n6 STUDIES IN INSECT LIFE, ETC. 



tically these skeletons belonged to organisms on 

 the border-line between animals and plants, and 

 these organisms are so minute that they are 

 only occasionally caught by the finest-meshed 

 nets that sweep the surface of the sea. So minute 

 are they, that to separate them from the sea- 

 water we have to use either an ultra-fine filter 

 or an apparatus known as the centrifuge, which, 

 whirling round at a great rate, is able to drive 

 the solid matter suspended in the water to the 

 furthest limits of the bottom of two rotating 

 test-tubes. In the " Science of the Sea " it is 

 stated that " only the waters of the Mediterranean 

 have been investigated for these forms by methods 

 other than the ordinary tow-nets : it would be 

 of great interest to discover their distribution in 

 oceanic waters generally/' This Dr. Gran has done. 

 On the " Challenger " Sir John Murray used 

 to obtain these microscopic creatures, now known 

 as the Coccolithophoridse, by allowing a glass of 

 sea-water to stand for some hours, when the 

 organisms sank and attached themselves to fine 

 threads placed at the bottom of the tumbler. 

 As we have seen on more than one occasion, 

 organisms of the minutest size have been also 

 discovered by examining Nature's own filter, the 



