REPORT ON THE RADIOLARIA. xcix 



the ground-form of the skeleton either polyaxon or isopolar monaxon, two fundamental 

 and variously combined directions of growth are recognisable ; firstly, the concentric 

 growth (equal increase of volume in all directions), and secondly, multipolar or diametral 

 growth (hypertrophy of certain parts in the direction of definite pairs of radii). A 

 different state of things obtains, however, for the most part, in the two legions of the 

 Osculosa (NASSELLARIA and PH/EODARIA), in which the central capsule possesses a vertical 

 main axis with different poles, and the structure of the skeleton is determined by this 

 allopolar monaxon ground-form. The two fundamental directions of growth here com- 

 bined in the most various ways are, firstly, unipolar growth (starting from the basal pole 

 of the vertical main axis), and secondly, radial or pyramidal growth (characterised by 

 the different development of separate parts in the direction of definite radii). Whilst 

 the growth of the malacoma is dependent on intussusception (as in most organic structures 

 capable of imbibing), the growth of the skeleton in all Radiolaria takes place by apposition 

 (see note B). 



A. The earliest investigations into the modes of growth in the Radiolaria are due to J. Miiller 

 (L. N. 12, pp. 21-33). More detailed communications I gave myself in my Monograph (L. N. 16, 

 pp. 150159). The relations there sketched have now, in consequence of the examination of the 

 Challenger collection, undergone many important additions, and in some divisions, important modifi- 

 cations ; these are for the most part treated of in the general account of the separate families. 



B. The view here maintained, that the skeleton of all Eadiolaria grows only by apposition, 

 appeared formerly to have certain exceptions. I thought I had shown that in Ccdodendrum the 

 thin-walled tubes grew not only in length but also in thickness, with continuous increase in the 

 lumen (L. N. 16, pp. 152, 360). Further K. Brandt concluded, from the varying size of the median 

 bars in the twin-spicules of Sphcerozoum, that these siliceous structures grow by intussusception 

 (L. N. 38, p. 401). Both suppositions have been proved erroneous, and I have come to the 

 opinion that in all Eadiolaria the skeleton grows by apposition. 



151. Regeneration. Whilst the general course of individual development (perhaps 

 without any exception in the Radiolaria), begins with the formation of zoospores in the 

 central capsule, there yet occurs in some groups a different form of ontogeny, introduced 

 by simple division of the unicellular organism, and coming under the term " regeneration " 

 in its wider sense. This spontaneous division occurs quite commonly in the Polycyttaria 

 (or social SPUMELLARIA), and produces their colonies (compare the chapter on Reproduc- 

 tion, 273). On the contrary, it has not been observed in the solitary SPUMELLARIA, nor in 

 the ACANTHARIA and NASSELLARIA ; possibly, however, the peculiar ACANTHARIAN family, 

 Litholophida, has arisen by the division of Acanthonida (compare p. ' 734). Among the 

 PH^EODARIA division is commonly observed in the order Phaeocystina (which have 

 an incomplete Beloid skeleton or none), and also in the Phaeoconchia. In all these 

 cases the increase by division is nothing else than an ordinary case of cell-division, in 

 which bisection of the nucleus precedes that of the central capsule. The regeneration by 



