RADIOLARIA 8 3 



The four rays of adjacent circles alternate, so that the " polar " 

 and " equatorial " rays are on one set of meridians 90° apart, 

 and the " tropical " spines are on the intermediate meridians, as 

 shown in the figures. By tangential branching, and the meet- 

 ing or coalescence of the branches, reticulate (Figs. 23, 24, 25) 

 and latticed shells are formed in some families, with circles 

 of openings or pylomes round the bases of the spines. In the 

 Sphaerocapsidae the spines are absent, but their original sites 

 are inferred from the 20 circles of pylomes. 



In the Spumellaria the simplest form of the (siliceous) 

 skeleton is that of detached spicules, simple or complex, or 

 passing into a latticed shell, often with one or more larger 

 openings (pylomes). Eadiating spines often traverse the whole 

 of the cavity, becoming continuous with its latticed wall, and 

 bind firmly the successive zones when present (Fig. 23). 



Calcaromma calcarea was described by Wyville Thomson as 

 having a shell of apposed calcareous discs, and MyxohracMa, 

 by Haeckel, as having collections of the calcareous Coccoliths and 

 Coccospheres. In both cases we have to do with a Eadiolarian 

 not possessing a skeleton, but retaining the undigested shells 

 of its food, in the former case {AcMssa) in a continuous layer, 

 in the latter (ThalassicoUa) in accumulations that, by their 

 weight, droop and pull out the lower hemisphere into distinct 

 arms. 



The (siliceous) skeleton of the Xassellaria is absent only in 

 the Nassoidea, and is never represented by distinct spicules. Its 

 simplest form is a " tripod " with the legs downward, and the 

 central capsule resting on its apex. The addition of a fourth 

 limb converts the tripod into a " calthrop," the central capsule in 

 this case resting between the upturned leg and two of the lower 

 three regarded as the " anterolateral " ; the odd lower leg, like 

 the upturned one, being " posterior." Again, the skeleton may 

 present a " sagittal ring," often branched and spiny (Fig. 26, A), 

 or combined with the tripod or calthrop, or complicated by the 

 addition of one or more horizontal rings. Another type is 

 presented by the "latticed chamber" surrounding the central 

 capsule, with a wide mouth ("pylome") below. This is termed 

 the "cephalis"; it may be combined in various ways with the 

 sagittal ring and the tripod or calthrop ; and, again, it may be 

 prolonged by the addition of one, two, or three chambers below, 



