PROTOZOA — KADIOLARIA 
27 
losa, with pores grouped ; skeleton ot strontium sulphate. 
3. Nassellaria : Osculosa, with one basal osculum ; skeleton, 
when present, of solid silica. 4. Phaeodaria : Osculosa, 
with the capsular wall projecting around the basal osculum 
as a tube ; two or more smaller oscula may be at the 
apical pole, extracapsular protoplasm contains dark pigment 
granules (phaeodium) ; skeleton of silica combined with 
organic matter. 
Only the glassy skeletons of pure silica — as occurring in 
Orders 1 and 3 — are found fossil. These skeletons, though 
manifesting extreme diversity of shape, conform in general 
plan to the structure of the central capsule, as may be seen 
from the greatly enlarged models in Table-case 15. Thus in 
Spumellaria the skeleton is usually a sphere of lattice-work 
(Fig. 6 c), or several such spheres one within the other 
(Fig. 6 b), and joined by cross-bars which radiate from the 
innermost sphere but do not meet at the centre, and which 
project as spines (Fig. 6 h and c). The sphere may be pulled 
out to an ellipsoid, or compressed to a discoid. In Nassellaria 
the skeleton is generally in the shape of a bell or of an 
elongated cone, which may be transversely constricted at 
intervals (as in Fig. 6 d ) ; there may be a spine at the apex 
and others projecting from the basal margin. All these 
skeletons are so minute that their form can scarcely be 
distinguished by the naked eye. 
Radiolaria live only in the sea, where they float in all 
parts and at all depths, but mainly near the siirface of 
tropical oceans. On death their skeletons sink to the bottom, 
but those not made of pure silica are dissolved by the sea- 
water ; and where the ocean is very deep the calcareous shells 
of the Foraminifera are also dissolved as they sink. Hence 
at depths of from two to four miles the ooze of the ocean- 
floor is formed almost entirely of the siliceous skeletons 
of Radiolaria. Similar radiolarian ooze has been deposited 
in past geological epochs, and vvhen found among the 
rocks bears witness as a rule to an upheaval of that 
part of the earth’s crust from a great depth. In the rocks 
of Tertiary age, such dried oozes are known as Tripoli 
stone (Kieselguhr), and occur in many parts of the world. 
The Radiolaria from several of them were described by 
Ehrenberg (1838-1873) under the name Polycistines. On 
the bottom shelf of Wall-case 9 b, in the corner, is a large 
core of the Miocene radiolarian marl of Barbados, from which 
400 species have been described ; and a glass slide with 
Gallery X. 
Table-case 
15. 
Wall-case 
9b. 
