… Wherever animals live, in fresh water, salt water, or on land, their body fluids are similar; all are salty. In marine invertebrates, whether es or… - Ann Haven Morgan

At Kartabo the waters of the meet those of the and soon join the great lake-like flowing north through the low lying country of British Guiana until it runs into the sea at . These rivers are hedged in by the jungle whose undergrowth has invaded the border waters in a persistent attempt to gain more territory. roots swing so far out from the banks that fish swim in and out among them. Moccamoccas, the giant s, have grown out still further. Their clublike stems standing in close rank provide on gigantic scale the kind of animal shelter furnished in more modest form by our own and s. Many slow flowing creeks feed into these larger streams but their mouths are hidden by creepers, tangles of mangrove, and prickly shrubs, and if the currents are gentle enough they are choked by lush growths of .
… Even at Kartabo Point the daily tides are insistent reminders of the ocean forty miles away.The low tide lay bare stretches of muck and ooze, silt and silty sand ... the likely dwelling place of the burrowing and crawling Mayflies ...

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 .

... 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.