REPORT ON THE RADIOLARIA. XXXIII 



either becomes itself the nucleus of a swarm-spore, or by repeated division gives rise to 

 a group of spore-nuclei. All those Radiolaria which are uninuclear during the greater 

 part of their existence, and in which the process of division is late, and takes place rapidly, 

 are called " serotinous " or late-dividing forms. To this category belong all Ph^odaria and 

 Nassellaria, as well as all the solitary or monozoic Spumellaria and some Acahtharia. 

 On the other hand, the name " precocious," or early dividing, is applied to those Eadio- 

 laria in which the division of the nucleus takes place very early, and in which, therefore, 

 the cell is multinuclear during the gi-eater part of its existence. This is the case in all 

 the social or polyzootic Radiolaria (Polycyttaria, Pis. 3-8), and also in the great majority 

 of the AcANTHARiA, botli Acanthometra and Acanthophracta. In the last 

 two groups, however, there are numerous exceptions, and these are seen in remarkably 

 large species, characterised by the great size of the central capsule. From a phylogenetic 

 point of view, the conclusion is allowable that the precocious forms are secondary, and 

 have arisen by adaptive modification from the primitive serotinous stem. In the 

 Polycyttaria (or social Spumellaria, i.e., the three families Collozoida, Sphserozoida, and 

 Collosphserida), the cause of the adaptation lies most probably in the formation of the 

 colony itself, for all these three families are so closely related to three corresponding 

 families of serotinous, monozootic Radiolaria (Thalassicollida, Thallassosphserida, Ethmo- 

 sphserida), that certain species of the latter are hardly to be distinguished from isolated 

 individuals of the former. Perhaps the remarkable formation of the large central oil- 

 globule, which particularly characterises the Polycyttaria, is the prime cause of their 

 early nuclear division. In the Acantharia the cause is most likely to be found in the 

 characteristic centrogenous development of their acanthin skeleton, whose radial bars first 

 of all appear in the centre of the capsule. Hence arises directly the excentric position of 

 the nucleus, which in the archaic stem of Acantharia {Actissa f) was prol^ably central. 

 In any case, but little weight is to be laid upon the precocious division of the 

 nucleus in the Acantharia in general, inasmuch as in certain species (both Acantho- 

 metra and Acanthophracta) the more usual serotinous division persists. 



64. Central and Excentric Nuclei. — The position of the nucleus in the interior of 

 the central ca^^sule was no doubt primitively central, and this situation in the geometrical 

 centre of the original spherical central capsule has been accurately retained in all mono- 

 zootic Spumellaria ; in the polyzootic families of this legion (Polycyttaria), on the 

 contrary, it is obscured by the precocious division of the nucleus. In the other three 

 legions, which may be phylogenetically derived from the Spumellaria, the position of 

 the nucleus is rarely central, but usually excentric, or at most subcentral. In the 

 Acantharia (both Acanthometra and Acanthophracta) the central position of 

 the nucleus is at once excluded by the constantly centrogenous development of the 

 skeleton ; the nucleus is therefore always excentric, and may lie at either side ; it usually 



(ZOOL. CHALL. EXP. PART XL. — 1886.) Ere 



