Super order Lepidosauria :-

This super order is divided into two orders.

Order - Rhynchocephalia :

– Most of the species of this order are found in the form of fossils.

• Body is small and lizard like.

–    Tail is laterally compressed.

– Limbs are pentadactyl and digits clawed.

– A functional third eye or pineal eye is found in the head.

– Vertebrae are amphicoelous type.

– Teeth are acrodont type.

– Penis or copulatory organ is absent in male.

– Long life span living 100 years

Sphenodon punctatum - Tuatara (living fossil) It is found only in NewZealand. Only living member.

Order - Squamata :

– Largest number of species of reptiles are found in this order in modern era. All the lizards & snakes are included in this order.

– One or two pairs of temporal fossae are found in temporal region of skull, which disappear after sometimes.

• Limbs clawed.

–    Some of the lizards and all snakes are limbless.   

– Scales are found on body.

– Vertebrae are procoelus type.

– Teeth are pleurodont, i.e. tooth is situated at lateral side of jaw bone.

– Copulatory organs are paired (hemipenis).

– Order squamata is divided into two suborders

Suborder  (i) Lacertilia ; Suborder (ii) Ophidia

Suborder  (i) Lacertilia :-

– Normally the members of this suborder is ‘‘Lizards’’

– Study of lizards is called ‘‘Saurology’’

– Girdles and pendactyle clawed limbs are well developed.

– Eyelids are movable and nictitating membrane is found in eye.

– Auditory aperture or auditory opening and tympanum is present.

– Autotomy (for breaking of tail) is found in lizards only.

– Urinary bladder is present.

– Foramen of panizzae is present in the heart of lizard.

– Both lungs are well developed.  

–    Hemidactylus       -      Common lizard, wall lizard. Power of regeneration is well marked.

–    Calotes       -         Blood sucker, Garden lizard/Girgit. it can change its colour according to environment.

–    Draco   -      Flying lizard. Its body skin expands in the form of 2 wings or patagia. With the help of these patagia, it can glide from one tree to another tree or its branches. It can not fly.

–    Chameleon   -      Arborial lizard (Viviparous) (Girgit)

–    Varanus      -      Goh, Monitor lizard. Varanus komodoensis - Ferocious Dragon - Largest living lizard.

–    Rhineura     -      Limb - less lizard.

–    Ophiosaurus    -      It is limbless lizard. It is also called glass - snake.

–    Anguis     -      limbless lizard

–    Burkudia    -      limbless lizard found in south India.

–    Heloderma   -      Gila - monster/Beaded lizard. It is the only poisonous lizard.  Its poison glands are modified sublingual glands (Maxico & USA). two sps (H. suspectum, H. horidema).

–    Mobuya    -      Viviparous lizard.

–    Phrynosoma   -      Horned toad (viviparous)

–    Uromastix     -      Sand lizard or Sanda

Suborder (ii) Ophidia :-

– Members commonly called ‘‘Snakes’’

– The study of snakes is called ophiology or serpentology.

– Body long, thin, smooth and limbless.

– Eyelids are immovable and nictitating membrane in eyes are absent.

– Girdles, sternum and urinary bladder absent.

– Auditory opening and tympanum absent. No middle ear.

– Tongue thin, long and bifid and sensitive to odour and vibration.

– Left lung is ill developed.

– Snake poison is proteinaceous and good digesting enzyme.

–    Python molurus -     Azgar. It is the largest snake, its length is about 25 feet. Rudiments of hind limbs are found on the body. It is non-poisonous snake.

–    Ptyas mucosus  -          Zamenis or Rat snake. It is commonly called Dhaman. It feeds on rats, so it is also called ‘‘Friend of farmers’’. It is a non-poisonous snake.

–    Eryx Johni        -         Sand boa. It is also called Dumuhi snake. It is a non-poisonous snake.

–    Typhlops      -      Blind snake. Non poisonous.

–    Hydrophis     -   Marine snake. It is a deadly poisonous snake. Its tail is laterally compressed. It is a viviparous snake.

–    Enhydrina      -      Sea snake (Poisonous)

–    Naja    -   Indian cobra. Poisonous snake. Its poison is neurotoxic cause paralysis of respiratory muscles.

–    Naja naja or N.hannah     -    King cobra, poisonous snake. It is the largest snake among poisonous snakes (Head with one or two circular mark).

–    Bungarus    -      Krait : Poisonous (neurotoxic) snake

–    Vipera      -     Viper snake : Head is differentiated from body. Poisonous snake (viviparous). Its venom is haemotoxic/Cardiotoxic Proteolysin is present and damages tissue Loreal pit is found which is a thermoreceptor. Largest viper is Russel viper (V mark on head).

–    Micrurus   -   Coral snake.

–    Crotalus   -   Rattle snake : It produces a characteristic rattling sound of ‘‘Rate - rate - rate’’, so it is called rattle snake. It is poisonous and ovoviviparous snake.

Characteristic features of poisonous snakes :-

– Small scales are found on head or hood.

– Laterally compressed tail is present in marine snake.

– Ventrally placed scales of the body are broad.

Two teeth mark is of poisonous snake. (V-shaped - Non poisonous)

– Poison glands of poisonous snakes are modified labial glands. Probably these glands are homologous to parotid salivary glands of Mammals.

– Poisonous teeth (fangs) are modified maxillary teeth.

– Treatment of poisonous snake bite is done by Antivenom dose. Antivenom is produced at

(i)   Central Research Institute Kasauli - Shimla

(ii)  Hoffkins Institute, Mumbai.

Biggest Serpentorium is located in India - Chennai

(2)  Super order - Archosauria

This super order is classified into

(i)   Order - Crocodilia

(ii)  Order - Saurischia

Order - Crocodilia

– Crocodiles, Alligator etc. are included in this order.

– Amphibious in nature.

– These are largest modern reptiles.  

– Skin is covered by lines of bony scutes.

– Snout is long. External nares are situated at the distal end of snout and nares have cover also.

– Diaphragm is present in between thorax and abdomen.

• Sternum and abdominal bifid ribs are present.

–    Heart is completely four chambered

–    Ventricle is completely divided into two chambers.

–    Teeth are thecodont type.

– Urinary bladder absent.

– A median erectile grooved penis is present in male animals.

–    Crocodiluss (Crocodile) - It is only found in Indian subcontinent.

–    Gavialis - Gharial. Snout very long.

• Alligator - Maxican crocodile.