Organs, organ systems and the frog's body

In a many-celled animal, similar cells, together with the material between them (the intercellular matrix), form tissues. Animals have only four basic types: epithelial (coverings and linings), connective (support and binding), muscular and neural. Tissues group into organs (heart, stomach, lungs, kidney), and organs into organ systems. Each part has its own job: this is division of labour.

Morphology is the study of an animal's outer form; anatomy is the study of its inner organs. The book's example is the common Indian frog, Rana tigrina, an amphibian and a chordate.

Frogs are cold-blooded (poikilotherms): their body temperature follows the surroundings. They can change colour to blend with their background (camouflage). To escape extreme heat and cold they go into deep burrows and sleep: aestivation is summer sleep and hibernation is winter sleep.

A frog's skin is smooth and slippery with mucus, and always moist. A frog never drinks water: it absorbs water through its skin. The back (dorsal side) is olive green with dark spots; the belly (ventral side) is pale yellow.

The body has only a head and a trunk: no neck and no tail. The eyes bulge out and are covered by a nictitating membrane, a thin see-through eyelid that protects the eyes in water, like natural swimming goggles. On each side of the head is a round eardrum, the tympanum. The forelimbs end in four digits; the larger hind limbs end in five webbed digits, for swimming and jumping.

Males and females look different (sexual dimorphism). Males have vocal sacs, which make their croaking loud in the breeding season, and a copulatory pad on the first digit of each forelimb. Females have neither.