(2) Formation of the Ova.—The ovarian cells arising from the ovary first become mature after their entry into the oötype by the formation of three polar bodies, fertilization then taking place. At the same time as the ovarian cell a number of yolk cells from the vitellarium and secretion, drop by drop, from the shell gland reach the oötype.262 The shell is then formed during the generally active contractions of the oötype walls and then passes on into the uterus. In the uterus of the endoparasitic trematodes the eggs accumulate more and more, often in large quantities, while in ectoparasitic species generally only one or some few eggs can be found. The completed ova are of various forms and sizes. They are mostly oval, at all events in the digenetic trematodes, and the yellowish or brown shell is provided with an opening at one pole which is closed by a watch-glass-shaped lid (operculum). Appendages (filaments) on the shell—at one or both poles—are uncommon, but are the rule in the ova of the Monogenea (ectoparasitic species).

(3) Deposition of the Ova.—Soon after their formation, the Monogenea (ectoparasitic trematodes) deposit round the place of their attachment on the skin or the gills or other organs of their hosts, eggs which attach themselves by means of their filaments. The embryonic development thus takes place outside the parent. This also holds good for the eggs of many endoparasitic species, although as a rule in these the eggs are always retained for a longer time in the uterus. Moreover, they usually here undergo a part or a whole of their development, and are eventually deposited in those organs in which the adult forms are parasitic, but this is not always the case, as the egg, e.g., of F. hepatica appears in bile (and fæces) quite unchanged. By the natural passages they eventually get out of the body, and in cases where such do not exist, as in the case of the blood-vessels, the eggs pass out by means of the kidneys.

(4) The embryonic development, after irregular segmentation of the ovum into a number of blastomeres, leads to the formation of a solid blastosphere or morula, which is surrounded by a cellular investing membrane (yolk envelope), while the principal mass of the cells forms the embryo, which uses for its nourishment the yolk cells, which have in the meantime disintegrated (cf. footnote, p. 223). Usually, after the ova have reached water the embryos hatch out, leaving the yolk envelope in the egg-shell; in other cases, however, the embryos only hatch out after having been subjected to the influence of the intestinal juices, that is to say, in the intestine of an intermediate host which has ingested with its food the ova that have escaped from the primary host.

(5) The post-embryonic development of the Trematodes is accomplished in various ways; the process is the most simple in the ectoparasitic species (Monogenea), the young of which should certainly be regarded as larvæ, because they possess characteristics (cilia, simple gut, etc.) that are lacking in the adult worms, but which, nevertheless, pass into the adult state direct after a relatively simple metamorphosis. In the Holostomata,263 a group found chiefly in the intestine of aquatic birds, and which rarely occur in other vertebrates, the ova develop in water. The young are ciliated all over, and, after having entered an intermediate host (leeches, molluscs, arthropods, amphibians, fishes) living in the water, they undergo a metamorphosis into a second larval stage; they then encyst and await transmission into the final host, where they become adult Metastatic trematodes, i.e., trematodes without asexually produced generations (p. 229).

In the remaining so-called digenetic trematodes (p. 230) one or two asexual generations interpose between the miracidium and terminal stage, so that quite a number of adult worms may originate from one egg. Usually the young, which are termed MIRACIDIA264 (fig. 129), hatch in water, where they move with the aid of their cilia. Sooner or later they penetrate into an intermediate host, which is always a snail or a mussel, and while certain of their organs disappear, they grow into a gutless germinal tube (SPOROCYST, fig. 131). These are simple elongated sacs with a central body cavity. They may or may not have excretory tubules. In these, according to the species, the larval stages (CERCARIÆ) that will ultimately become adult worms are produced, or another intermediate generation is first formed, viz., that of the REDIÆ265 (figs. 132, 133), which are always provided with an intestine, and these then give rise to cercariæ (figs. 130, 134). The cercariæ, as a rule, leave their host and move about in the water with the assistance of their rudder-like tails. After a little time, however, they usually again invade an aquatic animal (worms, molluscs, arthropods, fishes, amphibians), then they lose their tails and become encysted (fig. 135); here they wait until they attain, together with their host, the suitable terminal host, and in this new situation they establish themselves and reach maturity. Or, again, the cercariæ may themselves encyst in water or on foreign bodies (plants) and wait until they are taken up directly by the terminal host, e.g., sheep.

