320 ELEMENTS OF COMPARATIVE ZOO LOOT. 



these cells is microscopic in size, with an average diameter 

 of about ^ny of an inch ; and each consists of a semi-fluid 

 substance known as protoplasm, in the centre of which is a 

 mass of slightly different protoplasm, known as the 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 mus- 

 cular, 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 " branches" 

 which may be arranged in order of their complexity in the 

 following manner: 



BRANCH I. CCELENTERATA (p. 310). 



BRANCH II. SPONGIDA (p. 316). 



BRANCH III. VERMES (p. 242). 



BRANCH IV. MOLLUSCA (p. 269). 



BRANCH V. ARTHROPODA (p. 226). 



BRANCH VI. ECHINODERMA (p. 291). 



BRANCH VII. CHORD ATA (p. 153). 



