To give an air of completeness to this treatise, I shall devote the few pages remaining at my disposal to a brief summary of the general facts of parasitism as witnessed in birds, reptiles, fishes, and evertebrated animals. For details I must refer to the separate original works and memoirs quoted in the appended bibliographies.
Part I (Aves).
A prodigious number of entozoa are known to infest birds. So far from birds being less victimised than mammals, the contrary is the case. Every now and then avian epizoötics, due to parasites, sweep off hundreds of these attractive hosts, and in some cases even nestlings are not secure from entozoal invasion. It might be supposed that predacious birds would be more liable to invasion than the graminivorous species. Such is not the case. The eagles, hawks, vultures, and owls certainly harbor a great variety of helminths, but as much may be said of the grain-feeding game birds, and still more of the water birds. Pheasants and land-fowl, grouse and partridges, are largely infested; whilst, of water-fowl, herons and plovers, rails and snipe, ducks and geese, cormorants and divers, gulls and awks, play the rôle of host to a practically infinite variety of parasitic guests. The presence of the worm-guests does not imply any previously diseased condition of the host. Shoot any water bird, say an oyster catcher (Hæmatopus), or, still better, a grebe (Podiceps), and then carefully examine its intestinal contents. You will probably find in its interior flukes and round worms, tapeworms and Echinorhynchi. Capture and examine a frog or a salamander. The result is the same, except that the cestodes would probably be absent. As for fishes, if entozoa be a proof of cachexia, then it follows that the normal condition of all piscine hosts is a diseased state. Examine any tolerably well-grown salmon, trout, pike, perch, roach, chub, carp, or barbel, and probably any one of them will contain at least three different kinds of parasites, each of which will be present in more or less considerable numbers. From what is stated above it would be obviously futile to attempt even an enumeration of the species of avian entozoa—a remark which applies almost equally to the other groups of hosts that remain for consideration. Confining our attention to a few of the more noteworthy facts, I may observe that we have no very trustworthy data respecting the power for mischief possessed by flukes. From what we know of their destructiveness in man and certain other mammals, it would be hazardous to pronounce them harmless. Scientifically, they furnish particulars of great interest. One of the most striking facts of recent study relates to Zeller’s discovery that the little cercariæ (C. exfoliata) which are contained in a peculiar sporocyst (Leucochloridium paradoxum), infesting the tentacles of a snail (Succinea amphibia), are in reality larval forms of a fluke (Distoma macrostomum) which resides in the intestinal canal of warblers (Sylviadæ). By experiment Dr Zeller reared the Distoma in question in the intestines of whitethroats (Curruca garrula), in blackcaps (C. atricapilla), and in wagtails (Motacilla). Six days after transference the Cercariæ acquired sexual maturity. The odd thing is, that as the sporocystic Leucochloridia resemble insect larvæ, they are attacked and swallowed by the birds under delusion. It is a curious example of mimetism in favor of the fluke’s welfare. To this and other equally brilliant scientific results helminthologists were gradually led up by the earlier researches of Steenstrup and Van Beneden. As remarked in my ‘Entozoa,’ ever since Steenstrup’s discovery of the fact that Cercariæ found in the bodies of water-snails were larval flukes, a peculiar interest has attached itself to this subject. Not only were the conclusions which he elicited novel in themselves, but they formed a basis for the enunciation of that interesting “law of alternate generation” with which the famous Danish naturalist’s name will ever remain associated. In all essential particulars Steenstrup’s statements have been verified.
Fig. 72.—Head of Tænia paradoxa. a, Proboscis retracted; b, end of the rostellum expanded. Highly magnified. From an oyster-catcher (Hæmatopus ostralegus). Original.
By way of illustrating the phases of development through which the distomes pass I cannot do better than recapitulate in an abridged form the account I have previously given of Distoma (Echinostoma) militare of the snipe and curlew. This account is based on the investigations of Van Beneden, Von Siebold, and Pagenstecher. I regret that it is out of my power to reproduce the illustrations that have already appeared on this head in my introductory treatise (see ‘Entozoa,’ figs. 5 to 9 inclusive). Our Echinostoma militare produces oval-shaped eggs, which give birth to a free ciliated embryo, and this embryo produces a sporocyst or scolex by internal budding. When the sporocyst separates itself from the embryo it presents a very simple appearance, but showing already a cæcal digestive tube. The tail end is fissured, indicating an early stage of formation of caudal appendages. In the next stage we have a well-developed head and body, the tail becoming strongly pronounced. Limb-like caudal lobes project on either side, and an oral sucker makes its appearance in front. This sucker communicates with the œsophageal bulb and passes directly into the digestive cæcum, which contains a variable number of rounded particles. At this stage, also, incompletely developed Cercariæ may be seen in the perivisceral cavity. These Cercariæ are at first shapeless organisms, but after passing through a series of gradations they ultimately assume a definite form, which, in many cases, is sufficiently distinctive to enable us to refer the Cercariæ to particular species of Distoma. The older writers regarded many of the cercarians as adult flukes. In the early state these larvæ are furnished with tails. They may be seen lodged within the cavity of the body of the sporocysts, being twisted and folded in various attitudes. The Cercariæ not only exhibit a cephalic and ventral sucker, but also a dark forked line representing the digestive system. At a still further stage other structures come into view, until the perfect Cercaria displays an oral sucker, a pharyngeal bulb, an œsophagus, two alimentary cæca, a ventral sucker, a water-vascular system consisting of two main excretory ducts, and a contractile vesicle, by means of which the ducts communicate with the external surface. The tail is conspicuous and furnished with a fringe. The alimentary organs conform to the general trematode type, but before passing into the sexually-mature condition other changes are undergone. The Cercariæ part with their tails, and subsequently they encyst themselves on or within the surface of the body of some mollusk. Their pupa condition is thus arrived at. The pupa itself differs from the cercaria in presenting a double crown of hooks surrounding the head, but the other organs correspond with those already described. According to Van Beneden the hooks make their appearance immediately after encystation. In this condition it is next transferred to the intestine of some higher animal, and in this final situation it gradually acquires all those organs the possession of which will entitle it to be called a sexually-mature or adult distome. In the immature fluke we may now discern the mouth, the buccal or cephalic sucker, the pharyngeal bulb, the œsophagus, the digestive cæca, the coronal spines, the contractile vesicle, the aquiferous system of vessels, the matrices of the yelk-forming glands, and also a central mass of cellules, from which all the other reproductive organs will in due time be developed. In the adult Echinostoma militare the upper third of the body is clothed with little spines. Taking this example as illustrative of the ordinary mode of fluke development we find that a change of hosts is necessary, and that in the intermediate state they occupy the bodies of mollusks. Thus, for the continuation of the species, there must needs be a contemporaneity of vertebrate and evertebrate hosts. Surely no reasonable person can ascribe this concurrence to merely fortuitous circumstances. In this connection I may remark that Villot, in his account of the migrations of the trematodes, states that the cercarian forms of Distoma leptosomum and D. brachysomum of Tringa alpina occur in Scrobicularia and Anthura. These parasites are also found encysted in the gizzard of Tringa.