Accordingly the following conditions are necessary for the completion of the entire development: (1) The terminal host in which the adult stage lives; (2) an intermediate host into which the miracidia penetrate and in which they become sporocysts; (3) a second intermediate host in which the cercariæ become encysted. In certain species, as in Fasciola hepatica, this second host is omitted, as the cercariæ spontaneously encyst on plants, or again (in other species) encystment may occur within the first intermediate host, when, in fact, the cercariæ (which in this case do not acquire an oar-like tail) do not swarm out of, but encyst themselves within their sporocysts. The development, moreover, may be further complicated by rediæ appearing in addition to the sporocysts, though this occurs in the first intermediate host and not in a second one.

Animals that harbour adult digenetic Trematodes thus become infected by ingesting encysted cercariæ, which either occur (1) in certain animals (second intermediate hosts) on which they feed, or (2) in water, or (3) on plants, or finally (4) in the first intermediate host; whereas animals harbouring encysted cercariæ have been directly infected by the corresponding tailed stage, and animals harbouring germinal tubes (sporocysts or rediæ) have been infected by the miracidia.

(1) The MIRACIDIA of the digenetic Trematodes are comparatively highly organized, and the mode of their formation from the segmentation cells of the ovum is only imperfectly known. They have a cuticular epithelium (fig. 129) entirely or partly covered with cilia, beneath this a dermo-muscular tube composed of circular and longitudinal muscles; also, a simple gut sac with an œsophagus, occasionally also with pharynx, salivary glands and boring spine, also a cerebral ganglion on which, in some species, there are eyes (fig. 131, a). As to the excretory organs, they are represented by two symmetrically placed terminal flame cells, with excretory vessels opening separately; there is a more or less ample (primary) body cavity between the parietes of the body and the gut; from the cellular parietal lining of this cavity single cells (germ cells) become free (fig. 131, a, b), and become rediæ or cercariæ.

[The germ cells of the miracidium and the germ balls of the sporocyst arise, according to some observers, by further division of undifferentiated blastomeres; according to others from the cells of the lining wall of its body cavity. It is from these free germ balls that the redia stage is developed.

[In the germ ball or morula appears an invagination, giving rise to the cup-shaped gastrula stage. This elongates and forms the REDIA (fig. 131, c).

[In the interior of the redia cells are budded off and develop into gastrulæ, as in the case of the sporocyst. These become a fresh generation of rediæ or give rise to the third stage (CERCARIA).]

(2) The SPOROCYSTS, on the contrary, which are produced direct from the miracidia, are very simple, as all the organs of the latter disappear, even to the muscles and excretory organs, during or after penetration into the intermediate host, whereas the budded and still budding cells of the wall of the (primary) body cavity continue to develop rapidly and form germ balls. The sporocysts when fully developed have the appearance of tubes or fusiform bodies with rounded edge; they are frequently of a yellow colour. Their length rarely exceeds a few millimetres; in some species their size increases exceedingly through proliferation, and they then occupy a large portion of the body of the intermediate host.

(3) The REDIÆ (figs. 132, 133), on the other hand, are more cylindrical and always have a simple intestine of varying length, provided with a pharynx; they likewise possess, situated near the circular ridge, a “birth pore” which serves for the exit of the cercariæ originating within them.

