156 SYSTEMATIC ZOOLOGY. 



it so as to bring out details we will discover another 

 fact of great importance. Every one of these animals 

 will be found to be made up of small parts or cells, essen- 

 tially like each other, just as the wall of a building is built 

 up of separate bricks. Each of these cells is micro- 

 scopic in size, with an average diameter of about -g-^Vo" f 

 an inch; and each consists of a semi-fluid protoplasm, in 

 the centre of which is a nucleus. Now, since each and 

 every metazoan is built up of cells, we 'may speak of the 

 Metazoa as many-celled animals. 



These cells vary greatly in shape, but no matter how 

 different they may appear at first sight, they all agree 

 with the description given in the last paragraph. Some 

 may be spherical, others cubical or flattened, and still 

 others branched, yet in all there is the same nucleus. 

 Cells of the same general shape are united together to 

 form tissues, so that we have bone-tissue made up of what 

 may be called bone-cells, muscular tissue of muscle-cells, 

 and nervous tissue of nerve-cells, etc. 



In the Metazoa the tissues are built up into organs for 

 the performance of certain purposes; and usually a single 

 organ is composed of several kinds of tissues, while the 

 same kind of tissue may reappear in different organs. 

 Thus the hand of man is an organ of grasping; in it we 

 find muscular, bony, connective, and nervous tissues; 

 while in the heart of the shark muscular, connective, and 

 nervous tissues appear. 



The Metazoa are subdivided into groups, or phyla, which 

 may be arranged in order of their complexity in the 

 following manner: 



PHYLUM I. SPONGIDA. 

 PHYLUM II. CCELENTERATA. 

 PHYLUM III. VERMES. 



