Historical Sketch.— Some of the more salient points in the history of our knowledge of these animals have already been alluded to in the article Parasitism ( q.v.); a few additional facts must, however, be mentioned here. The Trematoda were first formed into a group by Rudolphi (2), who included in it the following genera :— Monostoma, Amphistoma, Distoma, Tristoma, Pentastoma, and Polystoma ; the name had reference to the suckers, which Rudolphi regarded as being for the most part openings into the body (Gr. τρῆμα, an aperture). Some of these forms were soon perceived to have but small connexion with the others; and Cuvier (3) reduced the whole to one genus, for which he adopted the name Fasciola, Linn. The Pentastomes have since been transferred to the Arachnida (q.v. )∙

Our scientific acquaintance with the group may be said to date from 1831, when Mehlis noticed that the eggs of certain Distomes hatched into a minute ciliated body with an eye-speck resembling an Infusorian, an observation which gave the key to the life-history of these forms. Von Siebold in 1835 (4) supplemented this discovery by the observation that the ciliated embryo of Monostomum mutabile contained, as a “necessary parasite,” as it was termed, an organism identical with the “kingsyellow worm” (Redia), found by Bojanus in pond-snails, and Von Baer had previously shown (5) that these gave rise to free-swimming organisms not unlike tailed Trematodes. The materials were thus ready to hand for a co-ordination of the whole life-history, and Steenstrup recognized it as an instance of the so-called “alternation of generations” (6). These researches received important additions at the hands of Pagenstecher (7) and others, who showed experimentally that encysted Distomes grow mature directly after their transference from one host to another, and thus that a migration is necessary to the attainment of their maturity, Diesing’s great work (8) appeared in 1850, and has formed the groundwork of all subsequent treatises on the systematic arrangement of parasitic worms, although it included forms which really belong to quite different groups. In 1861 Van Beneden gained a prize offered by the French Academy by his elaborate memoir on the intestinal worms (9), in which he not only described many new and interesting forms, but gave anatomical details regarding others previously known, and entered into detailed comparisons between the Cestodes and Trematodes, both in their adult and immature states. Of recent years the chief additions to our knowledge have been more in the direction of further details regarding the structure and life-history of special forms than the elaboration of new general principles.

Anatomy.— In endeavouring to give a very brief account of the more salient points in the anatomy of the Trematoda it has been thought expedient to select some well-known form as a type, and afterwards to indicate the characters in which other species differ from it; for this purpose the common liver-fluke, Fasciola {Distomum) hepatica has been chosen, as it is not unfrequently found in the bile-ducts of sheep and other domestic animals, and constitutes a scourge much dreaded by farmers. The account here given is in the main abstracted from Sommer (10).

External Appearance. — The animal has a flattened oval shape, with a sub-triangular process on the broader end, which represents the head. The total length varies from 20 to 35 mm., the breadth from 6 to 12 mm. On superficial examination two narrower lateral areas may generally be distinguished from a broader median one; the former are occasionally of a coarsely granular appearance and reddish-brown or orange in colour, and increase in breadth towards the posterior end of the body, where they commonly unite. The median area is commonly greyish-yellow in colour, sometimes spotted with black; its anterior portion corresponds to the uterus, the posterior to the testes. Two suckers (fig. 1, A, o, s) are in the middle line of the body; one is at the anterior extremity, and is directed forwards and somewhat downwards; it is known as the anterior or oral sucker, being perforated by the oesophagus. The posterior or ventral sucker is situated, as its name implies, on the inferior surface of the body, just behind the head-papilla. The suckers measure on an average about 1 mm. in diameter, the ventral being slightly the larger. The internal organs communicate with the outer world by four apertures :—(1) the mouth (o), situated at the anterior pole of the body and perforating the oral sucker; (2) the excretory pore (x ), placed at the opposite extremity, and giving exit to the effete products; (3) the porus genitalis (fig. 1, B, p), leading into a sinus into which the ducts of both sets of genital organs open,—it is to be found on the under surface of the headpapilla at or near its centre; (4) the opening of the Laurer-Stieda canal, situated on the dorsal surface of the animal, near the junction of the two portions of the median area,—it is excessively minute and difficult of detection, and leads by a narrow canal into the duct of the yolk-gland.

