Unit 4: Amphibia and Reptilia
Table of Contents
1. Class Amphibia
Amphibians (from Greek: amphi = double, bios = life) are the first vertebrates to transition from water to land, but they are not fully terrestrial. They lead a "double life," typically breeding in water and living on land.
1.1 General Characteristics and Classification
General Characteristics
- Habitat: Require moist environments; found in freshwater and on damp land.
- Skin: Smooth, moist, glandular, and scaleless. It serves as a major respiratory organ (cutaneous respiration).
- Respiration: By gills (in larvae), lungs (in adults), skin, and the lining of the mouth (buccal respiration).
- Heart: Three-chambered (two atria, one ventricle), leading to mixed (oxygenated and deoxygenated) blood.
- Reproduction: Fertilization is usually external. Eggs are anamniotic (lack a protective shell and membranes) and must be laid in water.
- Development: Indirect, with a free-swimming aquatic larval stage (e.g., tadpole) that undergoes metamorphosis.
- Thermoregulation: Poikilothermic (cold-blooded).
Classification (Up to Order)
| Order | Common Name | Key Characteristics | Examples |
|---|---|---|---|
| Apoda (Gymnophiona) | Caecilians | Limbless, worm-like, burrowing amphibians. Skin has dermal scales embedded. | Ichthyophis |
| Urodela (Caudata) | Salamanders & Newts | Body with a long tail. Two pairs of well-developed limbs. | Salamandra, Axolotl (shows neoteny) |
| Anura (Salientia) | Frogs & Toads | Body short, tailless (in adults). Hind limbs are long and adapted for jumping. | Rana (Frog), Bufo (Toad) |
1.2 Parental Care in Amphibians
Because their eggs are vulnerable, many amphibians have evolved complex ways to protect them.
- Nest Building: Frogs like Rhacophorus build foam nests hanging over water, so the tadpoles can drop in upon hatching.
- Guarding Eggs: Many frogs and salamanders (e.g., male Green Frog) will guard their egg clusters.
- Carrying Eggs on Body:
- The male Midwife Toad (Alytes) wraps the egg strings around his hind legs and carries them until they are ready to hatch.
- The female Surinam Toad (Pipa pipa) carries her eggs embedded in pits on her back.
- Brood Pouches: The male Darwin's Frog (Rhinoderma) broods the young inside his vocal sacs.
- Viviparity: Some caecilians and salamanders are viviparous, giving birth to live young.
2. Class Reptilia
Reptiles (from Latin: repere = to creep) are the first truly terrestrial vertebrates. Their key innovation is the amniotic egg, which allowed them to break their dependence on water for reproduction.
2.1 General Characteristics of Reptiles
- Skin: Dry, scaly skin (epidermal scales) that prevents water loss.
- Reproduction: Fertilization is internal. They lay amniotic eggs with a protective leathery shell and four extraembryonic membranes (amnion, chorion, yolk sac, allantois).
- Respiration: Exclusively by lungs.
- Heart: Incompletely four-chambered (two atria, one partially divided ventricle). Crocodiles have a fully four-chambered heart.
- Thermoregulation: Poikilothermic (cold-blooded), but many use behavior (like basking) to regulate body temperature.
- Skeletal System: Well-ossified skeleton.
2.2 Sphenodon (Tuatara): Characteristics, Distribution, and Affinities
Sphenodon is a unique reptile and a classic "living fossil." It is the only surviving member of the Order Rhynchocephalia, which flourished alongside dinosaurs.
Characteristics
- Lizard-like appearance but is not a true lizard.
- Possesses a well-developed parietal eye ("third eye") on top of its head, which is light-sensitive.
- Teeth are acrodont (fused to the jawbone) and arranged in two rows on the upper jaw and one row on the lower jaw.
- Lacks external ear openings.
Distribution
It is found only on a few remote, protected islands off the coast of New Zealand. It is highly endangered.
Affinities (Evolutionary Significance)
Sphenodon is called a living fossil because it retains many primitive reptilian features that have been lost in modern reptiles like lizards and snakes. It shows affinities with:
- Lizards (Superficial): General body form.
- Crocodiles & Turtles: Primitive skull structure (diapsid skull).
- Amphibians (Primitive): Certain features of the vertebrae and skull.
- Its existence provides a living window into the anatomy of reptiles from over 200 million years ago.
2.3 Difference between Poisonous and Non-poisonous Snakes
This is a critical identification skill. While exceptions exist, the following general rules are helpful:
| Feature | Poisonous Snakes (e.g., Cobra, Viper) | Non-poisonous Snakes (e.g., Rat Snake, Python) |
|---|---|---|
| Fangs | Have large, specialized fangs (maxillary teeth) at the front of the upper jaw to inject venom. | Lack fangs. Have many small, solid teeth of uniform size. |
| Head Shape | Often (but not always) triangular, due to large venom glands (e.g., Vipers). | Usually narrow and elongated. |
| Pupil of Eye | Often vertical or elliptical (e.g., Vipers). *Exception: Cobras have round pupils.* | Usually round. |
| Pit Organ | Pit Vipers have a heat-sensing loreal pit between the eye and nostril. | Absent. |
| Ventral Scales | In many (like vipers), the scales on the belly are small and do not cover the full width. | Often have large, transverse scales (ventral plates) that cover the full width of the belly. |
Important Exception: The "round pupil, narrow head" rule is unreliable. The Coral Snake is highly venomous but has a narrow head and round pupils, mimicking non-poisonous snakes.
2.4 Poison Apparatus and Biting Mechanism in Snakes
Poison Apparatus
The venom apparatus in a poisonous snake consists of:
- Poison Glands: A pair of modified salivary glands (parotid glands) located on the roof of the mouth, behind the eyes.
- Poison Ducts: Tubes that carry venom from the glands to the fangs.
- Fangs: Enlarged, hollow or grooved teeth on the maxillary bones.
- Proteroglyphous: Short, fixed fangs at the front (e.g., Cobra, Krait).
- Solenoglyphous: Long, hollow, erectile fangs that fold back when the mouth is closed (e.g., Vipers).
- Muscles: Compressor muscles that squeeze the glands to eject venom.
Biting Mechanism (in a Viper)
- Strike: The snake launches its head forward.
- Mouth Opening: The lower jaw drops, and a special set of bones (quadrate, pterygoid) pushes the maxilla forward.
- Fang Erection: This pushing action rotates the maxillary bone, causing the folded fangs to swing forward and lock into an erect position.
- Penetration & Injection: The fangs stab the prey. Muscles around the poison gland contract, squeezing venom through the hollow fangs like a hypodermic needle.
- Release: The snake retracts its fangs, the bones pull back, and the fangs fold back into the mouth. This entire process is incredibly fast.