THE SARCODINA 213 



maturity. In the Protozoa no such criterion is available, and the 

 distinction between young and adult is based on differences in size 

 and growth, or on phases of the life-cycle selected in an arbitrary 

 manner. In many cases the distinction presents no difficulty ; it 

 is perfectly easy to distinguish young from adult stages in such 

 forms as the Foraminifera and Radiolaria among Sarcodina, or the 

 genus Noctiluca among Mastigophora. But in other cases it is 

 purely a matter of opinion which phase in the life-cycle is to be 

 regarded as adult. Such a form as Pseudospora has a flagellated 

 and an amoeboid phase (Robertson), and can be placed in either 

 the Sarcodina or the Mastigophora with perfect propriety. The 

 amoeba-like genus Mastigamc&ba is placed in the Mastigophora 

 because the flagellum is retained ; but if any species of this genus 

 were to lose its flagellum when adult, rigid adherence to verbal 

 definitions would necessitate its being classed in the Sarcodina. 



The difficulty of separating and defining the stems of the Sarco- 

 dina and Mastigophora at their root is only to be expected on the 

 theory of evolution. The two classes are undoubtedly descended 

 from a common ancestral type, which has become modified in 

 two divergent directions, giving rise to two vast groups of organisms 

 which may differ from one another very slightly or very greatly in 

 selected examples. The systematist may meet with many obstacles 

 when it is required to lay down verbal distinctions between the two 

 classes, but it is easy to recognize, in a general way, two principal 

 morphological types, round which each class is centred, and which 

 may be realized to a greater or less extent in given cases. 



1. Sarcodine Type. Protozoa which grow to a relatively large 

 size ; in the so-called " adult phase " permanent organs of loco- 

 motion are wanting, and the naked protoplasmic body moves or 

 captures food by means of pseudopodia ; the young stages may be 

 flagellate or amoeboid. 



2. Mastigophoran Type. Protozoa usually of minute size, seldom 

 with a large adult phase (as, for example, Noctiluca) ; flagella 

 retained throughout active life, only lost in resting phases ; body 

 amoeboid or corticate. 



THE SARCODINA. 



The name Rhizopoda is sometimes used for this class but this 

 name is only applicable, strictly speaking, to the first four orders 

 recognized below, in which the pseudopodia are more or less root- 

 like, and not to the orders Heliozoa and Radiolaria, characterized 

 by stiff radiating pseudopodia. 



General Characteristics. As stated above, the Sarcodina are Pro- 

 tozoa for the most part of relatively large size. Many Sarcodina 



