112 



THE MICROSCOPE AND ITS KEVELATIO.NS. 



made-out. The emission of flagellated zoospores, very similar to those 

 of Clatlirulina (Fig. 288), has been observed in many Kadiolarians; but 

 of the mode in which they are produced, and of their subsequent history, 

 very little is at present known. Until the structure and life-history of 

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

 cidated, no satisfactory classification of them can be framed; and nothing 

 more will be here attempted than to indicate some of the principal forms 

 under which the Radiolarian type presents itself. 



502. Discida. Among the beautiful siliceous structures which are met 

 with in the Radiolarian sandstone of Barbadoes (Fig. 345) there is none 

 more interesting than the skeleton of Astromma (Fig. 340) ; in which we 

 have a remarkable example of the range of variation that is compatible 

 with conformity to a general plan of structure. As in other forms of 

 Haeckel's group of Discida, there is in this skeleton a combination of 



FIG. 346 



Varietal modifications of Astromma. 



radial and of circumferential parts; the former consisting of solid spoke- 

 like rods, whilst the latter is composed of a siliceous network more or 

 less completely filling up the spaces between the rays. The radial part 

 of the skeleton predominates in the beautiful 4-rayed example represented 

 at D, having the form of a Maltese cross; 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 5-rayed specimens 

 A and B, on the other hand, the radial portion is much less developed, 

 whilst the circumferential becomes more discoidal. And in c and E, 

 while the circumferential network forms a pentagonal disk, the radial 

 portion is represented only by solid projections at its angles. The transi- 

 tion 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 the difference in the number of rays is probably 



