MORPHOLOGY AND TAXONOMY OF MASTIGOPHORA 251 



Colonies thus arise of gregaloid, sphseroid, arboroid or catenoid nature. 

 In many of the phmt-hke forms, the motile stage is frequently 

 transitory, the cells throwing off or withdrawing their flagella, 

 secreting jelly and passing into a resting stage. Here, they may 

 reproduce by division, giving rise to aggregates known as the 

 PalmeUa phase. In some cases, this phase is dominant, the motile 

 stage being extremely short; in other cases, the motile stage is 

 dominant and the Palmella-stage short. This condition must be 

 distinguished from encystment where the cells secrete a resistant 

 membrane within which they are able to withstand adverse external 

 conditions. 



A typical flagellum consists of a periplastic sheath continuous 

 with the periplast of the cell, and an axial, highly contractile fila- 

 ment. The latter penetrates more or less deeply into the endoplasm 

 where it arises from the substance of a definite kinetic center. 

 The number of flagella is higlJy variable, sometimes 1, 2, 3, 4, 6, 

 8 and many being characteristic of different species. When more 

 than one are present, they may be similar in size and structure or 

 diverse. In the latter case there may be one main locomotor 

 flagellum and a second more minute accessory flagellum. In some 

 cases, one flagellum is directed anteriorly, the other posteriorly. 

 The latter is usually larger, heavier, and less active, behaving like 

 a trailing flagellum or a runner on which the cell moves over the 

 surface. It is possible that such a trailing flagellum may remain 

 or become attached to the body periplast which may be drawn out 

 ledge-like, and cause it to vibrate or undulate with the movements 

 of the flagellum. Such may be the possible origin, in some cases 

 at least, of the undulating membrane found in a number of types. 

 The insertion of the flagellum varies. In some cases a simple endo- 

 plasmic basal granule or blepharoplast is the sole representative of 

 a kinetic complex. In other cases the basal body, giving rise to 

 the flagellum, is separated from the blepharoplast, the two kinetic 

 centers being connected by a rhizoplast. In other cases all kinetic 

 bodies may be concentrated in one relatively large kinetic center; 

 or the parabasal body, basal body and blepharoplast may be separate 

 elements in the cell connected or not by rhizoplasts. This matter 

 of insertion is one of the essential difterences between the so-called 

 flagella of bacteria or spirocha^tes and animal flagellates. In the 

 former the axial fllament is absent, the so-called flagellum arising 

 from the periplast (see Chapter II). 



Pseudopodia, in addition to flagella, are common throughout the 

 entire group, and as in Sarcodina, may be of the axopodia or lobo- 

 podia type. In forms with axopodia the close resemblance between 

 the flagella and the pseudopodia in respect to origin and motility 

 lends support to the view that the two are homologous structures 

 and that axial filaments of heliozoon pseudopodia and axial fila- 



