XC1V THE VOYAGE OF H.M.S. CHALLENGER. 



of the small roundish or ovoid cells protrudes one or more vibratile flagella. The 

 fully developed spores, which commence their vibrations even within the central capsule, 

 emerge when it ruptures, and swim about freely in the surrounding water by means of 

 the flagellum. At this stage of its existence the young Radiolarian represents essentially 

 the simplest form of the Flagellata, such as Astasia or Euglena ; the unicellular body is 

 for the most part ovoid or subcylindrical, sometimes fusiform or reniform, usually from 

 0-004 to O'OOS mm. in diameter (PL 1, fig. Ic ; PL 129, fig. 11). In the anterior part 

 of the flagellate cell, immediately behind the base of the flagellum, lies a homogeneous, 

 spherical nucleus, whilst in the posterior part are usually several small fat-granules 

 and often also a small oblong crystal (hence the name " crystal-spore," " Krystall- 

 Sch warmer "). The number of vibrating flagella, which are extremely long and fine, seems 

 to be variable, usually one, sometimes two, occasionally perhaps three, or even four or 

 more (see note B). 



A. The formation of the motile spores in the central capsule was first observed by J. Muller in 

 Acanthometra (1856, L. N. 10, p. 502), then by A. Schneider in Thalassicolla (1858, L. N. 13, p. 41), 

 and finally by myself in Sphccrozoum (1859, L. N. 16, p. 141). These older observations were, 

 however, incomplete, for the origin of the motile corpuscles from the contents of the central capsule 

 was not observed. The first complete and detailed observations upon the formation of spores in 

 the Kadiolaria were published in 1871 by Cienkovvski (L. N. 22, p. 372, Taf. xxix.); they relate to 

 two different Polycyttaria, Collosphccra and Collozoum. These investigations were supplemented by 

 E. Hertwig on Collozoum and Thalassicolla (1876, L. N. 26, pp. 28, 43, &c.); on Collozoum he made the 

 important discovery that the Polycyttaria form two kinds of spores, one with and the other without 

 crystals, and that the latter are divided into macrospores and microspores (compare the chapter on 

 " Eeproduction," 212-216). Quite recently Karl Brandt has confirmed these observations, and 

 has extended them to all the genera of Polycyttaria (1881, L. N. 38, p. 393, and 1885, loc. cit.). 



B. The number of flagella, projecting from each spore, is very difficult to determine, owing to 

 their extraordinary length and slenderness. It appeared to me that in the majority of those 

 Eadiolaria whose spores I investigated only a single flagellum could be demonstrated with certainty, 

 although sometimes two, springing from a common base, seemed to be present. Compare the 

 chapter on " Eeproduction," ( 215) and the recent work of Karl Brandt on Sphseozoea (1885, 

 L. N. 52, pp. 145-174). 



143. The Actinophrys- Stage. The fate of the flagellate zoospores which emerge 

 from the mature central capsule of the Eadiolaria has not hitherto been decided by actual 

 observation ; all attempts to rear the swarming zoospores have been in vain, for they 

 have soon died. From what we know, however, of the comparative morphology of the 

 Protista, the hypothesis is fully justified, that between the Astasia-stage of the flagellate 

 swarm-spores, and the well-known Actissa-stage of the simplest Radiolaria, there lies an 

 intermediate developmental stage, which may be regarded as being essentially the 

 simplest Heliozoan form, Actinophrys or Heterophrys. The swarm-spore is very pro- 

 bably converted directlyin to a simple floating Heliozoon by its elongated or ovoid body 



