74 



COLLEGE ZOOLOGY 



cells, and that when the germ-cells become mature they separate 

 from the body, giving rise to a new generation, whereas the 

 somatic cells die. 



2. Tissues 



The somatic or body cells of the Metazoa are of various kinds, 

 and are grouped together into tissues. A tissue is an association 

 of similar cells originating from a particular part of the embryo 

 and with special functions to perform. Some of the simple 

 Metazqa possess only two kinds of tissue; others are made up 

 of a great number. The many different kinds of tissues may be 

 classified according to their structure and functions into four 

 groups. 



(i) Epithelial tissue (Fig. 46, C) consists of cells which cover 

 all the surfaces of the body both without and within. In the 

 simpler animals this is the only kind of tissue present. In the 

 more complex animals epithelial cells become variously modi- 

 fied because they are the means of communication between the 

 organism and its environment ; nutritive material passes through 

 them into the body, and excretory products pass through them 

 on their way out of the body; they also contain the end organs 

 of the sensory apparatus, and protect the body from physical 

 contact with the outside world. In man the cuticle and the 

 lining of the alimentary canal are examples of epithelium. 



(2) Supporting and Connective Tissues (Fig. 46, D) may be 

 encountered in almost any part of the body. Their chief func- 

 tions are (a) to bind together various parts of the body, and 

 (b) to form rigid structures capable of resisting shocks and pres- 

 sures of all kinds. These tissues consist largely of non-living 

 substances, fibers, plates, and masses produced by the cells 

 either within the cell wall or outside of it. The tendons which 

 unite muscles to bones, and the bones and cartilage, illustrate 

 the two kinds of tissue in this group. 



(3) Muscular tissues (Fig. 46, E, F) are the agents of active 

 movement. In certain Protozoa there are contractile fibrils 



