ence. The termite individual is one of the most defenseless, and, for those animals that prey on insects, one of the most toothsome luxuries to be found in the insect world. But the termite is one of the most abundant and widespread and successfully living insect kinds in all the tropics. Where ants are not, few insects are. The honeybee is a popular type of a successful life. The artificial protection afforded the honey-bee by man may aid in its struggle for existence, but it gains this protection because of certain features of its communal life, and in Nature the honey-bee takes care of itself well. The Little Bee People of Kipling's Jungle Book, who live in great communities in the rocks of Indian hills, can put to rout the largest and fiercest of the jungle animals. Co-operation and mutual aid are among the most important factors which help in the struggle for existence. Its great advantages are, however, in some degree balanced by the fact that mutual help brings mutual dependence. The community or society can accomplish greater things than the solitary individuals, but co-operation limits freedom, and often sacrifices the individual to the whole.

CHAPTER X

COMMENSALISM AND SYMBIOSIS

90. Association between animals of different species.-The living together and mutual help discussed in the last chapter concerned in each instance a single species of animal. All the various members of a pack of wolves or of a community of ants are individuals of the same species. But there are many instances of an association of individuals of different kinds of animals. The number of individuals concerned, however, is usually but two-that is, one of each of the two kinds of animals. In many cases of an association of individuals of different species one kind derives great benefit and the other suffers more or less injury from the association. One kind lives at the expense of the other. This association is called parasitism, and is discussed in the next chapter. In some cases, however, neither kind of animal suffers from the presence of the other. The two live together in harmony and presumably to their mutual advantage. In some cases this mutual advantage is obvious. This kind of association is called commensalism or symbiosis. The term commensalism may be used to denote a condition where the two animals are not so intimately associated nor derive such obvious mutual advantage from the association, as in that condition of very intimate and permanent association with obvious co-operative and marked advantage that may be called symbiosis. A few examples of each of these interesting conditions of association between which it is impossible to make any sharp distinction, will be given.

91. Commensalism.-A curious example of commensalism is afforded by the different species of Remoras (Echenidida) which attach themselves to sharks, barracudas, and other large fishes by means of a sucking disk on the top of the head (Fig. 103). This disk is made by a modification of

the dorsal fin. The Remora thus attached to a shark may be carried about for weeks, leaving its host only to secure food. This is done by a sudden dash through the water. The Remora injures the shark in no way save, perhaps, by the slight check its presence gives to the shark's speed in swimming.

Whales, similarly, often carry barnacles about with them. These barnacles are permanently attached to the skin of the whale just as they would be to a stone or wooden pile. Many small crustaceans, annelids, mollusks, and other invertebrates burrow into the substance of living sponges, not for the purpose of feeding on them, but for shelter. On the other hand, the little boring sponge (Cliona) burrows in the shells of oysters and other bivalves for protection. These are hardly true cases of even that lesser degree of mutually advantageous association which we are calling commensalism. But some species of sponge are never found growing except on the backs or legs of certain crabs." In these cases the sponge, with its many plant-like branches, protects the crab by concealing it from its enemies, while the sponge is benefited by being carried about by the crab to new food supplies. Certain sponges

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and polyps are always found growing in close association, though what the mutual advantage of this association is has not yet been found out.

Among the coral reefs near Thursday Island (between New Guinea and Australia) there lives an enormous kind of sea-anemone or polyp. Individuals of this great polyp measure two feet across the disk when fully expanded. In the interior, the stomach cavity, which communicates freely with the outside by means of the large mouth opening at the free end of the polyp, there may often be found a small fish (Amphiprion percula). That this fish is purposely in the gastral cavity of the polyp is proved by the fact that when it is dislodged it invariably returns to its singular lodging-place. The fish is brightly colored, being of a brilliant vermilion hue with three broad white cross bands. The discoverer of this peculiar habit suggests that there are mutual benefits to fish and polyp from this habit. "The fish being conspicuous, is liable to attacks, which it escapes by a rapid retreat into the sea-anemone; its enemies in hot pursuit blunder against the outspread tentacles of the anemone and are at once narcotized by the thread cells' shot out in innumerable showers from the tentacles, and afterward drawn into the stomach of the anemone and digested."

Small fish of the genus Nomeus may often be found accompanying the beautiful Portuguese man-of-war (Physalia) as it sails slowly about on the ocean's surface (Fig. 104). These little fish lurk underneath the float and among the various hanging thread-like parts of the Physalia, which are provided with stinging cells. The fish are protected from their enemies by their proximity to these stinging threads, but of what advantage to the man-ofwar their presence is is not understood. Similarly, several kinds of medusæ are known to harbor or to be accompanied by young or small adult fishes.

In the nests of the various species of ants and termites

many different kinds of other insects have been found. Some of these are harmful to their hosts, in that they feed on the food stores gathered by the industrious and provident ant, but others appear

to feed only on refuse or useless substances in the nest. Some may even be of help to their hosts. Over one thousand species of these myrmecophilous (ant-loving) and termitophilous (termite-loving) insects have been recorded by collectors as living habitually in the nests of ants and termites. The owls and rattlesnakes which live with the prairie-dogs in their villages afford a familiar example of commensalism.

92. Symbiosis. Of a more intimate character, and of more obvious and certain mutual advantage, is the wellknown case of the symbiotic association of some of the numerous species of hermitcrabs and certain species of sea-anemones. The hermitcrab always takes for his habitation the shell of another animal, often that of the common whelk. All of the hind part of the crab lies inside the shell, while its head with its great claws project from the opening of the shell. On the surface of the shell near the opening there is often to be found a sea-anemone, or sea-rose (Fig. 105).

FIG 104.-A Portuguese man-of-war (Physalia), with man-of-war fishes (Nomeus gronovii) living in the shelter of the stinging feelers. Specimens from off Tampa, Fla.