254 BIOLOGY OF THE PROTOZOA 



metrical to asymmetrical stages and back again; many of these are 

 amoeboid, especially in the Chnsomonadida and Cryptomonadida; 

 others, particularly the Euglenida, are sufficiently plastic to undergo 

 form changes without break or rupture of the periphery as happens 

 in pseudopodia formation. Such plastic organisms are said to be 

 metabolic and the phenomenon is usually called metaboly.^ Pseudo- 

 podia formation is found in many different types of flagellates and 

 conversely in some rhizopods flagella are present at some stage of 

 the life history. Whether to classify such forms as flagellates or 

 rhizopods is largely an academic cjuestion which may be answered 

 in an arbitrary- way by including all colored forms with pseudopodia 

 amongst the Phytomastigoda provided a flagellum is present at any 

 stage, while in some sub-orders (Rhizochrysidina, Phytodinidfe, etc.) 

 even flagella are unknown, the yellow, chromatophore-bearing stages 

 alone having been described. 



Amoeboid and Metabolic Types.— In all cases of amoeboid and meta- 

 bolic forms the cell symmetry is variable by reason of the form 

 changes due to transformation of energy as a result of destructive 

 metabolism, in the absence of firm and resistant membranes, pel- 

 licle, or products of the cortex in the form of lifeless membranes, 

 shells, etc. When such cortical structures are present the cell form 

 is usually constant, although metaboly may still be observed 

 (pAiglenida). The cell membrane may be delicate or heavy; plain 

 or striated; smooth or ridged or spinous, and the outline may be 

 simple or spirall^' twisted. It may be covered by gelatinous secre- 

 tions or impervious cellulose membranes (Dinoflagellida, Phyto- 

 monadida, Chloromonadida) or by calcified shells {Phacotus) or 

 plates {C()ccoUifu)})hori(la) or by silicious plates {MaUomonas) or 

 skeleton {T)iste])hanus). In some cases the pseudopodia that are 

 formed are tentacle-like and are regularly arranged about the base 

 of the flagelhnn (Cyrtoplwra, Pedinella, etc.). 



Flagella.— Flagella Aary in number from 1 to 4, very exceptionally 

 more than 4. If 2 are i)resent they may be similar in length and 

 in function (isomastigote) in which case both are primary flagella; 

 or they may be dissimilar in form and size (heteromastigote) and 

 directed forward, in which case the larger is the primary, the smaller 

 the secondary, or there may be 2 or 3 secondary flagella. In many 

 cases one (primary) flagellum is directed forward, while a second 

 which may be either larger or smaller than the primary, is directed 

 backward, and is known as a trailing flagellum (Schleppgeissel) . 

 In Dinoflagellida one flagellum moves freely in the surrounding 

 water while a second undulates in a characteristic transverse groove 



1 The iise of the term rneiahohj in this connection is misleading, metabolism 

 meaning the sum-total of chemical processes in the upbuilding and breakdown of 

 living protoplasm; all living things are metabolic in this sense and in using such a 

 term with a double meaning the significance is carried with the connotation. 



