THE SARCODINA 251 



and extracapsular regions of the body. The intracapsular medulla 

 contains the nucleus or nuclei, and is the seat of reproductive 

 processes. The extracapsular cortex is the seat of assimilation, 

 excretion, food-capture, and of such locomotor processes as these 

 organisms are able to perform, consisting chiefly of rising or sinking 

 in the water by means of changes in a hydrostatic apparatus 

 presently to be described. 



The Radiolaria are an exceedingly abundant group represented by a great 

 number of species both at the present time and in past ages ; over vast 

 tracts of the ocean-floor their skeletons are the principal, almost the sole 

 constituents of the ooze ; and the same must have been true in past times, 

 since in many geological deposits the rocks are composed of the same materials. 

 Every microscopist is familiar with their skeletons, which on account of 

 their beauty and variety of form are favourite objects for microscopic study 

 and demonstration. Corresponding with the variety of forms and species, 

 the internal structure shows a range of variation and differentiation which 

 it is impossible to deal with adequately in a short space ; it must suffice, 

 therefore, to describe here the main structural peculiarities of this group 

 in a general manner, and to indicate briefly the principal variations of structure 

 which are of importance for the classification of the group For further 

 information the reader must be referred to the larger treatises and special 

 monographs. 



Structure. Tne central capsule, absent in rare cases, may be a 

 thin, delicate structure, visible only after treatment with reagents, 

 or may be fairly thick. In homaxon forms it is generally spherical, 

 but may assume various shapes correlated with the general body- 

 form, and even may be lobed or branched. It is perforated by 

 openings which place the intracapsular protoplasm in communica- 

 tion with the extracapsular ; the openings may take the form of 

 fine pores scattered evenly over the whole surface (Peripylaria) ; of 

 similar pores aggregated into localized patches, pore-areas or pore- 

 plates (Acantharia) ; of a single pore-plate at one pole of an asym- 

 metrical capsule (Monopylaria, Fig. 106) ; or of one principal and 

 two lateral apertures (Tripylaria). 



The intracapsular protoplasm contains the nuclear apparatus, 

 either one nucleus of very large size or a number of smaller nuclei 

 (Fig. 105). In addition, various bodies of metaplastic nature, 

 serving as reserve-material for the reproductive processes, are 

 found in this region, in the form of fat-globules, oil-drops, concre- 

 tions, crystals, etc. 



The extracapsular region consists of three zones, from within 

 outwards : (1) an assimilative layer or matrix immediately sur- 

 rounding the capsule ; (2) a vacuolated layer, known as the " cal- 

 ymma," hydro static 'in function; (3) a protoplasmic layer from 

 which the pseudopodia arise. 



1. The assimilative layer contains pigment, representing ex- 

 cretory substances and ingested food-material in the shape of small 



