THE RADIOLARIA 



in 



be an early stage in species of the two families Thalassophysidae 

 and Physematiidae. Even the presence of developmental stages is not 

 decisive proof that the fertile protoplast or coenobium in question 

 is a final stage in the life-history, since in certain forms 1 an early 

 and variable reproductive stage is intercalated between the earliest 

 phase and that of full growth. Fission introduces further com- 

 plexities. The Acantharian genus Litlioloplms was founded on 

 stages of growth or fission products belonging to other genera ; and 

 the division of the Collozoidae by fission leads to minute forms 

 that might easily be mistaken for young stages, although they are 

 reproductive individuals. We are thus led to the conclusion that 



FIG. 8. 



Racial dimorphism in Aulacantha scolymantha, x 26. (After Hacker.) A, deep-sea form ; 13, 

 pelagic form from Naples, 100 fathoms. C.c, central capsule ; Exo, ectoplasm ; Pk, phaeodium ; 

 R, radial spicules ; Tf, tangential spicules. 



a knowledge of the life-history is essential to the construction of a 

 permanent classification, and that when this is obtained the species 

 will be groups segregated about their several types. 



The dimorphism of Radiolaria is of two kinds : somatic and 

 gametic. Somatic dimorphism is at present known only in few 

 instances. It consists in the development of a small race of a 

 widely ranging species in warmer surface water, and of a large race 

 (usually three times the size of the former) in cold and deep water. 

 Associated with these differences of size there is structural diversity. 

 The spicules of the small race are fewer and simpler, the ectoplasm 

 they support is delicate and limp, often sagging between the 



1 E.g. Collosphaera (Fig. 15, A). 



