112 



THE RADIOLARIA 



FIG. 9. 



Radial spicules of A, abyssal form of Auloscena 

 vertitillatus ; B, pelagic form. (After Hacker.) 



siliceous appendages. The skeleton of the large race ends in more 

 elaborate constructions, and stretches more tightly the tougher, 

 thicker ectoplasm that covers the animal. Such racial dimorphism 



is known in Aulacantha scoly- 

 mantha (Fig. 8), Circoporus sex- 

 fuscinus, in Auloscena verticilla- 

 tus, and probably will be found 

 more commonly when looked 

 for. Both races are capable 

 of reproduction, and it is im- 

 probable that they merge into 

 one another, but it is not 

 known whether the mode of 

 reproduction is the same in 

 both. 



Gametic dimorphism is 

 more general and perhaps uni- 

 versal, but is unaccompanied 

 by any known diversity of 

 somatic structure. It is there- 

 fore comparable with the di- 

 morphism of such Foraminifera 

 as Discorbina and Truncatulina, 

 and is signalised by the formation of isospores and of heterospores 

 in distinct and differently constituted individuals. These processes 

 involve the contents of the central capsule and are followed by the 

 death of the ectoplasm. An individual Radiolarian is therefore only 

 a phase in the life-cycle of its race, but the changes which lead up to 

 the formation of isospores are so distinct from those that precede the 

 development of heterospores, and involve such deep-seated nuclear 

 transformations, that it is difficult to believe that similar individuals 

 of any one generation can give rise to both forms of spore. On 

 this ground Brandt has been led to formulate the view that 

 isosporous and heterosporous individuals of any one species belong 

 to alternate generations. Direct evidence of this alternation has not 

 been obtained, and therefore the case of the Radiolaria is on a very 

 different footing from the observed alternation in Foraminifera. 



Distribution : A, Vertical. The recently published reports of the 

 German Plankton expeditions, though not yet complete, enable us 

 to picture the vertical distribution of the Radiolaria more accur- 

 ately than was formerly possible. The older records were derived 

 from surface townettings and from Ehrenberg's researches on 

 Radiolarian deposits at varying depths. They represented the 

 group as occurring at all depths, even on the sea-bottom, and as 

 increasing in variety with depth. The more recent exploring ex- 

 peditions give a very different result. From them it appears that 



