REPORT ON THE RADIOLARIA. eliii 



20 to 30 fathoms (§ 233). The complicated conditious of existence created by the keen 

 struggle for existence at the surface of the sea, give rise to the formation of very 

 numerous pelagic species, especially of Porulosa (Spumellaeia and Acantharia). The 

 abyssal Radiolaria are very different from those just mentioned ; they live at the bottom 

 of the deep-sea, not resting upon nor attached to it, but probably floating at a little 

 distance above it, and are adapted to the conditions of existence which obtain there 

 (§ 235). Here the Osculosa (Nassellaria and Ph.eodaria) seem to predominate. The 

 zonarial Eadiolaria live floating at various depths between the pelagic and aljyssal species 

 (§ 234). In their morphological characters they gradually apj)roach the pelagic forms 

 upwards and the abyssal downwards. 



The views which have hitherto been held regarding the bathymetrical or vertical distribution 

 of the Eadiolaria have been entirely altered by the magnificent discoveries of the Challenger, and 

 especially by the important observations of Sir Wyville Thomson (L. N. 31) and John Murray 

 (L. N. 27). These two distinguished deep-sea explorers have, as the result of their wide experience, 

 been convinced that Eadiolaria exist at all depths of the ocean, and that there are large numbers 

 of true deep-sea species which are never found at the surface of the sea nor at slight depths 

 (L. N. 31, vol. i. pp. 236-238 ; L. K 27, pp. 523, 525). The result of my ten years' work upon 

 the Challenger Eadiolaria, and the comparative study of more than a thousand mountings from all 

 depths, has only been to confirm this opinion, and I am further persuaded that it will some day be 

 possible by the aid of suitable nets (not yet invented) to distinguish different faunistic zones in the 

 various d^ths of the sea. In this connection may be mentioned the specially interesting fact that 

 the species of Eadiolaria of one and the same family present in the different depths characteristic 

 morphological distinctions, which obviously correspond to their different physiological relations in 

 the struggle for existence. Owing to those extensive discoveries, the representation which I gave 

 in my Monograph (1862, L. N. 16, pp. 172-196) of the vertical distribution of the Eadiolaria, and 

 of their life in the greatest depths of the sea, has been entirely changed. Compare also Butschli 

 (L. N. 41, p. 466). 



233. The Pelagic Fauna. — The surface of the open ocean seems everywhere, at a 

 certain distance from the coast at least, to be peopled by crowds of living Radiolaria. 

 In the tropical zone these pelagic crowds consist of many difiierent species, whilst in the 

 frigid zones, on the other hand, they are made up of many individuals belonging to but 

 few species. Most of these inhabitants of the surface may be regarded as truly pelagic 

 species, which either remain always at the surface or descend only very slightly below it. 

 Probably most Porulosa (both Spumellaria and Acantharia) belong to this group; 

 whilst but few Osculosa occur in it, and fewer Ph^odaria than Nassellaria. In 

 general the pelagic Radiolaria are distinguished from the abyssal by the more delicate 

 and slender structure of their skeletons ; the pores of the lattice-shells are larger, the 

 intervening trabecute thinner; the armature of spines, spathillse, anchors, &c., is more 

 various and more highly developed. Numerous forms are to be found among the pelagic 



(ZOOL. CHALL. EXP. — PART XL. — 1887.) Er U 



