Introduction

Across earlier chapters you met a wide range of organisms — from single-celled forms to complex multicellular animals — and saw how they meet their basic needs. In a unicellular organism, one single cell has to carry out every activity by itself: digestion, respiration, reproduction, and everything else. A multicellular animal solves the same set of problems very differently. The very same basic functions are still needed, but instead of one cell doing it all, different groups of cells specialise and share the load. Even in a comparatively simple animal like Hydra, the body is built from several distinct types of cells, each type numbering in the thousands; the human body scales this up enormously, with billions of cells contributing to its many functions.

This raises a natural question: how do all these cells, working in different groups, actually cooperate as one functioning body? The answer begins with a basic organisational unit you have already met in earlier classes — when a group of structurally similar cells, together with the intercellular substance surrounding them, works together to perform one specific function, that organisation is called a tissue. Remarkably, however varied and complex an animal's body plan is, it is built from only four basic types of tissue. In the chapter ahead, you will see how these few tissue types combine, in different proportions and arrangements, to build organs and organ systems, and then look in detail at how this organisation plays out in the body of the frog.