Organ and Organ Systems in Animals

Every animal body, however small or large, is built the same fundamental way: cells group into tissues, tissues combine into organs, and organs in turn associate into organ systems. This layered organisation is essential for the efficient, well-coordinated activity of the millions of cells that make up an organism.

This chapter studies that principle concretely through three animals placed at different points on the evolutionary ladder: the earthworm and the cockroach — two invertebrates with very different body plans, a soft segmented worm and a hard-shelled, jointed-legged insect — and the frog, a vertebrate. For each animal the chapter looks first at its morphology, the study of its externally visible form, and then its anatomy, the study of its internal organs and organ systems.