Internal Structure.—All Trematoda have been commonly regarded, like other flat-worms, as devoid of a body-cavity (coelom), and as consisting of parenchymatous tissue, in which the various organs were embedded. Recent researches of Fraipont (15) appear to show, however, that the intercellular spaces in this tissue are to be regarded as the homologue of a coelom. The body is enclosed by a complex sheath (cortex), which may be resolved into several layers, which will be discussed in order, proceeding from without inwards. (1) The cuticle, which encloses the whole body, is a thin, pellucid, structureless membrane; at the margin of the mouth it is reflected so as to form a lining for the oesophagus, and similarly at the opening of the genital sinus it passes inwards to form a lining to the vagina. The same phenomenon is observed at the excretory aperture. By the application of ammonia the cuticle may be separated from the subjacent tissues and its peculiarities demonstrated; although apparently smooth to the naked eye, it presents under the microscope numerous sharp backwardly directed processes, each of which encloses a hard stylet-shaped body. These prominences are closely set over the whole body except immediately around the suckers, extending even into the interior of the sinus genitalis. The cuticle is furthermore perforated by innumerable fine pores, directed outwards and somewhat backwards. With regard to the homology of the cuticle of Trematodes the same uncertainty prevails as in the case of Cestodes (see Taγe-Worms); the general opinion is that it is not comparable with the chitinous cuticle of Arthropoda, but is either a specially developed basement-membrane (Herbert, 11) or a layer of modified cells (Ziegler, 12, and Schwarze, 13). (2) The outer cellular layer is the matrix of the cuticle. (3) The muscular coat consists of three different layers:—(i.) a thin layer of circular fibres; (ii.) the longitudinal muscles, which form a series of separate bundles; (iii.) the oblique muscles, confined to the anterior half or third of the body, and crossing so as to form a rhomboidal lattice-work,—they are especially strong on the anterior ventral aspect of the animal. (4) The inner cellular layer consists of elements which closely resemble those of the outer, but are somewhat larger; they have been mistaken by various observers for cuticular glands. The suckers may be considered as parts of the cortical layer; speaking generally, each has the form of the segment of a sphere, although the anterior one is shallower at the lower than at the upper margin, and is penetrated by the oesophagus. Each consists of three sets of muscles,—a thin outer equatorial layer, a second meridional, and a mass of radially disposed fibres forming the greater part of the substance. It would appear that the function of the first two of these groups is to flatten out the sucker, whilst the radial ones restore its cavity and thus produce a suctorial action. To the ventral sucker are attached a number of muscular fibres belonging to the dorso-ventral system, and in particular a strong bundle, which passes from behind downwards and forwards.

The digestive system (fig. 1, A), the presence of which furnishes one of the most characteristic differences between Trematodes and Cestodes, extends throughout the body on a plane between the peripheral nervous and reproductive systems. It has only one aperture, as above mentioned, in the centre of the anterior sucker. The anterior portion or pharynx, although very short, measuring not much more than 1 mm. in length, is again divisible into two sections. The hinder of these is the larger, and is sometimes spheroidal but more commonly fusiform in shape; it has strong muscular walls, which, in conjunction with protractor and retractor muscles, bring about a kind of pumping action whereby nutritive fluids are taken into the stomach, which name may be applied to the larger posterior section of the alimentary tract, since in it the digestive processes are carried on. The canal, which leads from the posterior end of the pharynx, divides almost immediately into two branches, which diverge at first rapidly and then run almost parallel, as far as the hinder end of the body. Each of these gives off from its outer aspect some 16 or 17 lateral branches (c), which divide and subdivide till their ramifications fill nearly the whole area of the body. The digestive tract is lined by a layer of simple cells, resembling a cylinder epithelium. These behave towards the blood corpuscles and other contents of the intestine exactly as would a number of Amoebae, putting out processes or pseudopodia, which ingest them,— so that, in common with many of the lower Invertebrates, the liver-fluke lives by “intracellular digestion” (see Metschnikoff, 14).

