Another kind of parasitic adaptation is technically termed host manipulation. By subtle chemical influence a parasite may gain some control over the machinery of the host’s body and cause that machin- ery to serve the interests of parasite rather than host. Many curious examples are known from many groups of organisms. The tobacco mosaic virus causes its host to enlarge the pores between adjacent tobacco cells enough to allow the virus particles to pass through and infect other cells. One kind of parasitic worm alternates its life stages between ants and sheep, just as malarial parasites must alternate between vertebrate hosts and mosquitoes. The worm is effectively transmitted from an ant to a sheep because it enters certain sites in the ant’s nervous system where it causes the ant to climb to the top of a blade of grass and hang on, unable to let go. This greatly increases the likelihood that the ant will be eaten by a sheep. Another kind of worm alternates between snails and gulls. It causes the snail, which is ordinarily hard to find in the tangled growths of shallow coastal waters, to crawl up to a high level of bare rock or sand and stay there. It is then easily seen and eaten by a gull.
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