114 THE RADIOLARIA 



genera appear identical with those now living. Pre- Cambrian 

 Radiolaria are still doubtful (Hinde [44]). The Sphaerellaria (Poly- 

 cystina) and Nassellaria are the chief contributors, since the 

 strontium skeletons of the Acantharia are readily soluble, and 

 therefore are unknown in recent deposits or in a fossil state, and 

 the hollow siliceous spicules of the Phaeodaria also appear in- 

 capable of resisting decomposition. Many skeletons formerly identi- 

 fied as Radiolarian (such as Dictyota and Mesoscena) are now referred 

 to the Flagellata or to other orders, but the Nassellaria Cyrtoidea 

 form the majority, the Sphaerellaria, Discoidea, and Sphaeroidea 

 the minority, of Jurassic Radiolaria in quartzites and coprolites. 

 In later deposits of Miocene ages this predominance is maintained, 

 but the species found are identical with or closely akin to living 

 forms. 



Central Capsule. The cytoplasm of Radiolaria is distinguished 

 from that of other Protozoa by the great development, specialisa- 

 tion, and delimitation of its ectoplasm. The boundary between 

 this peripheral layer and the central nucleated plasma is almost 

 always a distinct one ; and the few cases amongst the Acantharia 

 and Sphaerozoa in which no limiting membrane can be traced, 

 serve to show that this separation is the outcome of more primitive, 

 undifferentiated conditions, which the Radiolaria display in early 

 life, to which they revert during fission, and occasionally retain 

 throughout life. 



The central capsule is the sign of this plasmic differentiation, 

 and the mark of a Radiolarian. It consists of a single, or in 

 Phaeodaria of a double, porous membrane of either chitinoid or 

 mucinoid nature. Usually the capsule is of such tenuity as to be 

 visible only after the use of reagents, or, as in Thalassicolla, it may 

 be comparatively thick and areolated by the growth of ridges on 

 its inner surface (Hertwig). 



The shape of the capsule is in general correlated with that of 

 the configuration of the animal. In homaxonic Spumellaria and 

 Acantharia it is spherical ; in lenticular and discoid forms it is 

 ellipsoidal. In the bilateral Nassellaria it is elongate, and in the 

 Phaeodaria spheroidal; but in the recently discovered spherical 

 Thalassothamnidae it is lobate or branched (Fig. 10). The 

 consistency of the central capsule, however, is not that usually 

 associated with chitinoid structures. It is capable of extension, 

 and in the concentric Sphaeroidea and Discoidea it is lobate and 

 may enclose the inner shells one after another. In the helmet- 

 shaped Nassellaria it throws out lobes through the basal plate of 

 the shells. During the processes of fission and sporulation the 

 central capsule in all Radiolaria becomes more or less completely 

 dissolved to allow of the separation or escape of the endoplasmic 

 contents. These phenomena show that the capsule is no per- 



