This sea-anemone is fastened securely to the shell, and has its mouth opening and tentacles near the head of the crab. The sea-anemone is carried from place to place by the hermit-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 the sea-anem

FIG. 105.-IIermit-crab (Pagurus) in shell, with a sea-anemone (Adamsia palliata attached to the shell.-After HERTWIG.

one.

In the tentacles there are many thousand long, slender stinging threads, and the fish that would obtain the hermit-crab for food must first deal with the stinging anemone. There is no doubt here of the mutual advantage gained by these two widely different but intimately associated companions. If the sea-anemone be torn away from the shell inhabited by one of these crabs, the crab will wander about, carefully seeking for another anemone, When he finds it he struggles to loosen it from its rock or from whatever it may be growing on, and does not rest until he has torn it loose and placed it on his 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 perfect harmony and to their mutual benefit.

There are a few extremely 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 pos

sess 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 con

dition 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 alga, which live habitually in the bodies of the sea-anemone. It is a case of true symbiosis. The algæ, or plants, use as food the carbonic-acid gas which is given off in the respiratory breathes processes of the sea-anemone, and the sea-anemone in the oxygen given off by the alge in the process of extracting the carbon for food from the carbonic-acid gas. 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. 107). They penetrate into and lie in the interior of the cells of this layer whose special function is that of digestion. They give this inner

most layer of cells a distinct green

color.

There are other examples known of the symbiotic association of plants and animals; and if we were to follow the study of symbiosis into the plant kingdom we should find that in

one of the large groups of plants, the familiar lichens which grow on every member lives

rocks and tree trunks and old fences, symbiotically. A lichen is not a single plant, but is always composed of two plants, an alga (chlorophyll-bearing) and a fungus (without chlorophyll) living together in a most intimate, mutually advantageous association.

CHAPTER XI

PARASITISM AND DEGENERATION

93. Relation of parasite and host. In addition to the various ways of living together of animals already described, namely, the social life of individuals of a single species and the commensal and symbiotic life of individuals of different species, there is another kind of association among animals that is very common. In cases of symbiosis the two animals living together are of mutual advantage to each other; both profit by the association. But there are many instances in the animal kingdom of an association between two animals by which one gains advantages great or small, sometimes even obtaining all the necessities of life, while the other gains nothing, but suffers corresponding disadvantage, often even the loss of life itself. This is the association of parasite and host; the relation between two animals whereby one, the parasite, lives on or in the other, the host, and at the expense of the host. Parasitism is a common phenomenon in all groups of animals, although the parasites themselves are for the most part confined to the classes of invertebrates. Among the simplest animals or Protozoa there are parasites, as Gregarina, which lives in the bodies of insects and crustaceans; there are parasitic worms, and parasitic crustaceans and mollusks and insects, and a few vertebrates. When an animal can get along more safely or more easily by living at the expense of some other animal and takes up such a life, it becomes a parasite. Parasitism is naturally, therefore, not confined to any one group or class of animals.

94. Kinds of parasitism. The bird-lice (Mallophaga), which infest the bodies of all kinds of birds and are found especially abundant on domestic fowls, live upon the outside of the bodies of their hosts, feeding upon the feathers and dermal scales. They are examples of external parasites. Other examples are fleas and ticks, and the crustaceans called fish-lice and whale-lice, which are attached to marine animals. On the other hand, almost all animals are infested by certain parasitic worms which live in the alimentary canal, like the tape-worm, or imbedded in the muscles, like the trichina. These are examples of internal parasites. Such parasites belong mostly to the class of worms, and some of them are very injurious, sucking the blood from the tissues of the host, while others feed solely on the partly digested food. There are also parasites that live partly within and partly on the outside of the body, like the Sacculina, which lives on various kinds of crabs. The body of the Sacculina consists of a soft sac which lies on the outside of the crab's body, and of a number of long, slender root-like processes which penetrate deeply into the crab's body, and take up nourishment from within. The Sacculina is itself a crustacean or crab-like creature. The classification of parasites as external and internal is purely arbitrary, but it is often a matter of convenience.

Some parasites live for their whole lifetime on or in the body of the host, as is the case with the bird-lice. Their eggs are laid on the feathers of the bird host; the young when hatched remain on the bird during growth and development, and the adults only rarely leave the body, usually never. These may be called permanent parasites. On the other hand, fleas leap off or on a dog as caprice dictates; or, as in other cases, the parasite may pass some definite part of its life as a free, non-parasitic organism, attaching itself, after development, to some animal, and remaining there for the rest of its life. These parasites may be called temporary parasites. But this grouping or classification,