THE PELAGIC FAUXA AND FLORA. 213 



the particular spot best adapted to conceal them ; and in a 

 few minutes only the practised eye of the naturalist can detect 

 their presence. Of course they do not all succeed in their first 

 attempt at finding the fittest sheltering-place, and an occasional 

 striking contrast formed by a white checkered crab upon a dark 

 background, or some other equally great opposition, will reveal 

 the presence of a straggler. But these stragglers quickly cor- 

 rect their error. 



The Sargasso Sea of the North Atlantic covers a rather in- 

 definite area between 22° and 36° of north latitude, and, accord- 

 ing to the statements of the older navigators, the amount of 

 Sargassum to be met with varies from occasional patches to 

 masses large enough to impede the progress of sailing-vessels. 

 The Sargasso Sea was seen by Columbus, and alarmed his com- 

 panions, who looked upon it as an insuperable obstacle to their 

 expedition. The Sargassum probably changes its position con- 

 stantly, according to the seasons, the currents, and the dii'ec- 

 tion of the wind ; but within the area bounded by the Gulf 

 Stream on the west, the equatorial current on the south, and 

 the return current from the Azores and Canaries, the Sargas- 

 sum has always been found in larger or smaller quantities. 



We are perhaps justified in considering the great banks of 

 Sargassum in the Atlantic, Pacific, and Southern Oceans as 

 survivors of a single bank, which probably swept round the 

 globe in the great equatorial current of pre-tertiary times. The 

 origin of this ancient bank may be due to the littoral species 

 of Sargassum still living to-day in the track of its former 

 path. 



Some of the pelagic animals we have spoken of are specially 

 interesting from a physiological point of view, owing to their 

 association with vegetable organisms. As early as 1851 Max 

 Schultze noticed the presence of chlorophyll in planarians, in 

 some infusoria, in fresh-water sponges, and in a few worms and 

 Crustacea. 



Peculiar yellow cells were observed by Johannes Miiller in 

 radiolarians, and were at first supposed by him to be connected 

 with their reproduction, while Haeckel, who detected the pre- 

 sence of starch in these cells, considered them as digestive 



