THE RADIOLARIA 133 



shell. By repetition of the process farther along the radii a second 

 and succeeding concentric shell may arise. In the most modified 

 case (Sphaerocapsidae) the lattice alone is present, but the place of 

 the radii is shown by twenty large pores distributed according to 

 Miiller's law. 



In the Spumellaria the skeleton is either absent, spicular, or 

 shelly. Both spicules and perforated shells are often present 

 simultaneously, and have evidently developed independently in 

 two of the main subdivisions Sphaerozoa and Sphaerellaria. 

 But whilst in the former the shell is single, in the latter it often 

 becomes multiple, interconnected by radial bars, and flowers out 

 into a wealth of appendicular growths that characterise this vast 

 group, which numbers two-fifths of the known Radiolaria. The 

 Sphaeroidea retain the homaxonial form ; the Discoidea have only 

 the first or first and second chambers spherical, and farther outwards 

 become flattened and often cruciform, the arms of the cross being 

 frequently divided into a large number of chambers, into all of which 

 the endoplasm and its associated pigmented oil-globules may pass. 

 Other modifications are mentioned in the conspectus (pp. 144-145). 

 In the Nassellaria, the Radiolarian skeleton develops into its richest 

 expression of geometric form. Its simplest types consist of a single 

 or multiple ring and of a tripod or tetrad (see Fig. 5), and from 

 these a helmet-shaped perforated shell has arisen, apparently by 

 lateral extensions of the simpler plan. Such a cephalis may be 

 simple or divided both sagittally and transversely by one or more 

 constrictions, and in exceptional cases a spherical shell may be 

 developed. The most interesting feature of this group is that the 

 whole of its variety can be traced fairly confidently to the modifi- 

 cations of a single element which Biitschli (8) believes to be a 

 ring and Dreyer (15) a tetrad spicule. 



The skeleton of the Phaeodaria has followed another line of 

 evolution. It consists essentially of minute aciculate spicules 

 imbedded in a gelatinous matrix. Between these a jelly-like 

 substance is secreted ; the inner layer of this matrix becomes silici- 

 fied to form a tube, the cavity of which is often subdivided by one 

 or more septa ; or the intermediate jelly may also become silicified 

 as a porous plate or shell of porcellanous texture. Commencing 

 with the Phaeocystina, in which the skeleton is absent or composed 

 merely of isolated radial and tangential spicules, the formation of a 

 lattice-shell has come about in several ways. The simplest mode is 

 that seen in the Aulosphaeridae, in which the tangential spicules 

 unite to form an open peripheral network. To this a second shell 

 may be added by the formation of a reticulum immediately outside 

 the central capsule (Cannosphaeridae). If the outer shell is absent, 

 a condition found in the Castanellidae is obtained. In these 

 Phaeodaria the single shell is composed of two conjoined membranes 



