CHAPTER VII 

 AN INTRODUCTION TO THE METAZOA 



i. CELLULAR DIFFERENTIATION TISSUES 



THERE is no sharp line between the Metazoa and Protozoa. 

 We have seen (Chap. VI) that the Volvocaceae, which are often 

 included in the Phylum Protozoa, are many-celled colonies, the 

 cells of which are either entirely independent, as in Spondy- 

 lomorum (Fig. 44), or are connected by protoplasmic strands, as 

 in Volwx (Fig. 46). Here the cells of the colony have ceased to 

 be independent, but have united to form a distinct body which 

 carries on nutritive processes, produces germ cells, and then dies. 

 The cells of this body are called somatic cells. Their functions 

 are locomotor and nutritive, but not reproductive. Such an 

 aggregation of cells is known as a tissue. A tissue is an associa- 

 tion of similar cells originating from a particular part of the 

 embryo and with special functions to perform. In Volwx there 

 is only one kind of tissue ; in some of the simple Metazoa there 

 are two kinds of tissue resulting from the differentiation of the 

 somatic cells; in the majority, however, there are at least three 

 and usually more. The many different kinds of tissue in the 

 Metazoa may be classified according to their structure and 

 functions into four groups. 



(i) Epithelial tissue (Fig. 48 A, B) 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. The 

 somatic cells of Volwx may be considered epithelium. In the 

 more complex animals epithelial cells become variously modified 

 because they are the means of communication between the or- 

 ganism and its environment; nutritive material passes through 



