The most interesting fixed messmates are those cirripeds or barnacles which, under the names of Coronula and Tubicinella (Figs. 3 and 4), cover the skins of whales. They are, like all the rest, free while young, but later they take shelter on the back or on the head of one of these huge cetaceans, and, having once chosen their abode, are after-ward permanent tenants. Each whale lodges a particular species, and
Fig. 3.—Coronet Barnacle (Coronula diadema).
the manatee, marine turtles, and various sea-snakes, have also their different sorts. Others establish themselves on their own immediate relations and on other crustaceans. A pretty genus found near Cape Verd, living on the carapace of a large lobster, spreads itself over the
Fig. 4.—Burrowing Barnacle (Tubicinella trachealis).
centre of the lobster's back, and looks not unlike a bouquet of flowers. Fig. 5 shows a fixed messmate attached to a sertularian.
Mutualists, as the name suggests, are animals which live on each other; and, though usually confounded with messmates and parasites, they differ from both in making some sort of return for benefits ob-
