PRIMARY FUNCTIONS OF THE ORGANISM 33 



described a more or less permanent pseudopodium 



which extends from one edge of the cell-body 



freely into the water " and waves back and forth, 



serving as a sort of feeler or antenna." l In yet 



another form, Masti- 



gamceba, there is a per- 



manent lash or flagellum 



projecting from one por- 



tion of the body, the 



rest of the creature re- 



taining " amoeboid " 



movements. In the 



group of Protozoa called 



" Flagellata," the amoe- 



boid habit is not found, 



but the animal moves 



very swiftly by the 



lashing of one or more 



permanent whiplike fla- 



gella. In these three 



types We See three dlf- 

 ferent grades of Special- 



, Mastigamcebaaspera: 



f, flagellum; p, pseudopodium. 

 (From Calkins, after Schultze.) 



1 In the comparisons that follow, the reader must not understand that 

 one type has been transformed into another in any way whatever. 

 The different steps have been arranged side by side much as one might 

 form an exhibit of different models of the telephone or the phonograph 

 from the first crude type to the modern improved machine. In one 

 sense, though not in a. material or genetic sense, the perfected phono- 

 graph has been derived from the earlier model. In the case of specializa- 

 tion of cells, however, as we shall see from the consideration of differ- 

 entiation in development there is often a very direct genetic relationship 

 between cells of a specialized type and those of the most generalized 

 types. 



D 