(4) The CERCARIÆ266 are very different; typically they consist of the anterior body and the oar-like tail at the posterior end (fig. 134). The former, even to the genitalia, has the organization of the adult digenetic Trematodes, and thus allows the easy recognition of at least the characters of that large group to which the species in question belongs. On the other hand, however, there are also organs that are lacking in the adult form, such as, in many, the boring spine in the oral sucker, or the eyes situated on the cerebral ganglion; moreover, also, cutaneous glands (fig. 134), the secretion of which forms the cyst membrane. The oar-like tail may be long or short (stumpy-tailed cercaria) or entirely absent; its free end may be partly split (furcate cercaria), or split to its base (bucephalus); in various forms also the anterior end of the tail is hollow, and has enclosed within it the anterior body, which is otherwise free. The size also of the cercaria belonging to the different species is very diverse; in addition to forms swimming in the water that have the appearance of minute milky-white bodies, there are forms which measure as much as 6 mm. in length.

The encysted cercariæ (fig. 135) are globular or oval, and are surrounded by a homogeneous membrane, which may be striated or contain granules. The tail is always cast off when encystment occurs, and organs peculiar to the cercaria stage (boring papilla, eyes) almost entirely disappear. On the other hand, the genitalia appear or become more or less highly developed, in extreme cases to such an extent that they become functional, and after autocopulation the creatures produce ova within the cysts.

The cycle of development of the digenetic Trematodes has hitherto been generally explained as a typical ALTERNATION OF GENERATIONS, one sexual generation regularly alternating with one or two asexually reproducing generations. Recent authors, however, regard the cells in the sporocysts from which rediæ or eventually cercariæ arise as parthenogenetically developing ova, and the sporocysts as well as the rediæ as generations propagating parthenogenetically. In this case, however, it is an alternation of a sexual not with an asexual but with firstly a parthenogenetic generation (the sporocyst), the central cells of which are regarded as ova which develop parthenogenetically into the redia, and this the second parthenogenetic generation finally produces larvæ (cercariæ) capable of developing into the sexually mature form.

Other authors, again, regard the development of the Digenea as only a complicated metamorphosis (p. 283), which is distributed over several generations before it is concluded.

Biology.

Endoparasitic Trematodes, as fully developed organisms, occur in vertebrate animals only, with very few exceptions; they inhabit almost all the organs (with the exception of the nervous and osseous systems and the male genitalia), but by preference the intestine in all its extent from the oral cavity to the anus; and, further, certain species or groups inhabit only quite restricted parts of the intestine. Besides in the intestine other species live in the liver, or in the bile-ducts, or in the gall-bladder; other accessory organs of the intestine, such as the pancreas, bursa Fabricii (of birds), are only infected by a few species. Many inhabit the lungs, or the air sacs in fowls, a few the trachea. Trematodes have also been known to occur in the urinary bladder, the urethra and the kidneys of all classes of vertebrates; they are also present in the vascular system of a few tortoises, birds and mammals; in birds they even penetrate from the cloaca into the oviducts, and are occasionally found enclosed in the laid eggs; one species is known to occur in the cavum tympani and in the Eustachian tube of a mammal (Dugong), another in the frontal sinus of the polecat; several species infest the conjunctival sac under the membrana nictitans of birds, one species even lives in cysts in the skin of song-birds. In an analogous manner the ectoparasitic Trematodes are not entirely confined to the surface of the body or the trachea of the lower vertebrate animals; a few species appear exclusively in the urinary bladder, in the œsophagus, and in the case of sharks in an accessory gland of the rectum.

Trematodes live free and active within the organs attacked, though they may attach themselves by suction for a longer or shorter period; in other cases, however, they bore more or less deeply into the intestinal wall with their anterior end, or lie in cysts of the intestinal wall which only communicate with the lumen through a small opening; in those species living in the lungs of mammals the host likewise produces a cyst, which usually encloses two specimens; such association of a pair is also observed in other situations, and, though this is the rule in species sexually distinct, it is not entirely confined to these.

As regards the AGE attained by endoparasitic Trematodes, there are but few reliable records, and these differ considerably; the overwhelming majority of species certainly live about a year, or perhaps a little longer, but there are some whose term of life extends to several or many years.

