EECENT BESEARCHES ON THE RADIOLARIA. 171. 



also inhabit the deeper parts of the ocean. Haeckel, when off 

 the coast of Sicily, found them disappear from the surface after 

 much disturbance of the water by wind, after prolonged rain 

 (though they do not seem much disturbed by a small quantity of 

 it), or after many days of the sirocco. They seem to be particu- 

 larly intolerant of dirty water, and delight in a smooth sea and 

 pure transparent water at a moderately warm temperature. The 

 voyage of the ' Challenger,' however, has convinced Sir C. Wyville 

 Thomson* that though Foraminifera are apparently confined to a 

 comparatively superficial stratum the Eadiolariaus exist at all 

 depths ; and he tells us, " in the deposit at the bottom, species 

 occur which have been detected neither on the surface nor at 1000 

 fathoms, the greatest depth at which the tow-net has been sys- 

 tematically used ; and specimens taken from near the bottom of 

 species which occur on or near the surface give us the impression 

 of being generally larger and better developed." j 



As to their geographical distribution, the same author tells us t, 

 " Eadiolarians were met with throughout the whole of the Atlantic, 

 and often in great abundance, the sea being not unfrequently 

 slightly discoloured by them. The forms which appeared in such 

 numbers were usually species of the Acanthometridcd, but Polycys- 

 tina and the compound genera were also numerous. The remains 

 of Eadiolarians were found in all deep-sea deposits, usually in very 

 direct proportion to the numbers occurring on the surface and in 

 intermediate water. It was frequently observed, however, that 

 when, in deep water, certain species swarmed on the surface, very 

 few of their skeletons could be detected at the bottom. This 

 applies especially to Acanfhomefridce, and is probably owing to 

 the extreme tenuity of the siliceous wall of their radiating spi- 

 cules, which may admit of their being dissolved while sinking 

 to a great depth ; or probably the spicules may never become 

 thoroughly silicified, but may retain permanently more or less the 

 condition of acanthin. The Polycystina seem much less destruc- 

 tible, and occur in abundance on the bottom at the greatest depths. 

 Although the Eadiolaria are universally distributed, like the 

 Diatoms, but in a less marked degree, they seem to be most nu- 

 merous when the specific gravity of the water is low ; they spe- 

 cially swarm in the warm and comparatively still region of the 

 south-western Pacific and among the islands of the Malay archi- 



» 'Voyage of the Challenger,' 1877, vol. i. p. 236. 

 t L. c. vol. ii. p. 340. 



12* 



