NEET Biology Animal Kingdom Notes - Class-Reptilia

Class-Reptilia

Class Reptilia.

(i) General Characters: Reptiles represent the first class of vertebrates fully adapted for life in dry places on land. They have no obvious diagnostic characteristics of their own that immediately separate them for other classes of vertebrates. The characters of reptiles are in fact a combination of characters that are found in fish and amphibians on one hand and in birds and mammals on the other. The class name refers to the mode of locomotion (L., repere or reptum, to creep or crawl), and the study of reptiles is called Herpetology (Gr., herpeton, reptiles).

(1) Predominantly terrestrial, creeping or burrowing, mostly carnivorous, air?breathing, cold?blooded, oviparous and tetrapodal vertebrates.

            (2) Body bilaterally symmetrical and divisible into 4 regions-head, neck, trunk and tail.

(3) Limbs 2 pairs, pentadactyle. Digits provided with horny claws. However, limbs absent in a few lizards and all snakes.

            (4) Exoskeleton of horny epidermal scales, shields, plates and scutes.

            (5) Skin dry, cornified and devoid of glands.

            (6) Mouth terminal. Jaws bear simple conical teeth. In turtles teeth replaced by horny breaks.

            (7) Alimentary canal terminates into a cloacal aperture.

(8) Endoskeleton bony. Skull with one occipital condyle (monocondylar). A characteristic T?shaped inter clavicle present.

(9) Heart usually 3?chambered, 4?chambered in crocodiles. Sinus venosus reduced. 2 systemic arches present. Red blood corpuscles oval and nucleated. Cold?blooded.

            (10) Respiration by lungs throughout life.

            (11) Kidney metanephric. Excretion uricotelic.

            (12) Brain with better development of cerebrum than in Amphibia. Cranial nerves 12 pairs.

            (13) Lateral line system absent. Jacobson's organs present in the roof of mouth.

            (14) Sexes separate. Male usually with a muscular copulatory organ.

(15) Fertilization internal. Mostly oviparous. Large yolky meroblastic eggs covered with leathery shells, always laid on land. Embryonic membranes (amnion, chorion, yolk sac and allantois) appear during development. No metamorphosis. Young resemble adults.

            (16) Parental care usually absent

(ii) Classification of Reptilia  : According to Bogert, there are more than 7,000 living and several extinct species of reptiles, grouped into approximately 16 orders of which only 4 are living.

            (a) Subclass I Anapsida ? Primitive reptiles with a solid skull roof. No temporal openings.

          Order 1. Chelonia or Testudinata : (Gr., chelone, turtle; L., testudo, turtle)

            (1) Body short, broad and oval.

            (2) Limbs clawed and or webbed, paddle-like.

(3) Body encased in a firm shell of dorsal carapace and ventral plastron, made of dermal bony plates. Thoracic vertebrae and ribs usually fused to carapace.

            (4) Skull anapsid, with a single nasal opening and without a parietal Foramen.  Quadrate is immovable.

            (5) No sternum is found.

            (6) Teeth absent. Jaws with horny sheaths.

            (7) Cloacal aperture a longitudinal slit.

            (8) Heart incompletely 4?chambered with a partly divided ventricle.

            (9) Copulatory organ single and simple.

            (10) About 400 species of marine turtles, freshwater terrapins and terrestrial tortoises.

            Examples ? Chelone, Chrysemys, Testudo, Trionyx, Dermochelys.

(b) Subclass II Euryapsida (extinct) : Skull with a single dorso?lateral temporal opening on either side bounded below by postorbital and squamosal bones.

(c) Subclass III Parapsida (extinct) : Skull with a single dorso?lateral temporal opening on either side bounded below by the supra temporal and post frontal bones.

(d) Subclass IV Synapsida (extinct) : Skull with a single lateral temporal opening on either side bounded above by the postorbital and squamosal bones.

(e) Subclass V Diapsida : Skull with two temporal openings on either side separated by the bar of postorbital and squamosal bones.

          Order 2. Rhynchocephalia : (L., rhynchos, snout + Gr., kephale, head)

            (1) Body small, elongated, lizard-like.

            (2) Skull diapsid. Parietal foramen with vestigeal pineal eye present. Quadrate is fixed.

            (3) Vertebrae amphicoelous or biconcave. Numerous abdominal ribs present.          

            (4) Teeth acrodont. Cloacal aperture transverse.

            (5) Heart incompletely 4?chambered.

            (6) No copulatory organ is male.

            Example ? Represented by a single living species, the ?tuatara? or sphenodon punctatum of New Zealand.

            Order 3. Squamata : (L., squama, scale or squamatus, scaly)

            (1) Advanced, small to medium, elongated.

            (2) Skull diapsid. Quadrate movable.

            (3) Vertebrae procoelous. Ribs single ? headed.

            (4) Heart incompletely 4?chambered.

            (5) Cloacal aperture is transverse.

            Order 4. Crocodilia : (G., krokodeilos, Crocodile)

            (1) Skin thick with scales bony plates and scutes.

            (2) Skull diapsid. Quadrate immovable. No parietal foramen. A pseudopalate present.

            (3) Ribs bicephalous. Abdominal ribs present.

            (4) Heart completely 4?chambered.

            (5) Cloacal aperture is a longitudinal slit.

            Examples ? Crocodylus, Gavialis, Alligator