The s () of s repeat the essentials of kidney form and function, tubules closely associated with blood and body fluid, each one a guardian of the con… - Ann Haven Morgan

... Darwin ... became the official naturalist on the five-year voyage of the "" (1831–1836) ...
... He also read the essay in which Malthus told of the human populations that became too large for the space available to them (1838). This suggested a plan. Some organism must be winnowed out by their natural surroundings; thus, others would be benefited. There would be natural selection.

es posses much more beauty and interest than their reputation credits them with. Most of them are marked with concealing colors and patterns, browns, greens, and blacks, picturing upon them the broken shadows and water-soaked leaves of their natural background and hiding them in it. They are sensitive to the slightest vibration of the water, to shadows passing over them, and to small changes in the water around them. Their whole set up is one of exquisite efficiency for their mode of living. … The external features most essential to a leech are the strong muscular suckers at each end of its body and the sucking mouth which which may or may not be armed with jaws … Leeches are segmented worms like bristleworms and common earthworms and belong to the Phylum .

s are transparent microscopic animals which live in fresh or salt water. They abound in the surface waters of great lakes, and swarm through the shallows of ponds and bogs; there is scarcely any stand of soft water, whether transient puddle or rain-barrel or fountain-basin, where rotifers can not be found. They live in ponds and lakes, providing a large part of the food for small s and worms and are thus indirectly a large source of food for fishes.