Frog (Rana tigrina)
Why to study frog?
- To know ourselves & nature better.
Why frog as type?
- Easily available.
- Easy to procure & handle.
- Non-poisonous.
- Small size.
- Intermediate character of vertebrates.
Frog (Rana tigrina)
- Indian bull frog
- Size large.
- Croaking loud.
Classification
- Kingdom: Animalia
- Phylum: Chordata
- Class: Amphibia
- Species: tigrina
Difference between Frog and Toad
| Frog | Toad |
|---|---|
| Slippery, smooth, moist skin. | Rough, dry, warty skin. |
| Triangular jaw | Semicircular jaw. |
| Non-poisonous (poison glands few) | Poison glands are present; poisonous. |
| Vocal cord present in male frog. | Vocal cord absent. |
| Edible. | Non-edible. |
| Salivary (parotid) gland absent. | Parotid gland present. |
| Omastemum present. | Omastemum absent. |
| Teeth present only in upper jaw. | Teethless. |
Habitat
Amphibious (both land and water). Usually found in freshwater ponds, rivers, ditches, paddy field, under stones and in damp places.
[Death due to lack of oxygen: Asphyxiated death.]
They live near water sources for:
- Reproduction.
- Protection from predator.
- Cutaneous respiration.
Habits
- Resting: squatting posture. Hind limbs rest making Z-shape.
- Feeding: Tadpole → Herbivores (Aquatic plants). Adult frog → Carnivorous (Spider, Insect, Worms, Snails, or Cannibal). Generally capture alive prey in motion i.e. ignores motionless prey.
Locomotion
- Leaping / Jumping / Saltatorial → Faster
- Swimming → Webbed feet
- Walking (Diagonal walking) → slower
Breeding behaviour
- Breeds during spring season.
- Male frog croaks to attract female for copulation.
- Mating call
- Amplexus / Pseudocopulation occurs.
- Eggs fertilize in water.
Body Temperature
- Poikilothermic / Ectothermic.
- Hibernation: Winter sleep; Inactive phase / Minimal use of ATP
- Aestivation: Summer sleep
Body Division
Head and Trunk.
Head
- Triangular in shape; forms snout; bears mouth, external nostrils, eyes & tympanum.
Mouth: Terminal, semi-circular wide opening. Bounded by upper & lower jaws. Immovable lips.
Nostrils: Two in number; either side of snout; only respiratory in nature.
Eyes: Two, large, bulging (protruding), dorsolateral, on the top of head. (As eyes are large, they fulfill the absence of neck & frog can see all around without moving the body.) Protected by eyelid.
- Upper, thick, pigmented, immovable eyelid.
- Lower, thin, semi-transparent, movable eyelid.
- Third eyelid / Nictitating membrane present as an outgrowth of lower eyelid.
Tympanum: No external ears; behind & below the eye on either side of head; circular & obliquely placed.
Brown spot: Small lightly coloured spot just in front of eyes; mark third eye possessed by remote ancestors of frog.
Trunk
Forelimb (4 digits): Manus (Hand), Antebrachium. During breeding season, male frog develops "nuptial pads".
Hind limb (5 digits): Thigh → Femur; Shank → Crus; Foot → Pes. During resting position hind limbs are arranged in Z-shape.
Sexual Dimorphism
- Males are usually smaller than females.
- Male frogs have vocal sacs for croaking (mating call).
- Male frogs have copulatory pad on first inner finger of forelimb.
- (Vocal cord → Present in both.)
Skin
- Thin, smooth, slippery.
- Skin colour changes to match with surrounding → Metachrosis (Darker in dark places, lighter in light places.)
- 2 distinct layers: Epidermis and Dermis.
Epidermis
- Outer layer of skin; non-vascularised, nerve supply.
- Stratum corneum: outermost layer; ecdysis occurs; flat, dead, keratinised cells (4–5 layers).
- Stratum germinativum / Malpighian layer: one cell thick columnar cells; actively dividing cells.
Dermis
- Innermost layer of skin; connective tissue, muscle fibre, vascular nerve supply, glandular, chromatophores.
- Stratum spongiosum: loose & spongy, below basement membrane; cutaneous glands (mucous gland → mucus; poison gland → toxin).
- Melanophores: Dark (Brown, Black); Xanthophores: Yellowish; Guanophores: White/silvery.
- Intermedin and Epinephrine hormones regulate the pigmentation in the skin of frog.
- Stratum compactum: dense collagen fibre.
Largest organ in frog = Liver.
Digestive System
- Ingestion → Digestion (Polymer→Monomer) → Absorption → Assimilation → Egestion
- Alimentary canal: carnivores → shorter alimentary canal in comparison to herbivores. They have short AC as they are carnivores.
1. Mouth
- Wide, terminal, bounded by upper & lower jaws.
- Lower jaw movable (only upwards and downwards movement).
Buccopharyngeal cavity
- Lower jaw movable; upper jaw has teeth.
- Teeth prevent the escaping of prey. (Never does chewing.)
- Teeth inwardly directed: Maxillary teeth, Pre-maxillary teeth, Vomerine teeth.
Teeth: Homodont (all teeth alike); Acrodont (directly attached to jaw); Polyphyodont (throughout life). (In humans, heterodont, thecodont & diphyodont.)
