As members of the vertebrate subphylum, humans are generally more interested in the biology of vertebrates than in the biology of any other group of organisms. With this chapter, we begin our detailed examination of vertebrate biology. We start with how vertebrates are put together. In the following chapters we will see how the various parts carry out the myriad of vertebrate life processes. But first, the foundation: each vertebrate body is made up of a variety of organs and organ systems that carry out different life processes such as digestion, respiration, and reproduction. Organs are made up of groups of specialized cells that have similar structures and functions. These groups of cells are called tissues. Although there are between 50 and several hundred types of cells, and there are subdivisions within each tissue type, vertebrates have just four basic kinds of tissue: epithelium, connective, muscle, and nerve.