THE PELAGIC FAUNA 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 direc- 
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 J ohannes Müller 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 