The canals of the excretory system (m ) may be divided into three groups. (1) The collecting network consists of very fine tubules which anastomose freely with each other: they are situated on the boundary between the cortical and middle layers, and arc therefore visible from either side of the body. (2) Conducting vessels (v , d) receive the contents of this network. Each of these is formed by the union of a larger or smaller number of the delicate canals just described, and after a longer or shorter course opens into the median excretory canal (m ). On the way, however, it communicates with the neighbouring vessels, so that a second network is formed, which is distinguished from that of the collecting tubules by the greater size of its meshes and by the fact that it is specially visible from the dorsal surface of the animal. In the head four of these conducting vessels arise, which are disposed in two pairs, one situated dorsally and one ventrally. As they pass backwards they receive many branches, the dorsal unites with the ventral of its own side, and the two tubes thus formed unite to constitute the last division of the excretory system. (3) The median vessel (m ) passes along the body for the posterior two-thirds of its length, immediately beneath the dorsal cortical layer. It is widest near the commencement, where it measures about 0∙5 mm. in diameter, and finally opens at the posterior extremity of the body. The wall of the excretory apparatus is constituted everywhere by an exceedingly delicate elastic membrane, which exhibits neither a cellular lining nor cilia; furthermore, neither valves nor muscles have been demonstrated in connexion with it. It contains a thin colourless fluid, in which very small highly refractive drops are suspended.

The details of the termination of the excretory system seem to have been first clearly made out by Fraipont (15), who worked upon species in which they are more distinct than in the form now under consideration. The spaces between the round connectivetissue cells of the body are star-shaped in form, and into these the finest excretory tubules, above mentioned, open by funnels (fig. 1, D), into each of which projects a vibratile cilium, thus constituting the so-called “flame-cells.” These researches have given rise to numerous differences of opinion, as regards questions both of fact (16) and of priority (17).

