Page:EB1911 - Volume 27.djvu/255

Rh These words were sung by the men of Cornwall, who seem to have assembled during the bishop's short imprisonment in 1688. It is probable, however, that a similar threat was heard in 1628, when John Trelawny (1592-1665), grandfather of the bishop, was imprisoned by the House of Commons for opposing the election of Sir John Eliot to parliament. The “Song of the Western Men,” which contains the above refrain, was composed in 1825 by R. S. Hawker.  TREMATODES, or flukes (as they are called from their fish-like shape), one of the three classes that compose the phylum (q.v.). They are flattened organisms provided with two or more suckers, hence their name (, pierced with holes), and are exclusively parasitic both in their earlier and mature stages of life. Their structure has undergone little degeneration in connexion with this habit, and may be compared organ for organ with that of the (q.v.). The chief peculiarities that distinguish Trematodes from their free-living allies, the Turbellaria, are the development of adhering organs for attachment to the tissues of the host; the replacement of the primitively ciliated epidermis by a thick cuticular layer and deeply sunk cells to ensure protection against the solvent action of the host; and (in one large order) a prolonged and peculiar life-history. The only organs that exhibit any sign of degeneration are those of sense, but in the ectoparasitic Trematodes simple eye-like structures are present and perhaps serve as organs of temperature. The class as a whole is linked to the Turbellaria not only by its similarity of structure, but by the intermediation of the singular class the Temnocephaloidea (see ), which in habit and in organization form an almost ideal annectant group.

External Characters.—The body, which varies in length from a few millimetres to a couple of feet, is usually oval and flattened. In certain genera the margins are infolded either along their whole length (the male of Schistostomum haematobium; fig. 9, A) or anteriorly only (Holostomidae). The anterior third of the body is attenuated and sharply marked off from the bulbous trunk in Didymozoon. Trematodes never exhibit segmentation, though a superficial annulation may occur, e.g. in Udonella.

The ventral surface is characterized by one or more suckers and apertures. The mouth lies usually in the centre of the anterior

and sub-terminal sucker or between two adoral suckers, but in Gasterostomum and its allies it is mid-ventral. A second sucker of variable size and shape lies behind the oral one. In the ectoparasitic Trematodes this post-oral sucker is a complex disk placed near the hinder end and provided with suckerlets, hooks and a musculature arising from a special skeleton. In the majority of endoparasitic forms it is merely a muscular disk just behind the mouth; but in the Aspidocotylea this sucker forms a muscular ribbed sole extending over the greater part of the ventral surface (fig. 7).

The anterior and posterior ends of the body are well defined. The former is specially modified in a few genera in a manner analogous to the “proboscis” of certain Rhabdocoel Turbellaria. Thus in the recently discovered arctic genus Prosorhynchus the muscular and glandular extremity is protrusible, but in the allied Gasterostomum this organ is represented by a sucker with fimbriated or tentacular margins. Another form, Rhopalophorus, has two cephalic tentacles that are retractile and covered with hooks. The chief genital pore is placed anteriorly between the oral sucker and the ventral one, and is posterior only in Holostomidae, Gasterostomidae and a few Distomidae. Usually this aperture is median, but occasionally asymmetrical. Both male and female gonoducts open through a common atrium to the exterior by this pore, but in three bisexual genera the male and female ducts are developed in separate individuals (Bilharzia, Didymozoon, Koellikeria). A single or paired accessory gonopore is met with in many Trematodes just as in certain Turbellaria (e.g. Cylindrostomum, Trigonoporus). This accessory pore is not of uniform significance. In ectoparasitic Trematodes it is paired and usually ventral (fig. 4 B, v), but the two apertures may run into one, and may also open dorsally (Hexacotyle). In this group, the accessory gonopore is the opening of the “vagina,” in contradistinction to the median and atrial opening of the uterus which is a “birth-pore.” In most endoparasitic Trematodes the accessory gonopore is a median and dorsal structure. It is the opening of Laurer's canal and is homologous not with that of the “vagina” just mentioned, but with a totally distinct structure—the “yolk-receptacle”—which in ectoparasitic forms discharges into the gut instead of to the exterior (see fig. 3).

The excretory pore is terminal and posterior in endoparasitic forms: paired, anterior and dorsal in the ectoparasitic class.

Parasitic Habits.—The Trematodes with few exceptions select a vertebrate for their host. Speaking generally each species of parasite has a particular host, upon the blood of which it nourishes itself and matures its reproductive organs. This strange partiality is now to some extent intelligible. It has been shown in the mammals that blood-relationship, in the strict and literal sense, holds good. The blood of most species behaves differentially towards precipitants, and it is therefore conceivable that when blood is used as food and is elaborated into special compounds for the nutrition of the reproductive organs of a parasite, these specific or larger differences in the blood of animal hosts may prevent the ripening of the gonads of a widely diffused parasite and only one particular kind of blood prove suitable. It would seem that the Trematodes present various degrees of such adaptation, for whilst some—e.g. the common liver-fluke (Distomum hepaticum)—mature equally well in the bile-ducts of a man as in those of a sheep or rabbit, others and in fact the majority are restricted apparently to one host. It must, however, be borne in mind that a Trematode may develop in an “aberrant” manner in one host and “normally” in another; and unless we knew the initial stock, the two forms would be regarded as distinct species, each with its own host.

The position of the Trematode on its host is of far-reaching importance. If ectoparasitic and attached to the skin, apertures or gills, the Trematode adopts more elaborate adhesive organs and undergoes a less complex development than are required for the endoparasitic members of the class. The latter are almost invariably swallowed by their host in an immature state with its food, and front the stomach or intestine they work their way into the lungs, liver, body-cavity or blood vessels. These endoparasites have a peculiar larval development, the results of which are to increase their numbers and enhance the opportunity of their gaining the necessarily remote station in some fresh individual host. It is usual to consider the ectoparasitic habit as leading up to the endoparasitic one. From what we know of the Platyelmia, however, it is more probable that the two are quite independent and have been evolved separately.

The influence of Trematodes on their hosts is a varied one. Probably all of them secrete an active poison by the aid of their glands, but the effects of these substances are not readily perceptible. In addition to this, they constitute a drain upon the blood which may result in anaemia. If present in large numbers they may give rise to obstruction of the liver-ducts or to inflammation of other tissues. The most important of the Trematodes in its effect on man is Schistostomum (Bilharzia). This parasite is one of the plagues