REPORT ON THE RADIOLARIA. cxlvi 



live in the Baltic ; I found their skeletons in the alimentary canal of Aurdia, Ascidians and 

 Copepods. 



D. The so-called " fresh-water Radiolaria," which have been described by Focke, Greeff, 

 Grenacher and others, are all Heliozoa, without either central capsule or calymma. 



227. Local Distribution. — ^As regards tlieir local distribution and its boundaries the 

 Radiolaria sliow in general the same relations as other pelagic animals. Since they are 

 only to a very slight extent, if at all, capable of active horizontal locomotion, the 

 dispersion of the different species from their point of development (or " centre of creation ") 

 is dependent upon oceanic currents, the play of winds and waves and all the accidental 

 causes which influence the transport of pelagic animals in general. These passive 

 migrations are here, however, as always, of the greatest significance, and bring about the 

 wide distribution of individual species in a far higher degree than any active wanderings 

 could do. Any one who has ever followed a stream of pelagic animals for hours and seen 

 how millions of creatures closely packed together are in a short time carried along for 

 miles by such a current, will be in no danger of underestimating the enormous importance 

 of marine currents in the passive migration of the fauna of the sea. Such constant 

 currents may, however, be recognised both near the bottom of the sea and at various 

 depths, as well as at the surface, and are therefore of just as much significance for the 

 abyssal and zonarial as for the pelagic Radiolaria. It is easy to explain by this means 

 how it is that so many animals of this class (jirobably indeed the great majority) have a 

 wide range of distribution. The number of cosmopolitan species which live in the 

 Pacific, Atlantic and Indian Oceans is already relatively large. In each of these three 

 great ocean basins, too, many species show a wide distribution. On the other hand, there 

 are very many species which are hitherto known only from one locality, and probably 

 many small local faunas exist, characterised by the special development of particular 

 groups. The observations which we at present possess are too incomplete, and the rich 

 material of the Challenger is too incompletely worked out, to enable any definite 

 conclusions to be drawn regarding the local distribution of Radiolaria. 



The statements made in the systematic portion of this Eeport regarding the distribution of the 

 Challenger Eadiolaria are very incomplete. In most cases only one locality is mentioned, and that 

 is the station (§ 240) in the preparations or bottom deposit from which I first found the species in 

 question. Afterwards I often found the same species again in one or more additional stations (not 

 seldom in numerous preparations both from the Pacific and Atlantic), without the possibility of 

 adding them to the habitat recorded under the description. The necessary accurate determination 

 and identification of the species (measuring the different dimensions, counting the pores, &c.), would 

 have occupied too much time, and the "writing of this extensive Eeport would have lasted not ten 

 but twenty or thirty years. 



228. Horizontal Distribution. — From the extensive collections of the Challenger and 

 from the- other collections which have furnished a welcome supplement to them, it appears 



