INTRODUCTION TO THE METAZOA 61 



is usually built around this character. Even though differentia- 

 tion leads along numerous different lines in the metazoan body, 

 the general results are so nearly uniform in different animal 

 groups that but four chief classes of tissues are commonly 

 recognized, namely; epithelium, connective tissue, muscle tissue, 

 and nervous tissue. 



In addition to these chief classes of tissues, many other kinds 

 are recognizable but often these are modifications of one or more 

 general classes. Thus the nettling cells of a Hydra or the pig- 

 ment cells of a squid may be considered as special classes of cell 

 differentiation. But since these special types of cells are rather 

 readily referable to an epithelium or a connective tissue or 

 one of the other types, there is no distinct advantage in creating 

 additional classes of tissues. 



Both form and structure of an organism depend upon the 

 grouping of the component tissues. Thus both morphology and 

 physiology go back for their final basis to the study of the several 

 tissues and frequently to the form and structure of the individual 

 component cells. 



Epithelial Tissues 



Definition and Classification. — Any tissue which covers 

 a surface is called an epithelium. Phylogenetically, tissues of 

 this class are the most primitive, for the coelenterates are essen- 

 tially nothing but epithelial in structure. In like manner, in 

 individual development among all the Metazoa, the first division 

 of labor for the cells of the embryo occurs with the formation of 

 the gastrula stage when nothing but surface coverings is present. 



Form vs. Function. — Varied forms of cells and sundry func- 

 tions occur in tissues in this group. If an epithelium comprises 

 but a single layer of cells, it is termed simple, while an epithelial 

 layer two or more cells in thickness is designated as stratified 

 epithelium. The form of the component cells leads to the use of 

 such descriptive terms as cuboidal, columnar, and squamose, 

 but, since function is a much more important criterion for 

 classification, epithelia are grouped as protective, formative or 

 glandular, ciliated, communicative or sensory, and germinal. 



Ciliated and Flagellated Epithelium. — Even in the gastrula 

 stage of ontogeny some or all of the cells are frequently provided 

 with cytoplasmic threads, the movements of which are primarily 

 for locomotion. Ciliated or flagellated cells as a means of loco- 