Tongue: Bifid; large, muscular, sticky; taste papillae projections (taste bud absent); protrusive, retractile; protractor & retractor muscle.
- Gullet: Food opening → oesophagus → Stomach
- Glottis: Air opening
- Epiglottis, salivary gland absent but mucous glands present.
2. Oesophagus
- Short, less developed.
3. Stomach
Cardiac and pyloric parts. Glands are evenly distributed in cardiac and pyloric part but in human glands are major in pyloric part.
Gastric gland:
- Oxyntic cell: HCl
- Chief / Peptic / Zymogen cells: Pepsinogen (inactive form)
- Mucous gland: Mucus.
Function of stomach is storage of food & churning of food.
- HCl: acidic; activates pepsinogen; kills micro-organisms; dissolves hard substance.
- Pepsin acts on protein.
- The food is now called chyme. (Proteoses + Peptones)
In human right lobe is larger but in frog, left lobe is larger. In human gall bladder present in quadrate lobe while in frog, it is present in median lobe.
4. Small Intestine
- Duodenum and Ileum.
- Duodenum: U shaped → Brunner's gland absent. Receives hepatopancreatic duct (Bile juice + Pancreatic juice). Bile salts: emulsification of fat; neutralizes acidity of food. Absence of Ampulla of Vater and Sphincter of Oddi.
- Pancreatic juice: Trypsinogen (Inactive) → Enterokinase → Trypsin; Amylopsin / pancreatic amylase (digests carbohydrates); Lipase/steapsin (digests lipid/fat).
- Protein \(\xrightarrow{Trypsin}\) Amino acid; carbohydrate Amylase → Glucose; Fats \(\xrightarrow{Lipase/steapsin}\) Fatty acids and glycerol.
- (No crypts of Lieberkuhn but succus entericus is secreted by intestinal glands.)
- Ileum: Maximum absorption; complete digestion; intestinal folds called villi for increasing absorptive surface area.
5. Rectum (large intestine)
- Ileum → Rectum.
- Storage of undigested food.
- Absorption of water, salts/ions.
6. Cloaca
- Common opening of reproductive system, excretory system and digestive system.
- Rectum opens into cloaca.
- Anal sphincter present.
Liver and Pancreas
| Liver | Pancreas |
|---|---|
| Largest organ/largest gland in frog. 3 lobes: Right, Median & Left (Left lobe larger). | 2nd largest gland. Mixed gland: Endocrine (Islets of Langerhans — Insulin, Glucagon) and Exocrine (Acini). |
| Bile secretion (no enzymes). | Pancreatic secretion (enzymes). |
| Glisson's capsule absent. |
Respiration in frog
- Cutaneous respiration
- Buccopharyngeal respiration
- Pulmonary respiration
Tadpole → Branchial respiration → Gills (3 pairs of external gills; 4 pairs of internal gills).
Positive and Negative Pressure Lungs
| Positive Pressure Lung | Negative Pressure Lung |
|---|---|
| Pleural cavity pressure > atmospheric pressure (In frog). | PC < atmospheric pressure (In human). |
1) Cutaneous respiration
- Respiration through moist skin/integument by simple diffusion; highly vascular (dermis).
- Skin of frog is slimy, moist, thin which aids in cutaneous respiration. Mucous glands of dermis; dermis of frog acts as alveoli of mammals.
- It takes place throughout the life even in hibernation and aestivation (~50% respiration).
- Absence of cutaneous respiration leads to death. Waxing/drying of skin prevents cutaneous respiration leading to death of frog known as "asphyxiated death".
2) Bucco-pharyngeal respiration
Respiration takes place through the moist mucous membrane lining of the bucco-pharyngeal cavity. Mouth, gullet and glottis are kept closed while nares are open. Lungs do not take part in it.
Mechanism: Mouth along with glottis are closed → Floor of buccal cavity is lowered by contraction of sternohyoid muscle → Air rushes into the buccal cavity (Inspiration) → Exchange of gases takes place through moist mucus membrane → Floor of buccal cavity is raised by contraction of petrohyal muscles (Expiration) → Air passes out.
3) Pulmonary respiration
- Respiration through lungs; occurs when demand of oxygen is high.
Mechanism:
- A: Nostrils open; sternohyoid muscles contract; buccal floor lowered; enlarged buccal cavity filled with fresh air; glottis closed; lungs empty.
- B: Nostrils closed; petrohyal muscles contract; buccal floor raised; buccal cavity reduced; glottis open; enlarged lungs filled with air.
- C: Nostrils closed; sternohyoid muscles contract; buccal floor lowered; foul air comes into buccal cavity; glottis open; lungs emptied.
- D: Nostrils open; petrohyal muscles contract; buccal floor raised; foul air released through open nostrils; glottis closed; lungs contracted.
A & B: Inspiration. C & D: Expiration.
Note: During contraction of sternohyoid muscle, petrohyal muscles relax and vice versa. Petrohyal — UP (raises buccal floor.)
Laryngo-tracheal chamber
- Larynx and trachea fused.
- Cartilages present: 2 Arytenoid, 1 Cricoid.
- Thyroid cartilage absent in frog.
Vocal cord present in both male and female frog. Vocal sac/resonators present only in male frog.
Circulatory System of Frog
- Blood Vascular System
- Lymphatic System
- Blood vessels → Lymph vessels; heart → Lymph heart