The advantages of communal or social life, of co-operation and mutual aid are real. Those animals that have adopted such a life are among the most successful of all in the struggle for existence. The termite worker is one of the most defenseless and for those animals that prey on insects one of the most toothsome insects, and yet the termite is one of the most abundant and successfully living insect kinds in all the tropics. Ants are everywhere and are everywhere successful. The honey-bee is a popular type of successful life. The artificial protection afforded it by man may aid it 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. Co-operation and mutual aid are among the most important factors which help in the struggle for existence. =Commensalism.=--TECHNICAL NOTE.--Examine ants' nests to find myrmecophilous insects. If on the seashore search for hermit-crabs with sea-anemones on shell. If inland, try to have some preserved specimens showing the crabs and sea-anemones. The phases of living together and mutual help just discussed concerned in each instance a single species of animal. All the members of a pack of wolves or of a honey-bee community belong to a single species. But there are numerous instances known of the mutually advantageous association of individuals of two different species. Such an association is called _commensalism_ or _symbiosis_. The hermit-crabs live, as has been learned (p. 154), in the shells of molluscs, most of the body of the crab being concealed within the shell, only the head and the grasping and walking legs protruding. In some species of hermit-crabs there is always to be found on the shell near the opening a sea-anemone. "This sea-anemone is carried from place to place by the crab, and in this way is much aided in obtaining food. On the other hand, the crab is protected from its enemies by the well-armed and dangerous tentacles of its companion. On the tentacles there are many thousand long slender stinging threads, and the fish that would eat the hermit-crab must first deal with the stinging anemone." If the sea-anemone be torn away from the shell the crab will wander about seeking another anemone. When he finds one, he struggles to loosen it from the rock to which it is attached, and does not rest until he has torn it loose and placed it on his shell.