PEOTOZOA — EADIOLAEIA. 



27 



losa, with pores grouped ; skeleton of strontium sulphate. Gallery X, 

 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 wdth 

 organic matter. 



Only the glassy skeletons of pure silica — as occurring in Table-case 

 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 e), or several such spheres one within the other 

 (Fig. 6 h), 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 b 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. 



Eadiolaria live only in the sea, where they float in all 

 parts and at all depths, but mainly near the surface 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 Radiolaria; examples of 

 this, dredged by H.M.S. ' Challenger,' are shown in a case in 

 the middle of this Gallery. Similar radiolarian ooze has 

 been deposited in past geological epochs, and when 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 {KieselgvJir), and occur in many parts of the world. 

 The Eadiolaria from several of them were described by 

 Ehrenberg (1838-1873) under the name Polycistines. On Wall-case 

 the bottom shelf of Wall-case 9b, in the corner, is a large 

 core of the Miocene radiolarian marl of Barbados, from which 



