STRUCTURAL DIFFERENTIATIONS 141 



found only in organisms belonging to the groups Heliozoa and 

 Radiolaria, in which they radiate out in all directions from a usually 

 spherical l)ody (Fig. 73). 



Unlike flagella, the outer coating of an axopodium is not a smooth 

 periplast-like sheath, but consists of fluid protoplasm in which the 

 movements of gramdes out on one side and back on the other are 

 clearly discernible. In this manner the outer protoplasm is continu- 

 alh' changing about the central axial filament, which alone is con- 

 stant or fixed. Upon prolonged irritation, or in preparation for 

 division or encystment, the axial filaments themselves, together 

 with the enveloping protoplasm, are withdrawn. 



Like flagella the axial filaments are formed as outgrowths from 

 endoplasmic kinetic elements. Gymnosphcera, Raphidiophrys, Sphco- 

 rasfruni, Acanthocystis, Dimorpha, etc., possess characteristic 

 "central granules" which, from their activities in cell division, are 

 unmistakably centroblepharoplasts (see p. 99) from the substance 

 of which the axial filaments are formed (Fig. 60, p. 117). J]'agner- 

 ella horeaUs, in addition to the central granule, possesses a zone of 

 basal bodies which give rise to the axial filaments and which at 

 times of retraction of the pseudopodia are drawn into the central 

 granule. In still other cases, as in Adlnosphoerium eichhornii the 

 axial filaments do not arise, apparently, either from central granules 

 or from nuclei, but appear to start indefinitely in the cytoplasmic 

 reticulum (Fig. 73, D). In this case, however, the nuclei are so 

 numerous that it would be difficult for an axial filament to escape 

 proximity to some of them, while in a similar multinucleated form— 

 Camptonema unions— each nucleus gives rise to a single axial fila- 

 ment (Fig. 53, p. 102). 



While the more common fc^rms of Heliozoa are quiescent, floating 

 types, some of the Heliozoa are freely motile, ^irtodiscus species, 

 according to Penard, moves actively about, in the manner of a 

 monad; Acanthocystis aculeata, as well as other species of the same 

 genus, turns slowly over and over in a rolling movement; Camptn- 

 noiia nutans, according to Schaudinn, bends and straightens its 

 axojxxlia in food-getting and in other activities, ^{ctinosphaerium 

 eichhornii and Actinophrys sol are practically motionless. The 

 active movements are due to the axopodia and the structure of 

 axopodia is strikingly like that of flagella. That the contractile 

 axial filament is the seat of this movement, and not the enveloping 

 protoplasm is not open to reasonable doubt. Structure, function 

 and mode of origin thus justify the inclusion of axopodia with the 

 kinetic elements of the cell. 



On the other hand, in types with axopodia which are practically 

 motionless, the axial filaments have ai)parently lost the vibratile 

 function and now serve as supporting elements for the long radiating 

 pseudopodia. There is little reason to doubt that such elements are 



