struggles to loosen it from its rock or from whatever it may be growing on, and does not rest until it has torn it loose and placed it on its shell.

There are numerous small crabs called pea crabs (Pinnotheres) which live habitually inside the shells of living mussels. The mussels and the crabs live together in apparent harmony and to their mutual benefit.

The relations between ants and aphids (plant lice) are often referred to in popular natural histories and books about

Hydractinia.

insects as examples of symbiosis of unusual interest. Unfortunately, however, not enough careful study has been given to many of these apparently true examples of symbiosis to enable us to be certain of the truth of the alleged care and guarding of the ant-cows, as Linnæus called these aphids, by their milkers, the ants. That ants do swarm about the aphids to lap up the "honey dew" excreted by them is wholly true, and the very presence of the sharp-jawed and pugnacious ants must keep away many enemies of the defenseless plant lice, toothsome morsels for the ladybird beetles, flower-fly larvæ and other predatory insects.

In the case of the interesting relations between the corn root aphid, Aphis maidisradici, of the Mississippi Valley States and the little brown ant, Lasius brunneus, however, we

have the careful observations of Professor Forbes to rely on. In the Mississippi Valley, this aphid deposits in autumn its eggs in the ground in corn fields, often in the galleries of the little brown ant. The following spring before the corn is planted, these eggs hatch. Now, the little brown ant is especially fond of the honey dew secreted by the corn root lice. So when the latter hatch in the spring, before there are corn roots for them to feed on, the ants carefully place them on the roots of certain kinds of grass and knotweed (Setaria, Polygonum), and there protect them until the corn germinates. They are then removed to the roots of the corn. It is probable that the ants even collect the eggs of the aphids in the autumn and carry them into their nests for protection and care.

The studies of Wheeler and others have revealed some interesting cases of the living together of different species of ants. In some cases one of the ant species may be living almost wholly at the expense of the other species, as does the little yellow thief-ant, Solenopsis molesta. Although this ant sometimes lives in independent nests, more often it is to be found living in association with some large ant species-it consorts with many different hosts-feeding almost exclusively on the live larvæ and pupa of the host. The thief-ant is so small and obscurely colored that it seems to live in the nests of its host practically unperceived. The Solenopsis nest may be found by the side of the host nest, around it, or partly in it, the tiny Solenopsis galleries ramifying through the nest mass of the host, and often opening boldly into these large galleries. Through their narrower passages, too narrow to be traversed by the hosts, the tiny thief-ants thread their way through the host nest in their burglarious excursions (Fig. 245).

But there are numerous cases of a less one-sided advantage in the association of different species. As an example the conditions exhibited by the red-brown ant, Myrmica brevinodes and the smaller Leptothorax emersoni (conditions made. known by Wheeler's careful observations) may be briefly described (Fig. 246). The little Leptothorax ants live in the Myrmica nests, building one or more chambers with entrances from the Myrmica galleries, so narrow that the large Myrmicas cannot get through them. When needing food the Leptothorax workers come into the Myrmica galleries and chambers and, climbing on the backs of the Myrmica workers, proceed

to lick the face and the back of the head of each host. A Myrmica thus treated, says Wheeler,

"paused, as if spellbound by this shampooing and occasionally folded its antennæ as if in sensuous enjoyment. The Leptothorax after licking the Myrmica's pate, moved its head round to the side and began to lick the cheeks, mandibles, and labium of the Myrmica. Such ardent osculation was not bestowed in vain, for a minute drop of liquid— evidently some of the recently imbibed sugar-water-appeared on the Myrmica's lower lip and was promptly lapped up by the Leptothorax. The latter then dismounted, ran to another Myrmica, climbed on its back, and repeated the very same performance. Again it took toll and passed on to still another Myrmica. On looking about in the nest I observed that nearly all the Leptothorax workers were similarly employed."

Wheeler believes that the Leptothorax get food only in this way. They feed their queen and larvæ by regurgitation. The Myrmicas seem not to resent at all the presence of their Leptothorax guests, and indeed may derive some benefit from the constant cleansing licking of their bodies by the shampooers. But the Leptothorax workers are careful to keep their queen and young in a separate chamber, not accessible to their hosts. This is probably the part of wisdom, as the thoughtless habit of eating any conveniently accessible pupa of another species is widespread among ants.

There are numerous interesting cases of symbiosis in which not different kinds of animals are concerned, but animals and plants. It has long been known that some sea anemones possess certain body cells which contain chlorophyll, that green substance characteristic of the green plants, and only in few cases possessed by animals. When these chlorophyll-bearing sea anemones were first found, it was believed that the chlorophyll cells really belonged to the animal's body, and that this condition broke down one of the chiefest and most readily apparent distinctions between animals and plants. But it is now known that these chlorophyll-bearing cells are microscopic, one-celled plants, green algae, which live habitually in the bodies of the sea anemone. It is a case of true symbiosis. The alge, or plants, use as food the carbon dioxide which is given off in the respiratory processes of the sea anemone, and

the sea anemone breathes in the oxygen given off by the algæ in the process of extracting the carbon for food from the carbon dioxide. These algæ, or one-celled plants, lie regularly only in the innermost of the three cell layers which compose the wall or body of the sea anemone (Fig. 231). They penetrate into and lie in the interior of the cells of this layer, whose special function is that of digestion. They give this innermost layer of cells a distinct green color. Even certain amoebalike protozoans have been found to contain individuals of a onecelled alga, Chlorella,

in their single-celled bodies, the tiny animal and smaller plants living together truly symbiotically.

Among the higher plants and animals, cases of symbiosis are not rare. There lives in the live-oak trees in the vicinity of Stanford University a certain scale insect, Cerococcus ehrhorni, which differs from the other two or three species of its genus in not having its body covered by a heavy, thick, protecting layer of secreted wax. But it gets the needed protection in another way. It is always covered by a thick felt like fungus growth, which has been found by investigation to germinate its spores and to find a constant food supply in the "honey dew" excreted by the scale insects. This feltlike covering of fungus, never found to be lacking in the scale insect, serves apparently as a sufficient substitute for the heavy waxen mass common to the related species.

FIG. 231.-Diagrammatic section of sea anemone: a, The inner cell layer contains alga cells, the two isolated cells at the right being cells of this layer with contained algæ; b, middle body wall layer; c, outer body wall layer. (After Hertwig.)

The ants show particularly well instances of interesting symbiotic life with plants. Fig. 232, drawn from a specimen sent to us from the Philippine Islands by the botanist Copeland, shows some details of one such instance. The Dischidias are milkweeds of the extreme Orient. They twine

upon trees by means of their flexible stems and branches and are especially noted for possessing appendages in the form of pitchers. These pitcherlike appendages are modified leaves: the normal Dischidia leaf is orbicular, thick, and fleshy. Each pitcher is the blade of a leaf folded so that the lower surface forms the inner surface of the pitcher. Into these pitchers

FIG. 232.-Leaves of Dischidia, which contain adventitious roots of the same plant and in which live colonies of small ants. (From specimens from Philippine Islands.)

grow adventitious roots that spring from the leaf peduncle. Also in these pitchers live colonies of ants. As rent for furnishing these comfortable cozy little ant homes, the Dischidia gets, by means of the adventitious roots in the pitcher, food from the excreta and cadavers of the ants. Hundreds of ants with larvæ and pupae can be found in these Dischidia leaves, and without doubt we have here a mutually advantageous symbiotic adaptation.

From Weismann's chapter on Symbiosis in his "Vorträge über Descendenztheorie," Vol. I, 1902, we translate the follow