Summary

In a multicellular animal, no single cell does everything -- work is parcelled out among cells, tissues, organs and organ systems, and this division of labour is what lets the whole body keep functioning efficiently. A tissue, at its core, is simply a cluster of similar cells (plus whatever non-cellular material sits between them) cooperating on a shared job. Epithelial tissue is the covering/lining type -- it sheets over the body's outer surface and lines internal cavities, ducts and tubes, always with one face open to a fluid or the outside environment, and its cells are locked together edge-to-edge by junctions so the sheet holds its integrity.

The Indian bullfrog (Rana tigrina) is the frog species most commonly studied as the representative amphibian body plan. A moist, glandular skin -- richly supplied with blood vessels -- lets it breathe partly through the skin itself, in addition to lungs, so it can respire both underwater and on land. Its body plan splits into a head and a trunk, with a forked, muscular tongue anchored at the front of the mouth for snapping up prey. Food travels through the oesophagus, stomach, intestine and rectum before reaching the cloaca, a shared terminal chamber; the liver and pancreas are the two main accessory digestive glands feeding into this tract. …