Fig. 2.1. Early students of the cell and some of 

 their instruments. A, Matthias Jacob Schleiden, 

 1804-1881. B, Theodor Schwann, 1810-1882. 

 (A and B, from W. A. Locy, Bioloay and Its 

 Makers, copyriajht 1908 by Henry Holt and Co.) 



CHAPTER 



2 



STRUCTURE 



Animals may be large or small, live in water or on land, lay eggs or give 

 birth to their young. Some have backbones as man has and are known as 

 vertebrates; others, lacking such a support, are called invertebrates. There 

 are more than a million kinds of animals but all of them have many common 

 features. We shall find, for example, that every animal is composed of one or 

 more cells, as the units of structure in living organisms are called; there are 

 unicellular animals and multicellular animals. In the multicellular animals, 

 the cells are not all alike. Instead, there are a number of different kinds of 

 cells, each type occurring in a group known as a tissue; the details of cells 

 and tissues are not visible to the unaided eye. The tissues are further grouped 

 to form organs which can be separated by dissection. 



Animals, whether single-celled or many-celled, must obtain food and oxy- 

 gen from their surroundings, or environment, in order to live. With very few 

 exceptions, animals cannot make their food. Each individual must break 

 down, or digest, food which he has obtained from some other animal or plant 



10 



