

AN INTRODUCTION TO THE METAZOA iol 



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 lining of the 

 alimentary canal are examples of epithelium. 



(2) Supporting and Connective Tissues (Fig. 48 C) may be en- 

 countered in almost any part of the body. Their chief functions 

 are two in number; (i) they bind together various parts of the 

 body, and (2) they form rigid structures capable of resisting 

 shocks and pressures 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 themselves, illus- 

 trate the two kinds of tissues in this group. 



(3) Muscular tissues (Fig. 48 D, E) are the agents of active 

 movement. We found (Chap. IV) that the locomotion of Ameba 

 could be explained by the presence of contractile fibers. In other 

 Protozoons belonging to the Classes Infusoria and Sporozoa 

 there are muscular fibrils called myonemes in the membranous 

 coverings. In most of the higher organisms special muscle cells 

 are differentiated for performing the various movements of the 

 body. These cells possess muscle fibrils which are able to con- 

 tract with great force and in quick succession. The fibrils are 

 usually of two kinds; (a) cross striated (D) and (b) smooth non- 

 striated (E). The latter form a less highly developed tissue than 

 the former and are found in the simpler inactive animals, and 

 in those internal organs of higher organisms not subject to the 

 will of the animal. 



(4) Nervous tissue is composed of cells which are so acted 

 upon by external physical and chemical agents that they are 

 able to perceive a stimulus, to conduct it to some other cell or 

 cells of the body, and to stimulate still other cells to activity. 

 All protoplasm is irritable; animals without nervous systems, 

 e.g. Ameba, are capable of reacting to a stimulus, but in more 

 complex organisms certain cells are specialized for the sole pur- 



