Page:Popular Science Monthly Volume 8.djvu/692

672 labor, prefer to install themselves in the mouth of a neighbor, and take tithes of the morsels which he swallows.

The little crab that makes its abode within the shell of the edible oyster (Fig. 2) is a true messmate, and the oyster is but one of many bivalve mollusks that give shelter and partial support to these diminutive crustaceans. These crabs, called by naturalists Pinnotheres, though in one sense dependents, are at the same time of great service to the animals within whose shells they receive protection. Van Beneden says of them: "The pinnothere is a brigand who causes himself to be followed by the cavern which he inhabits, and which opens only at a well-known watchword. The association redounds to



the advantage of both; the remains of food which the pinnothere abandons are seized upon by the mollusk. It is the rich man who installs himself in the dwelling of the poor, and enables him to participate in all the advantages of his position. The pinnotheres are, in our opinion, true messmates. They take their food in the same waters as their fellow-lodgers, and the crumbs of the rapacious crabs are doubtless not lost in the mouth of the peaceful mussel.... Little as they are, these crabs are well furnished with tackle and advantageously placed to carry on their fishery in every season; concealed in the bottom of their living dwelling-place, they choose admirably the moment to rush out to the attack, and always fall on their enemy unawares. Some pinnotheres live in all seas, and inhabit a great number of bivalve mollusks."

In the examples thus far cited, and in many more that have been observed, the dependent forms are free to depart whenever they choose, and are therefore called free messmates. Though for a time giving up their liberty, they sooner or later resume it, in possession of all their organs for fishing and locomotion, and in all respects fitted to live an independent life. There are others, however, that enter into the same sort of association, and make the relation a permanent one: these are known as fixed messmates. They are free in their youth, but, as maturity approaches, and the cares of a family are thought of, a host is selected in which they establish themselves, and, throwing aside their fishing and locomotive apparatus, they renounce the world, and even part with the most precious organs of animal life, not excepting those of the senses.