Commensalism

[Literally, "at the same table."] (Biol.) A term indicating the close, but not parasitic, a ociation of two organisms, for the benefit of one or both. Numerous examples of this a ociation have been observed in the animal and plant world. There are many accidental combinations, of no special advantage to either side, as when a sea shell is covered externally by sponges, worm tubes, acorn shells, and the like. But in cases where a crab is covered by such organisms the advantage is mutual, the crab being masked and able to approach its prey unobserved, while the sedentary organisms are benefitted by being carried about. There is a higher grade of commensalism where the partnership is deliberate, the bearer of the external form aiding the operation. This is seen in the case of certain hermit crabs, which carry about sea anemones in the sea-shells they inhabit. When these crabs change their homes, leaving one shell to take upa residence in another, they make efforts to induce the anemones to also change their place of residence, and are ill at ease until provided with partners. Here the stinging powers of the anemone are a benefit to the crab, while they, in a measure, divide the spoil between them. In other cases commensals are a ociated without being attached. Small fishes are sometimes found habitually dwelling within sea anemones, while little crabs make their homes within bivalves or sponges, and many worm types are found in constant free a ociation with other animals. So many of the insects which frequent plants are commensals, not feeding on the plant but on other visitors. They, in some cases, protect the plant from injury by other forms. Commensalism differs both from parasitism, in which one organism preys upon the other, and symbiosis, in which the partnership is much more intimate, as when an alga and a fungus combine to form a lichen, or algæ are internal a ociates of certain worms and other animals. Com'mentry, a mining town of France, dept. Allier, on the Œuil, 8 m. S.E. of Montlucon. It is seated in the midst of one of the largest of the French coal-fields. Pop . (1895) 9,316.