animals and
plants are abundant, and sometimes by their numbers colour great areas
of water; or, as in the drift of the Gulf Stream, make a tangle of
animal and plant life through which a boat travels only with
difficulty. The basis of the food supply of this vast and hungry
floating life is, as on land, vegetable life; for plants are the only
creatures capable of building up food from the gases of the air and
the simple chemical salts found dissolved in water. Occasionally, in
shallow or warm seas, marine floating plants, large and visible like
the sea-weeds of the coast, form the floating masses known as Sargasso
seas; more often the plants are minute, microscopic specks visible
only when a drop of water is placed under the microscope, but
occurring in incredible numbers, and, like the green vegetation of the
earth, forming the ultimate food-supply of all the living things
around them. Innumerable animals, great and small, live on the plants
or upon their fellows, and, however far he may be from land, the
naturalist has always abundant material got by his daily use of the
tow-net. This drifting population floats at the mercy of the waves.
Most of the animals are delicate, transparent creatures, their
transparency helping to protect them from the attacks of hungry
fellows. Nerves, muscles, skin, and the organs generally are clear,
pale, and hardly visible. Such structures as the liver, the
reproductive organs, and the stomach, which cannot easily become
transparent, are grouped together into small knots, coloured brown
like little masses of sea-weed. Other floating creatures are vividly
coloured, but the hues are bright blues and greens closely similar to
the sparkling tints of sea-water in sunlight. The different members of
this marine flotsam frequently rise and fall periodically: some of
them sinking by day to escape the light, others rising only by day;
others, again, appearing on the surface in spring, keeping deep down
in winter. Perhaps the majority of them are phosphorescent, sometimes
shining by their own light, sometimes borrowing a glory from
innumerable phosphorescent bacteria with which they are infested.
Nearly every class of the animal kingdom contributes members to this
strange population. The young forms of many fish, as for instance of
conger, flying gurnards, and some flatfish, are pelagic and have
colourless blood, and pale, transparent, gelatinous or cartilaginous
skeletons. The tadpole-like stages