parasitism is very widely spread, and there is probably no animal which does not unwillingly entertain unwelcome guests that make no return for services rendered. As De Morgan sings (in The Budget of Paradoxes):

"Great fleas have little fleas upon their backs to bite 'em,

And little fleas have lesser fleas, and so ad infinitum;

And the great fleas themselves, in turn, have greater fleas to go on,
Whilst these again have greater still, and greater still, and so on".

The general progress of evolution has been from the less to the more specialized as a result of adaptation to increasingly complex surroundings, but to this parasites constitute a striking exception. Free quarters and free rations having been provided for them, they have taken but little part in the active struggle for existence, and well illustrate the principle of Degeneration. They are on the down-grade, adapting themselves to comparatively simple conditions. Hence we find that complex organs of digestion, circulation, respiration, and locomotion, together with nervous system and sense-organs, have undergone more or less reduction in thoroughgoing parasites, though, on the other hand, they have frequently developed special piercing, sucking, and adhesive structures, enabling them to exploit their living food-supply, and to maintain their position. The great danger attending this particular mode of life is constituted by the smallness of the chance of transfer from one host to another. In the more helpless forms this difficulty is often met by the practice of living in two or more different hosts which eat or prey upon one another; the adult egg-producing stage, being the most important, is commonly associated with the strongest and most highly organized of these, the so-called "final host". The biological relations between the successive living refuges is always such as to maintain most surely "the vicious circle of parasitism". Even more important is the immense fecundity of parasites, a necessary provision, for the chances of survival are extremely small. Leuckart calculated, for example, that any one egg of a tape-worm has only one chance in some 83,000,000 of giving rise to an adult.

What is called Brood Parasitism, where an animal shirks the duty of bringing up its own young, will be considered in this section, although it is by no means the same thing as true parasitism.

BIRDS (AVES) AS BROOD-PARASITES

The Common Cuckoo (Cuculus canorus), as everyone knows, deposits her just-laid egg in the nest of some small bird, carrying it there in her bill. The proceedings of the young cuckoo, as observed by Mrs. Blackburn, are thus described by Lloyd Morgan (in Animal Behaviour):-" One of the most remarkable instincts of young birds is that of the cuckoo, which ejects eggs and nestlings from the home of its foster-parent. Mrs. Hugh Blackburn found a nest which contained two meadow-pipits' eggs, besides that of a cuckoo. On a later visit the pipits were found to be hatched, but not the cuckoo. At the next visit, which was after an interval of forty-eight hours, 'we found the young cuckoo alone in the nest, and both the young pipits lying down the bank, about ten inches from the margin of the nest, but quite lively after being warmed in the hand. They were replaced in the nest beside the cuckoo, which struggled about until it got its back under one of them, when it climbed backwards directly up the open side of the nest, and hitched the pipit from its back on to the edge. It then stood quite upright on its legs, which were straddled wide apart, with the claws firmly fixed half-way down the inside of the nest, among the interlacing fibres of which the nest was woven, and, stretching its wings apart and backwards, it elbowed the pipit fairly over the margin, so far that its struggles took it down the bank instead of back into the nest [fig. 1132]. As it was getting late, and the cuckoo did not immediately set to work on the other nestling, I replaced the ejected one and went home. On returning next day, both nestlings were found dead and cold, out of the nest' (Birds from Moidart and Elsewhere)." Similar habits have been described for the Cow-Birds (species of Molobrus) of America. One species of these (M. rufaxillaris) actually lays its eggs in the nest of a related species (M. badius), which is industrious enough to build one for itself. It may further be remarked in passing that some kinds of Cuckoo also construct nests, and bring up their young in the usual way.

Newton, after speaking of the social nesting-habits of certain birds, makes the following suggestions as to the origin of broodparasitism (in A Dictionary of Birds):-" In the strongest contrast to these amiable qualities is the parasitic nature of the Cuckows of the Old World and the Cow-Birds of the New, but

