REPORT ON THE RADIOLARIA. CXXXV 



may be explained as protective coverings, the radial spines as defensive weapons, and 

 the anchor-hooks and spathillse as organs of prehension, which are of advantage to their 

 possessors in the struggle for existence; the regular arrangement of the radial spines 

 in the Radiolaria may also be explained on hydrostatic grounds, it being advantageous 

 that the body should be maintained in a definite position of equilibrium, &c. The 

 well-known laws of direct or actual adaj^tation, which we designate cumulative, corre- 

 lative, divergent adaptation, &c., here explain a multitude of morphological phenomena. 

 The connection is less distinct in the case of the laws of indirect or 2^otential adapta- 

 tion, although this must play as important a part in the formation of the Radiolaria as 

 in that of other organisms (compare on this head my Generelle Morphologic, Bd. ii. 

 pp. 202-222). 



212. Reproduction. — The most common form of reproduction in the Radiolaria is 

 the formation of spores in the central capsule, which in this respect is to be regarded 

 as a sporangium (§215). In many Radiolaria (Polycyttaria and Ph^odaria), however, 

 there occurs in addition an increase of the unicellular organism by simple division 

 (§ 213); upon this the formation of colonies in the social Radiolaria is dependent (§ 14). 

 Reproduction by gemmation is much less common, and has hitherto been observed only 

 in the Polycyttaria (§214). In this group alone there also occur at certain times two 

 different forms of swarm-spores which copulate, and thus indicate the commencement 

 of sexual reproduction (Alternation of Generations, § 216). The general organ of 

 reproduction is in all cases the central capsule, whilst the extracapsulum never takes an 

 active part in the process. 



213. Cell-Division.- — Increase by cell-division among the Radiolaiia in the early 

 stage, before the formation of the skeleton, is widely distributed (perhaps even general ?); 

 in the adults of this class it is rather rare and limited to certain groups. It is most 

 readily observed in the Polycjrttaria ; the growth of the colonies in this social group 

 depends mainly (and in many species exclusively) upon repeated spontaneous division of 

 the central capsule ; all the individuals of each colony (in so far as this has not arisen 

 by the accidental fusion of two or more colonies) are descendants of a single central 

 capsule, which has arisen from an asexual swarm-spore (§ 215) or from the copulation 

 of two sexual swarm-spores (§ 216). Whilst the central capsules of the colonies continu- 

 ally increase by division, their calymma remains a common gelatinous sheath. Among 

 the Sptjmellaria reproduction by simple cell-division probably occurs also in many 

 monozootic C o 1 1 o d a r i a. Among the Acantharia the peculiar group Litholophida 

 has perhaps arisen by the spontaneous division of Acanthonida (see p. 734). 

 Among the Ph^odaria increase by ceU-division seems to occur commonly in many 

 groups, as in the Phseocystina, which have no skeleton (Phajodinida, PI. 101, 



