REPORT ON THE RADIOLARIA. Ixxvii 



114. The Astroid Skeleton. Under the name " Astroid" we place the peculiar star- 

 shaped skeletons of the ACANTHARIA in opposition to those of all other Radiolaria, for 

 they are separated from them not only fundamentally by reason -of the chemical nature 

 of their substance (Acanthin, 102), but also by their centrogenous origin, and the 

 resulting stellate form (Pis. 129-140). The ACANTHARIA are the only Radiolaria in 

 which the skeleton arises within the central capsule by the formation of numerous rays 

 or radial spines of acanthin which project on all sides from the centre. Originally these 

 are united at this point, their conical or pyramidal points meeting and being supported 

 one upon another. In the great majority of ACANTHARIA this loose apposition is constant, 

 so that when the soft parts are destroyed the skeleton falls to pieces. Only in a few 

 forms in this legion are the central ends of the spines fused so that the whole skeleton 

 forms a connected star (Astrolithium). The small group Chiastolida (or Acanthochi- 

 asmida) is characterised by the fact that the two rays which are opposite to one another 

 in each axis unite and form a diametral bar. The skeleton is almost always composed 

 of twenty radial spines, which are regularly disposed (Icosacantha), only in the small 

 primitive group Actinelidais the number variable (Adelacantha, 110). 



115. The Beloid Skeleton. As Beloid or spicular skeletons are grouped together all 

 those which consist of several disconnected portions ; these always lie outside the central 

 capsule, either within the calymma or on its surface. Such extracapsular Beloid skeletons 

 are entirely wanting in the ACANTHARIA and NASSELLARIA ; they occur only in the 

 B e 1 o i d e a among the SPUMELLARIA, and in the Phseocystina among the PH^EO- 

 DARIA; the individual Beloid portions of the former are solid, those of the latter hollow. 

 In both groups the simplest forms of the separate portions are simple unbranched needles 

 (Thalassos2)hcera, Thalassoplancta, Physematium, Belonozoum, among the SPUMELLARIA; 

 Cannobelos and Cannorrhaphis among the PH^EODARIA) ; usually these spicules are dis- 

 posed tangentially over the surface of the calymma. Among the Beloidea branched 

 spicules occur more commonly than these simple ones ; they are either stellate (with 

 many rays united in a centre) or twin-like, with a tangential bar, from each pole of which 

 two or three (seldom more) radial branches project (Pis. 2, 4), Among the PH^EODARIA 

 the subfamily Dictyochida is characterised by the annular shape of its Beloid portions, 

 either simple rings, or hat-shaped or pyramidal bodies with a latticed cap over the ring 

 (PI. 101, figs. 3-14 ; PL 114, figs. 7-13). The family Aulacanthida among the PH^O- 

 DARIA, alone possesses hollow radial tubes, which penetrate the whole calymma, and 

 project distally over its surface, whilst their proximal ends rest upon the surface of the 

 central capsule. Although in these cases the enclosed proximal end is always simple, the 

 free distal end develops the most various processes in adaptation to its prehensile 

 functions (Pis. 102-105). 



