notheres) which live habitually inside the shells of living mussels. The mussels and the crabs live together in perfect harmony and to their mutual benefit.

FIG. 203.-Hermit-crab (Pagurus) in shell, with a sea-anemone (Adamsia palliata) attached to the shell.-After HERTWIG.

272. Relation of parasite and host. 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 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; but parasites themselves are mostly invertebrates. 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.

273. 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 rootlike 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, like that of the external or internal parasites, is simply a matter of convenience, and does not indicate at all

any blood relationship among the members of any one group.

274. The simple structure of parasites.-In all cases the body of a parasite is simpler in structure than the body of other animals which are closely related to the parasitethat is, animals that live parasitically have simpler bodies than animals that live free, active lives, competing for food with the other animals about them. This simplicity is not primitive, but results from the loss or atrophy of the structures which the mode of life renders useless. Many parasites are attached firmly to their host, and do not move about. They have no need of the power of locomotion. They are carried by their host. Such parasites are usually without wings, legs, or other locomotory organs. Because they have given up locomotion they have no need of organs of orientation, those special sense organs like eyes and ears and feelers, which serve to guide and direct the moving animal; and most non-locomotory parasites will be found to have no eyes, nor any of the organs of special sense which are accessory to locomotion and which serve for the detection of food or of enemies. Because these im

portant organs, which depend for their successful activity. on a highly organized nervous system, are lacking, the nervous system of parasites is usually very simple and undeveloped. Again, because the parasite usually has for its sustenance the already digested highly nutritious food elaborated by its host, most parasites have a very simple alimentary canal, or even no alimentary canal at all. Finally, as the fixed parasite leads a wholly sedentary and inactive life, the breaking down and rebuilding of tissue in its body go on very slowly and in minimum degree, and there is no need of highly developed respiratory and circulatory organs; so that most fixed parasites have these systems of organs in simple condition. They often bear no resemblance to the complex forms from which they are descended.

275. Sacculina.-Among the more highly organized animals the results of a parasitic life, in degree of structural degeneration, can be more readily seen. A well-known parasite, belonging to the crustacea-the class of shrimps, crabs, lobsters, and cray-fishes-is Sacculina. The young Sacculina is an active, free-swimming larva much like a young prawn or young crab. But the adult bears absolutely no resemblance to

such a typical

crustacean as a cray-fish or crab. The Sacculina, after a short period of independent existence, attaches itself to the ab

domen of a crab,

and there com- FIG. 204.-Sacculina, a crustacean parasite of crabs. a, atpletes its devel- tached to a crab, with root-like processes penetrating the crab's body; b, removed from the crab. opment while

living as a parasite. In its adult condition (Fig. 204) it is simply a great tumor-like sac, bearing many delicate, rootlike suckers, which penetrate the body of the crab host and absorb nutriment. The Sacculina has no eyes, no mouth parts, no legs, or other appendages, and hardly any of the usual organs except reproductive organs. Degeneration here is carried very far.

276. Parasitic insects. In the order Hymenoptera there are several families, all of whose members live during their larval stage as parasites. We may call all these hymenopterous parasites ichneumon flies. The ichneumon flies are parasites of other insects, especially of the larvæ of beetles and moths and butterflies. In fact, the ichneumon flies do more to keep in check the increase of injurious and destructive caterpillars than do all our artificial remedies

for these insect pests. The adult ichneumon fly is fourwinged and lives an active, independent life. It lays its eggs either in or on or near some caterpillar or beetle grub, and the young ichneumon, when hatched, burrows about in the body of its host, feeding on its tissues, but not attacking such organs as the heart or nervous ganglia, whose injury

FIG. 205.-Parasitized caterpillar from which the ichneumon fly parasites have issued, showing the circular holes of exit in the skin.

would mean immediate death to the host. The caterpillar lives with the ichneumon grub within it, usually until nearly time for its pupation. In many instances, indeed, it pupates, with the parasite still feeding within its body, but it never comes to maturity. The larval ichneumon fly pupates either within the body of its host (Fig. 205) or in a tiny silken cocoon outside of its body. From the cocoons the adult winged ichneumon flies emerge, and after mating find another host on whose body to lay their eggs.

One of the most interesting ichneumon flies is Thalessa (Fig. 209), which has a remarkably long, slender, flexible ovipositor, or egg-laying organ. An insect known as the pigeon horn-tail (Tremex columba) (Fig. 207) deposits its eggs, by means of a strong, piercing ovipositor, half an inch deep in the trunk wood of growing trees. The young or larval Tremex is a soft-bodied white grub, which bores deeply into the trunk of the tree, filling up the burrow behind it with small chips. The Thalessa is a parasite of the Tremex, and when a female Thalessa finds a tree infested by Tremex, she selects a place which she judges is opposite