COMMENSALISM, the intimate connec tion or partnership between animals of quite different affinities; thus commensals, messmate.s, or fellow-boarders take up their abode to gether for their mutual benefit. A good ex ample is a large sponge, whose canals and pas sages shelter innumerable worms, crustacea, etc., which lodge there without expense to their host. Floating jellyfish shelter certain pelagic crustaceans (Hyperina) and little fishes. The oyster-crab (Pinnoiheres) is a typical case of commensalism; it does no hartn to the oyster or mussel in whose shell it lives, and which protects it from danger; while, as the result of its sheltered life, its shell is soft and thin. The pearl-oyster, besides taking in a Pia notheres as a boarder, admits a kind of shrimp. Certain small slender, eel-like fishes (Fieras fer) insir.uate 'themselves into the body of holothurians, whic-h also keeps open house for Pinnotheres and shrimps; hence a holothurian has been compared to a hotel with its table d'hote. Other crustaceans board in different animals; thus a little crab (Fabia chilensis) lodges in the cnd of the intestine of a sea urchin ; another (Porcellana) lives on the Brazilian coast in a starfish. Polyps and corals shelter various species of crabs, snails, etc., all being of the same color, while a crinoid (Comatula) takes in as a pertnanent lodger a decapod crustacean (Galathea). Hermit crabs, taking .up their abode in an empty snaiT-shell, are obliged to admit a variety of intruders who come to stay. Certain mollusks live in star fishes and other echinoderms and, as the result of their .semi-parasitic life, become more or
1Fss modified and degenerate. Thus Stylina lives on a crinoid (Comatala); a species of Stilifer becomes encysted on the rays of a starfish (Linckia), and on the underside of the arms of the same Linekia lives a limpet-like snail (Thyca), while S. astericola lives in the body of a Bornean starfish, and so on with a number of similar cases. Ascidians throw their *front hall* (atrium) wide open to a variety of forrns, such as small worms, polyps, mollusks, crustaceans of different orders, sea spiders, brittle-stars. The *Venus flower basket," a silicious sponge of the Philippine Islands, gives shelter to three different kinds of crustaceans: a prawn, a Pinnotheres and an isopod.
Rising to the higher animals there occurs on an island off the New Zealand coast the case of the interesting lizard fftuatara,2' which shares its deep burrows with a petrel, though the latter may at times be the work of the bird. Each builds its nest on opposite sides of the chamber, the lizard almost invariably choos ing the right and the petrel the left side. The former sits with its head close to the entrance ready for any attack; it feeds partly on worms and beetles, and in part on the remains of fishes and crustaceans brought to their com mon table by the petrel.
By far the most numerous assemblage of mesamates are the different lcinds of beetles and other insects which live in ants' nests, the number of kinds of which amotmt to upward of .1,500. See ANT ; COCKROACH ; SYMBIOSIS.