IX. PHYLUM PROTOZOA 



A. INTRODUCTORY REMARKS 



In the preceding sections of this manual we have studied in detail the anatomy 

 of a fairly complex animal, the frog. We have seen the systems of organs of 

 which the frog is composed, the cells and tissues which are the framework of 

 these organs, and for what purpose and in what manner these organs are used 

 in enabling the animal to continue its existence. We have further seen that all 

 of this complicated mechanism arises from a single undifferentiated cell, the egg, 

 which, stimulated to activity by the entrance into its substance of a sperm, 

 starts on a process of development in the course of which the multitude of 

 structures found in the adult animal come into existence. 



In this course of development in animals we further note that certain funda- 

 mental steps are involved. First the egg proceeds to produce a large number of 

 apparently similar undifferentiated cells by the process of cell division, which 

 we also studied. This continues until a ball of cells is produced. Then occurs 

 the first step in differentiation; part of the ball invaginates so that a layer of 

 cells, now called the entoderrn, lies within another layer of cells, the ectoderm. 

 So important are these layers for the future development that they are designated 

 as germ layers, i.e., layers from which certain systems are to arise. Owing to 

 their different positions these two layers have different relations to the external 

 environment and hence must take on different functions. The ectoderm, being 

 in contact with the environment, must necessarily receive stimuli from this 

 environment and act as protection against the harmful conditions which may 

 arise; hence it is destined for nervous and covering structures. The entoderrn 

 naturally takes on digestive functions, since food is essential to life, and an 

 animal can hardly digest food unless it takes the food into its interior. This 

 structural condition found in the gastrula stage of development is known as 

 diplobastic (meaning two germ layers), and, as we shall see, thousands of animals 

 exist whose structure has gone no farther than this. However, it is obvious that 

 no very great degree of complexity and differentiation can be attained in a gas- 

 trula; a third germ layer, the mesoderm, next arises between the other two, from 

 which is produced by far the greater part of the structures that we have seen in 

 the frog. This condition is known as triploblastic (meaning three germ layers). 

 The next advance is the splitting of the mesoderm, so as to leave a cavity, the 

 coelome, between its two layers; and finally the mesoderm becomes segmented, 

 that is, repeats itself along the axis. 



Having thus established in our minds a fairly complete picture of the make-up 

 of an animal and the manner in which its anatomical features have come into 



