n8 



THE RADIOLAR1A 



certain pseudopodia. They occur exclusively in the Acantharia 

 Acanthometrida, and form circular groups of short, rod-like bodies 

 clustered round each of the radial spicules (Fig. 11). Upon 

 careful examination they are found to connect the ectoplasm with 

 the pseudopodial covering of the spicule and to possess a high 

 degree of contractility. Their form varies accordingly. When 

 expanded the myonemes appear as homogeneous threads '006 to 

 013 mm. long. When contracted they not only become shorter 

 (012-'02) and thicker, but exhibit in many cases a very distinct 

 cross-striping. They are, in fact, muscular structures comparable 



Sh. 



M 



Fio. 11. 



Portion of a living specimen of Acanthometron pellucidum, one of the Acantharia, x900 

 (after Schewiakoff), to show endoplasm and ectoplasm. The latter consists of vacuolated 

 cytoplasm (E) slung up to the rod (S) by striated myonemes (.V), which are inserted into 

 the sheath (Sh) around the rod. In the endoplasm two nuclei (N) and zooxanthellae (Z) are 



with the contractile fibrillae of Gregarines and Infusoria (Schewia- 

 koff [33]), and they serve to raise or lower the hydrostatic, ecto- 

 plasmic apparatus of these Radiolaria, and so to facilitate their 

 ascent or descent. 



Another cytoplasmic modification of the Acantharia may here 

 be mentioned, namely, the axopodia. They consist of contractile 

 pseudopodia that radiate from near the centre of the endoplasm 

 to the periphery of the animal, and possess an axial fibre around 

 which an unceasing cyclosis of granules takes place. These 

 axopodia differ from the ordinary pseudopodia of the Acantharia 

 not only in their deeper origin but also in their more limited 



