INTRODUCTION. 7 



cell; for this reason Dobell* has lately proposed for 

 it the name " centroplast." 



PSEUDOPODIA. 



In the Heliozoa the pseudopodia are very attenuated^ 

 and scattered along them minute globules can generally 

 be discerned ; in drawings it is not practicable to 

 depict them truthfully ; a thin line o'f " gum-water " 

 would perhaps be the nearest approach to giving an 

 idea of their appearance. 



Their length varies from less than the diameter of 

 the animal's body to three or four times the diameter ; 

 their number is also very variable, in some species 

 being a thousand or more, but usually the number is 

 sufficiently few to be counted ; this variation is found 

 not only in difEerent species but to some extent in 

 individuals of the same species, and this applies not 

 only to those of various sizes but also to those of the 

 same size. 



Every pseudopod usually has a firm central axis, 

 but it is generally too filamentous and transparent to 

 be readily detected, except in the genera Actinophrys 

 and Actinosphaerium in which it is plainly visible. 

 This axis has some remarkable properties ; in those 

 species which possess a " central granule " and usually 

 also in the case of those in which the nucleus is 

 central, the axes of the pseudopodia are prolonged 

 within the margin of the ectoplasm and continued 

 through the plasma until the border of the nucleus 

 or the central granule is reached; this axis endows 

 each pseudopod with considerable rigidity and 

 elasticity and reminds the observer of a colourless 

 "spun" glass filament of great tenuity, but in spite 

 of this rigidity the animal is able to retract or extend 

 its pseudopodia with remarkable rapidity, their dis- 

 appearance being apparently due to some modifying 

 action of the endoplasm on the axis, which can be 



* ' Quart. J. Mior. Soi.' Ixii, 1917, p. 522. 



