134 THE RADIOLARIA 



imbedded in a porcellanous impregnation throughout which minute 

 aciculate spicules occur. It is provided with an oral opening on the 

 end of a projecting and often spiny peristome. Again, this inner shell 

 may assume a bi valvular form (Fig. 32), and then carries a number 

 of complex appendages. Some of these are branching hollow 

 species, terminating in anchor-like expansions ; others constitute the 

 "galea" and "rhizocanna" (see Fig. 32, p. 151). 



Biological Significance of the Skeleton. The results of recent 

 investigation point to the conclusion that the chief skeletal function 

 is a hydrostatic one and is effected by stretching or folding the 

 superficial ectoplasm. The older conception of the skeleton as 

 projecting freely beyond the cytoplasm has been shown to be a 

 mistaken one in many instances, and it is probable that the skeleton 

 is during life covered by the outermost delicate plasmic layer 

 in all Radiolaria. Between the characters of this layer and the 

 development of the supporting rods a definite relation holds for 

 certain forms. A few widely varying Radiolaria are dimorphic, a 

 small pelagic variety and a larger abyssal form being readily and 

 apparently rightly distinguishable (Aulacantlia scolymantha, Auloscena 

 and Sagosphaera-spQcies). In these cases the surface-form possesses 

 a delicate ectoplasmic layer, and the supporting rods are simpler 

 and shorter, whereas in the bathybial variety the outermost 

 cytoplasm is dense, more voluminous, and usually more stiffly sup- 

 ported by verticillate skeletal projections. The graceful and 

 elaborate skeletal appendages of other Phaeodaria are probably to 

 be explained not as a means of catching food, but as a support for 

 the ectoplasm ; and the whole plan and construction of the tubular 

 skeleton in these forms is no doubt related intimately to the 

 pressures that fall upon this limiting layer. 



In connection with this sustentative function of the Phaeodarian 

 skeleton, the mode of formation of its tubular systems offers some 

 features of special interest. The most general mode is that indi- 

 cated at the close of the last section (p. 133), and in this method 

 minute needle-like spicules form the centre around which tubular 

 developments of silica take place. But in addition to this intrinsic 

 centre, many Phaeodaria have adopted extrinsic objects, and around 

 these as catalysators, the tubular silica is deposited. Like other 

 Radiolaria, but to a greater extent, the Phaeodaria ingest quantities 

 of foreign bodies, with which their phaeodium is distended. Amongst 

 these ingesta, diatoms and Radiolarian skeletons are abundant. 

 From Phaeodaria, in which such gatherings are casual, we can trace 

 a series leading to forms in which diatom-selection becomes a regular 

 habit, associated directly with the formation of a radial skeleton. 

 Thus AulograpUis pandora and Auloceros arborescens from the Atlantic 

 and Indian Oceans contain in their phaeodia frustules of Rhizosolenia, 

 and spicules of many species of Aulacanthids picked up apparently in 



