POLYCYSTINA 



849 



B 



' 



consist ot'as many as a thousand zooids aggregated together in various 



forms, discnidal. cylindrical, spheroidal, chain-like, or even iiecklace- 

 like. The 'colonies ' seem to be produced, like the multiple segments 

 of the 1 todies of Foraminifera, by the non-sexual multiplication of a 

 primordial zo'oid ; but whether this multiplication takes place by 

 fission, or by the budding oft' of portions of the sarcode-body; has 

 not yet been clearly made out. The emission of flagellated zoospores. 

 provided with a very large nucleus, and in sonic cases with a rod- 

 like crystal, has been observed in many radiolarians ; but of the mode 

 in which they are produced, and of their subsequent history, very 

 little is at present known. Viitil the structure and life history of 

 the animals of this very interesting type shall have been more fully 

 elucidated, no satisfactory classification of them can lie trained : and 

 nothing more will be here attempted than to indicate xnne of the 

 principal forms under which the radiolarian type presents it-elf. 1 



Discida. Among the 

 beautiful silk-ions struc- 

 tures which are met with 

 in the radiolarian sand- 

 stone of Barbadoes (fig. 

 i)44) there is none more 

 interesting than the ske- 

 leton of Astl'OiillltH (fig. 



648). in which we have a 

 remarkable example of 

 the range of variation that 

 is compatible with con- 

 formity to a general plan ^ Y^i - 

 of structure . As i n < tther 

 forms of Haeckel's group 

 of Discida,, there is in 

 this skeleton a combina- 

 tion of radial and of cir- 

 cumferential parts, the 

 former consisting of solid 

 spoke-like rods, whilst the 

 latter is composed of a silicious network more or le>- completely 

 filling up the spaces between the rays. The radial part of the skele- 

 ton predominates in the beautiful four- rayed example represented at 

 D, having the form of a cross with equal arms ; whilst in F and G it 

 still shows itself very conspicuously, though the spaces between the 

 rays are in great part filled up by the circumferential network. In the 

 five-rayed specimens A and B. on the other hand, the radial portion 

 is much less developed, while the circumferential becomes more dis- 

 coidal. And in <-' and E. while the circumferential network forms a 

 pentagonal disc, the radial portion is represented only by solid projec- 

 tions "at its angles. The transition between the extreme forms is 

 found to be so gradual when a number of specimens are compared 

 that no lines of specific distinction can be drawn between them ; and 



1 Considerable attention has been given to the question of the classification of 

 the Rarliolaria by Haeckel and by R. Hertwig, JfiKtisch* Denkschr. ii. 1H79, p. 129. 



FIG. 646. Polycystina : A, Poiloryrtis Schoin- 

 burgkii', B, TXhopalocanium ornatnm. 



