THE ORDER AMONG ANIMAL TYPES 97 



stand in the close relationship of parent and offspring or brother 

 and sister. 



A natural and likewise sound method of analysis of any subject 

 is to proceed from the simple to the complex. The groupings of 

 the various animal types which follow are arranged on this rational 

 basis. A knowledge of this order is a necessary part of the working 

 equipment of all students of Biology, for not only does it indicate 

 the pattern of animal life as it exists today, but also in a general 

 way at least, it represents the order in which animal forms have 

 appeared in the world's history. The significance of these facts will 

 be discussed in more detail in a later chapter. 



Animal Kingdom 

 Sub-Kingdom Protozoa 

 All animals that consist of a single cell, either separate or as 

 colonies of cells each of which is a complete animal, are placed in 

 this sub-kingdom. But one phylum is recognized, the phylum Pro- 

 tozoa. It is divided into four classes. 



Phylum Protozoa 



Class I. SARCODINA (Fig. 28). This class includes forms which 

 move by means of temporary extrusions of the body substance, 

 known as pseudopodia. All are minute and may be seen only with 

 the aid of the microscope. Many varieties are quite common in 

 stagnant water containing decaying plant and animal materials. 

 The most commonly observed forms are members of the genus 

 Amoeba. Some, especially those of the genus Entamoeba, are para- 

 sites in the tissues of Man and other higher animals and are in 

 some cases the cause of disease, for example, amcebic dysentery. 

 Members of two orders, the Foramenifera and Radiolaria, and some 

 others are encased in hard shells composed of lime. 



Class II. MASTiGOPHORA (Fig. 29). Members of this class are dis- 

 tinguished by the fact that they move through the water propelled 

 by means of one or more long fibres known as fLACELL^E. All are 

 microscopic. They are common in stagnant water. A common ex- 



