/. RHIZOPODA. 



187 



flagella, cilia furnish the basis for the differentation of these classes, to 

 which are added forms the class of Sporozoa modified by parasitism. 



Class I. Rhizopoda. 



In the lowest position in the Protozoa must be placed those 

 organisms which lack permanent structures for locomotion and 

 nourishment, but in which the protoplasm of the body performs 

 these functions. The term Rhizopoda refers to the fact that the 

 protoplasm sends out root-like processes false feet or pseudopodia 

 for locomotion and for taking nourishment. These differ from 

 true appendages in that they are not constant cell organs, but are 

 formed according to demand and again disappear. A pseudopo- 

 dium arises when the protoplasm 

 streams to one point of the body and 

 extends as a process beyond the sur- 

 face. Since the process becomes 

 attached and draws the body after 

 it, or since the protoplasm of the 

 body may flow into it, a slow change ( 

 of place occurs. Thus the process 

 disappears and is absorbed in the 

 organism, and new pseudopodia are 

 formed at other places which after a 

 time are retracted in turn. This 

 type of locomotion is called amoeboid 

 after the Amceba, in which it was 

 first accurately studied. When the 

 Ehizopoda in their wanderings meet 

 particles of nourishment, they en- 

 close them with their protoplasm FIG. 116. Amoeba proteus. (After 



, Leidy.) cv, contractile vacuole; en, 



and take them into the interior OI entosarc; efc, ectosarc; n, nucleus; 

 ,, ' T , .,., ,^ x N, food-body. 



the body (fig, 116, J\ ). 



The form of the pseudopodia is approximately constant for 

 each species, but as a whole very variable, so that it may be used 

 not only for separating species but families and larger groups. Qn 

 the one hand, there are finger-like pseudopodia (fig. 116), on the 

 other, those of such delicacy that even under strong magnification 

 they appear like fine threads (fig. 117). Between these two 

 extremes are many intermediate forms. Thread-like pseudopodia 

 usually branch, and when the branches meet they may fuse and 

 form anastomoses, from which it follows that it is not true, as was 

 once supposed, that the psendopodia are .covered, by a membrane. 



