. Biologia animale. Zoologia; biologia. 524 CONSIDERAZIONI GENERALI 566. Simbiosi.-Se simbiosi deve essere chiaramente separato dal mutu- alism, la separazione deve essere sulla base di massima intimità e il carattere vitale dell'associazione. Essa è essenzialmente una forma estrema di mutuahsm. Un caso di simbiosi si è notato nel rapporto che esiste tra un hydra e una alga verde (Sec. 169), le cellule dell'alga che vive una vita simbiotica in quelli dell'organismo animale e arredamento di ossigeno per l'Idra nel ritorno per il proprio cibo. Altri casi analoghi sono noti. Simbiosi è rappresentato anche dall't
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. Animal biology. Zoology; Biology. 524 GENERAL CONSIDERATIONS 566. Symbiosis.—If symbiosis is to be clearly separated from mutu- alism, the separation must be on the basis of maximum intimacy and the vital nature of the association. It is essentially an extreme form of mutuahsm. One case of symbiosis has been noted in the relationship which exists between a hydra and a green alga (Sec. 169), the cells of the alga living a symbiotic life in those of the animal organism and furnishing oxygen to the hydra in return for its own food. Other similar cases are known. Symbiosis is also shown by the termite and the protozoan symbiont which lives in its intestine (Sec. 319). It was long a matter of speculation as to how termites are able to digest the cellulose of the wood. Fig. 365.—Hermit crab in a snail shell, which also bears two sea anemones. Con- sidered by some as an illustration of commensalism, but referred to here as one of mutualism. From a preserved specimen. X %. on which they feed, since other insects are not known to have this ability, Cleveland has recently discovered that this is due to the presence in the intestine of the termite of a protozoan which prepares the wood for digestion and absorption by the insect. In the absence of the protozoan the termite is unable to use this food. On the other hand, the protozoan finds appropriate conditions for existence only in the intestine of the termite, and thus the association is vital to both. 567. Parasitism.—The associations of organisms of different species which have so far been defined all involve benefit to one or both but injury to neither. If injury is done to one, then the association becomes either one of parasitism or predatism. Parasitism has already been defined as the association of two organisms of different species in which one, termed the parasite, lives at the expense. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability -
