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28 29 Parasitism and symBiosis the human question Many organisms develop relationships with others, either parasitic or symbiotic. In parasitic relationships only one species benefits from the arrangement, while symbiotic relationships suit both or all parties. Lice, fleas and worms, for example, afford no advantage to their hosts, but gain resources themselves, while the bacteria in our stomach help us digest, and get fed in the process. Although some deadly bacteria and viruses below seem to have a loselose dynamic with their hosts, they aim to infect new victims before both die. Some symbiotic relationships are so involved that they result in composite organisms. An example of an animalanimal composite is the Portuguese manofwar, which looks like a jellyfish. The components of its body are actually different species of organisms working in a cooperative colony. An example of a plantplant composite is a lichen, which is part alga and part fungus. There are also animalplant composites, such as upsidedown medusas top left, page 27, which are jellyfish that contain colonies of algae. Mankind may be increasingly likened to a parasite with respect to most life on Earth. Meanwhile, we are in a symbiotic relationship with daffodils, apple trees, dogs, cows, chickens, grasses and a few other species, all of which thrive at the expense of the many. Above, and top right page 27 Various species of lichen. Lichens are composite organisms consisting of a symbiosis between fungi and photosynthetic partners which derive food for the lichen from sunlight. Left A Portuguese manofwar. This is a siphonophore, a colony of specialised polyps and medusoids. Below The household flea. An example of a parasite, bringing no gain to its host, simply feeding off it.
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