The liver-fluke contains a complete set of male and female organs, which form the most conspicuous part of its anatomy, and both of which open into the genital sinus which has been described above. A. The Male Organs. (i.) The testes (fig. 1, B, t ) are two in number, situated one behind the other in the hinder division of the median area. They rest upon the ventral cortical layer in the parenchyma of the body, and immediately above them are the ramifications of the digestive tract. Each consists of a large number of ramifying tubes, often with slightly dilated extremities. These unite into three or four, and eventually into two, main excretory ducts (va, vp), which terminate at the base of the cirrus-pouch. Within the testicular tubules may be found spermatozoa in all stages of development; the first stage appears to consist of small roundish membraneless cells with a single nucleus; the nucleus then divides and the cells become polygonal from mutual pressure. These large cells lie in the middle rather than at the sides of the tube, and among them are a number which, while they possess on one side a smooth evenly rounded contour, are on the other very irregularly and deeply serrated. These serrations elongate until they become the delicate filaments of spermatozoa, the small shining heads of which are still embedded in the protoplasm of the cell. (ii.) The vasa deferentia (va, vp) are a pair of slender elongated canals, which lie on the two sides of the middle line, and unite at the inner extremity of the cirrus-pouch, which they penetrate in common. Their walls consist of a very delicate homogeneous but resistant membrane, upon which contractile fibres are disposed, close together and parallel to the axis. (iii.) The cirrus-pouch (cs) is a muscular egg-shaped organ; the upper pole, which receives the united vasa deferentia, is situated above the ventral sucker and separated by only a very slight interval from the dorsal cortical layer, whilst the position of the lower pole is indicated by the porus genitalis (p ). The muscles are disposed in two layers, of which the inner is thin and composed of circular fibres; the outer longitudinal layer is much thicker, and its fibres are disposed in bundles; furthermore its apex receives a large number of dorso-ventral fibres. Within the cirrus-pouch the two terminal sections of the male conducting apparatus are situated. (iv. ) The first of these is the vesicula seminalis (fig. 1, B, vs ; C, g), a large, spindle-shaped dilatation of the canal usually more or less curved upon itself. Its wall is somewhat more complex than that of the vasa deferentia, consisting of a layer of tissue with many nuclei but no distinct cell-boundaries, succeeded by a delicate layer of circular muscular fibres, which is again followed by a layer of longitudinal ones. (v. ) The ductus ejaculatorius (fig. 1, C, h ), which immediately succeeds the vesicula seminalis, is a long slender tube, disposed in coils, and usually projecting like a papilla into the base of the sinus genitalis. Its walls are furnished with a number of unicellular glands. B. The Female Organs. The female reproductive apparatus may be roughly divided into two portions, that which produces the eggs and that which conveys them to the outside of the body; in the former of these processes three organs take part—one producing the germ, another the secondary or food-yolk, and a third the egg-shell. (i. ) The germarium or ovary (fig. 1, B, o ) is situated between the anterior testis and the ventral sucker, in about three cases out of four on the right hand side of the body. It has the form of a branching tubular gland, the ramification being dichotomous throughout; in most cases the branches are about as large as the stems which give rise to them. The oviduct passes towards the shell-gland, narrowing as it approaches this, and finally unites with the excretory duct of the yolk-glands. (ii.) The yolk-glands (y) of the liver-fluke are paired organs of considerable size; they extend over both lateral areas, to which they impart the opaque appearance and reddish colour above alluded to. They are composed of innumerable small acini, spheroidal in shape and situated in groups on minute ductules, which unite to form a longitudinal canal on either side of the body. These canals are on the whole parallel to the margins of the animal and distant from it about one-fifth of its greatest breadth. At the anterior margin of the testicular area each longitudinal canal gives off a transverse branch, which unites with its fellow of the opposite side in the middle line to form a pear-shaped reservoir, situated just behind the posterior margin of the shell-gland. From this reservoir the common yolk-duct passes forwards in the substance of the shellgland and there unites with the oviduct. Previously to this, however, it gives off a minute canal, which after an upward course opens on the dorsal surface of the animal; it is known as the Laurer-Stieda canal, and its function has been the subject of much discussion. It has been supposed (1) “to serve for copulatory purposes,” as has been seen by Zeller (18) in Polystomum, and as is supported by its structure in Axine and Microcotyle, and (2) “to act as a safety tube for the escape of over-abundant or altered vitelline products and spermatozoa,” the main argument in support of which is that its calibre is too narrow to admit of copulation taking place by its means; compare Sommer (10), Kerbert (11), Poirier (19), Loose (20), and Lorenz (21). (iii.) The uterus or female conducting apparatus (u ) originates at the union of the ducts of the germarium and yolk-gland. Its first portion, which lies within the shell-gland, is a delicate narrow canal, except when it is distended either by eggs or by semen. The median section of the organ is by far the largest both in length and breadth; it occupies almost the whole of the anterior part of the median area of the animal, between the ventral sucker and the shell-gland, and forms four or five large coils lying alternately right and left, which as a rule are filled with completely formed eggs. The third section of this organ includes the coils which lie above and anterior to the ventral sucker; it is sometimes called the vagina. When it contains eggs these are generally in a single file, and thus give it a moniliform appearance; it lies entirely on the left side of the body, gradually approaching the middle line as it passes forward, until it ends below the cirrus-pouch at the left and posterior aspect of the genital pore (fig. 1, C, e ). (iv.) The shell-gland (fig. 1, B, s), which (as its name implies) furnishes the external coating of the eggs, has been already several times mentioned. In the Trematodes, as in the tape-worms, it forms a kind of central point of the female generative system; it is a spheroidal mass of unicellular glands, each of which opens by its own special duct into the commencement of the uterus. The secretion of the shell-gland is liberated in the form of small pellucid droplets, which unite to form drops; afterwards it becomes thick and viscid and of a mahogany brown colour. In this condition the drops are dispersed through the uterus mixed with the secretions of the other genital glands, and they apply themselves to the recently formed eggs, producing a delicate membrane around them. This process is carried on in those coils of the uterus which lie immediately outside the shell-gland, corresponding to the “ootype” described by Van Beneden in other Trematodes.

The eggs undergo a gradual development as they pass along the uterus. The ripe primitive ovum, on entering the female conducting apparatus, becomes coated with a larger or smaller number of spherules of secondary yolk, and then undergoes the process...