Why a Frog Never Needs to Drink Water

The frog lives both in water and on land, and its body shows every major organ system of a vertebrate in a simple, easy-to-follow form. That makes it an ideal model for understanding how an animal's body is organised.

This chapter covers division of labour, the frog's external features, its digestive and respiratory systems, and its circulation, excretion, nervous system and reproduction.

In multicellular animals, cells specialise for particular jobs, similar cells form tissues, tissues combine into organs, and organs work together as organ systems — each level dividing the work of staying alive into more specialised parts.

The levels:

- Cells — specialised units, such as red blood cells that carry oxygen and neurons that carry signals
- Tissues — groups of similar cells: epithelial, connective, muscular and neural tissue
- Organs — tissues arranged for a function, such as the stomach or heart
- Organ systems — organs working together, such as the digestive or circulatory system

An everyday example. A busy restaurant kitchen splits work among people who chop, cook and serve — no one does everything, but together they deliver a meal.

The substance. Specialised cells depend on one another — a neuron cannot digest food, so the whole body must cooperate.

**The Indian frog, Rana tigrina, has a head and trunk without neck or tail, moist mucus-covered skin, bulging eyes protected by a nictitating membrane, a tympanum for hearing and powerful webbed hind limbs, and males differ from females by vocal sacs and copulatory pads.

Skin. Smooth and slippery with mucus; olive green with dark spots above and pale yellow below. Frogs can change colour to blend with surroundings, and they absorb water through the skin instead of drinking.

Head and senses.

- Eyes bulge out and are covered by a nictitating membrane that protects them in water
- A membranous tympanum on each side receives sound

Limbs. Forelimbs have four digits and hind limbs five; the hind limbs are larger, muscular and webbed for swimming and leaping.

Temperature and seasons. Frogs are cold-blooded; they avoid extreme heat by aestivation (summer sleep) and extreme cold by hibernation (winter sleep).

Sexual dimorphism. Males have vocal sacs and a copulatory pad on the first digit of the forelimb; females lack both.

An everyday example. The loud croaking after the first monsoon showers comes from male frogs, amplified by their vocal sacs.

The substance. A frog's moist skin is essential**, since it is used both to absorb water and to breathe.

Food moves from the mouth through the buccal cavity, pharynx, oesophagus, stomach, intestine and rectum to the cloaca, helped by bile from the liver and juices from the pancreas; the frog breathes through its skin in water (cutaneous respiration) and through its skin, buccal cavity and lungs on land.

Digestive system:

- A bilobed tongue flicks out to catch insects
- The alimentary canal is short, as frogs are carnivores
- Stomach — hydrochloric acid and gastric juices partly digest food into chyme
- Duodenum — bile from the gall bladder emulsifies fat; pancreatic juice digests carbohydrates and proteins
- Intestine — final digestion and absorption through villi and microvilli
- Undigested waste leaves through the cloaca

Respiration:

- Cutaneous — through moist skin, in water and during aestivation and hibernation
- Buccal — through the lining of the mouth cavity on land
- Pulmonary — through a pair of pink, sac-like lungs in the upper trunk

Worked example — which mode? A frog buried in dry mud during summer breathes only by cutaneous respiration; a frog sitting on a rock breathes by buccal and pulmonary respiration as well.

An everyday example. The throat of a resting frog pulses rhythmically as it pumps air in and out of its mouth and lungs.

The substance. The frog's short gut matches its insect diet — plant-eaters need much longer guts.

The frog has a closed circulatory system with a three-chambered heart, a pair of kidneys that excrete urea, a nervous system of brain, spinal cord and nerves, and separate sexes with external fertilisation in water and a tadpole stage in its life cycle.

Circulatory system:

- Heart — two atria and one ventricle, covered by a pericardium
- Sinus venosus joins the right atrium; the ventricle opens into the conus arteriosus
- Hepatic portal and renal portal systems carry blood to the liver and kidneys
- Red blood cells are nucleated; a lymphatic system is also present

Excretory system. A pair of dark red kidneys, ureters, a urinary bladder and the cloaca. Frogs are ureotelic — they excrete urea. In males, the ureters also carry sperm as a urinogenital duct.

Nervous system. Brain with forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes) and hindbrain (cerebellum, medulla oblongata); spinal cord; cranial and spinal nerves; sense organs for touch, taste, smell, sight and hearing.

Reproductive system and life cycle. Males have a pair of yellowish testes attached to the kidneys; females have ovaries with separate oviducts opening into the cloaca. Fertilisation is external in water; eggs hatch into tadpoles with gills and a tail, which undergo metamorphosis into adults.

An everyday example. Wriggling tadpoles in rain-filled ponds and paddy fields are young frogs before metamorphosis.

The substance. With a single ventricle, oxygen-rich and oxygen-poor blood partly mix, unlike in the four-chambered hearts of birds and mammals.