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this peculiarity of theirs has already been dwelt upon. Enough to say here that the egg of the parasite is introduced into the nest of the dupe, and after the necessary incubation by the fond fool of a foster-mother the interloper successfully counterfeits the heirs, who perish miserably, victims of his superior strength. The whole process has been often watched, but the reflective naturalist will pause to ask how such a state of things came about, and there is not much to satisfy his enquiry. Certain it is that some birds, whether by mistake or stupidity, do not unfrequently lay their

eggs in the nests of others. It is within the knowledge of many that Pheasants' eggs and Partridges' eggs are often laid in the same nest, and it is within the knowledge of the writer that Gulls' eggs have

been found in the
nests of Eider-Ducks,
and vice versâ; that a
Redstart and a Pied
Fly-Catcher, or the
latter and a Tit-
mouse, will lay their
eggs in the same con-
venient hole the forest being rather deficient in such accom-
modation; that an Owl and a Golden-Eye will resort to the
same nest-box, set up by a scheming woodsman for his own
advantage; and that the Starling, which constantly dispossesses
the Green Woodpecker, sometimes discovers that the rightful
heir of the domicile has to be brought up by the intruding tenant.
In all such cases it is not possible to say which species is so
constituted as to obtain the mastery; but just as it is conceivable
that in the course of ages that which was driven from its home
might thrive through the fostering of its young by the invader,
and thus the abandonment of domestic duties would become a
direct gain to the evicted householder, so the bird which, through

Fig 1132.-Young Cuckoo (Cuculus canorus) ejecting a Fledgling
Meadow-Pipit from the Nest

inadvertence or any other cause, adopted the habit of casually dropping her eggs in a neighbour's nest, might thereby ensure a profitable inheritance for endless generations of her offspring. This much granted, all the rest will follow easily enough, but it must be confessed that this is only a presumption, though a presumption which seems plausible if not likely."

FISHES (PISCES) AS PARASITES

The Lampreys and Hags (Cyclostomata) are scaleless, eelshaped creatures, devoid of jaws, and intermediate in habit between carnivorous forms and external parasites. On the under side of the head is a bell-shaped sucker, the lining of which is thickened TONGUE into a varying number of sharp horny teeth. At the top of the bell is the true mouth-opening, provided with a projecting tongue", also tooth-bearing (fig. 1133). By means of the sucker these creatures are able to attach themselves to other fishes, the flesh of which they rasp away, using the tongue for the purpose, this

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being moved by means of powerful muscles.

MOLLUSCS (MOLLUSCA) AS PARASITES

Certain Sea-Snails afford the best illustrations of the parasitic habit as occurring among Molluscs. One of the Cap-Shells (Thyca ectoconcha, fig. 1134) is an external parasite upon a kind of StarFish (Linckia multiforis). It will be seen from the illustration that this form is still easily recognizable as a mollusc, though the influence of its particular mode of life is also obvious. The mouth has shifted backwards, and is on the end of a short proboscis, which penetrates into the body of the host, and is surrounded by an adhesive disc, formed by the fusion of parts of the foot with an outgrowth from the head. The characteristic raspingorgan (odontophore) has entirely disappeared, and the pharynx has been converted into a sort of suction-pump by which the juices of the star-fish are drawn in. The body of the same un

fortunate star-fish may also present a number of rounded swellings in which are lodged parasitic snails of another species (Stilaster Linckia) that have undergone much further modification, being practically endoparasites in which the proboscis has become very long, while the rest of the body is much smaller in proportion, and the shell has disappeared. Communication with the exterior is still kept up, however, by means of a small hole, in the interests of breathing and the getting rid of waste products. From this case we pass on to degenerate worm-like snails, which are true internal parasites, and have lost most of the typical organs of the group to which they belong, though the study of their life-histories renders no doubt pos D.G sible as to their classifi

catory position.

Their

bodies hang freely into the interiors of their hosts, one end being fixed to the inner side of the body-wall of the same. A degenerate

Gullet
Proboscis

Fig. 1134.-Parasitic Cap-Shell (Thyca ectoconcha) attached to the Skin of a Star-Fish (Linckia multiforis); diagrammatic section. D.G., Digestive gland; F.G., foot-ganglion; S., S., suctorial disc; ot., otocyst.

of the kind (Entocolax Ludwigii) lives within a species of seacucumber (Myriotrochus Rinkii), and one still more strongly modified (Entoconcha mirabilis) within another creature of the same sort (Syapta digitata).

The parasitic habit of the larvæ of Freshwater Mussels has been dealt with elsewhere (see vol. iii, p. 406).

INSECTS (INSECTA) AS PARASITES

Innumerable insects have adopted the parasitic habit, either for the whole of their lives or for some particular stage of existIt will only be possible to give a limited number of examples in illustration of the more interesting kinds of adaptation.

ence.

BUGS (HEMIPTERA) AS PARASITES.-The fact that the insects of this order possess piercing and sucking mouth-parts naturally suggests that some of them attack other animals, which is indeed the case, though the majority would appear to devote themselves