Trematodes are but rarely found encysted in the higher vertebrate animals; the condition, however, is more frequent in amphibians, and especially in fishes, as well as in numerous invertebrate animals.

Classification of the Trematodes of Man.

The following classification, partly artificial, partly natural, embraces only the flukes found in man:—

Order. Digenea, v. Beneden, 1858.

Sub-order. Prostomata, Odhner, 1905.

Group. Amphistomata, Rudolphi, 1801, ep., Nitzsch, 1819.

Family. Paramphistomidæ, Fischoeder, 1901.

Sub-family. Paramphistominæ, Fisch., 1901.

Sub-family. Cladorchiinæ, Fisch., 1901.

Family. Gastrodisciidæ, Stiles and Goldberger, 1910.

Group. Distomata, Retzius, 1782.

Family. Fasciolidæ, Railliet, 1895.

Sub-family. Fasciolinæ, Odhner, 1910.

Sub-family. Fasciolopsinæ, Odhner, 1910.

Family. Opisthorchiidæ, Braun, 1901, emend. auctor.

Sub-family. Opisthorchiinæ, Looss, 1899, emend. auctor.

Sub-family. Metorchiinæ, Lühe, 1909.

Family. Dicrocœliidæ, Odhner, 1910.

Family. Heterophyiidæ, Odhner, 1914.

Family. Troglotremidæ, Odhner, 1914.

Family. Echinostomidæ, Looss, 1902.

Sub-family. Echinostominæ, Looss, 1899.

Sub-family. Himasthlinæ, Odhner, 1910.

Family. Schistosomidæ, Looss, 1899.

The Trematodes Observed in Man.

Family. Paramphistomidæ, Stiles and Goldberger, emend. 1910.

Sub-family. Cladorchiinæ, Fisch., 1901.

Genus. Watsonius, Stiles and Goldberger, 1910.

Watsonius watsoni, Stiles and Goldberger, 1910.

Syn.: Amphistomum watsoni, Conyngham, 1904; Cladorchis watsoni, Shipley, 1905.

Female Organs.—Ovary dorso-posterior of posterior testis. Shell gland dorsal to ovary. Vitellaria ventral and lateral to gut cæca extending from gut fork to equator of acetabulum. Uterus dorsal to testes, ductus hermaphroditicus absent. Laurer’s canal opens in dorso-median line slightly behind anterior border of sucker.

Family. Gastrodisciidæ.

Genus. Gastrodiscus, Lkt., 1877.

Gastrodiscus hominis, Lewis and McConnell, 1876.267

Syn.: Amphistomum hominis, Lew. and McConn.

Body, reddish in the fresh, 5 to 8 mm. long; posteriorly, 3 to 4 mm. broad. The disc has incurved edges which are interrupted in front where it joins the anterior cylindrical portion and posteriorly behind the ventral sucker. The disc itself and ventral surface are covered with a number of (microscopic) papillæ. Pharynx provided with two diverticula or pouches. The bifurcation of the gut lies sometimes above, sometimes below the level of the genital pore. The gut cæca end about the level of the centre of the acetabulum.

Testes much lobed, the anterior is smaller than the posterior and lies at about the level where the anterior conical portion joins the disc. The posterior testis just in front of the anterior margin of the acetabulum separated from it by the ovary. The ovary, somewhat oval in shape or slightly constricted in the middle, lies slightly to the right of the median line. Dorsal to it lies the well-developed shell gland, Laurer’s canal opening in front of the excretory bladder. The excretory bladder is a long sac with its opening at its posterior extremity about the level of the middle of the acetabulum. The vitellaria are restricted in extent. They do not extend forward beyond the anterior border of the posterior testis. They are best developed in the area between the acetabulum and the termination of the gut cæca.

The eggs are oval and measure 150 µ in length by 72 µ in breadth.

Habitat.—Cæcum and large intestine of man. Also in the pig (5 per cent.) in Annam.

Distribution.—This parasite has been recorded from Assam (not uncommon), British Guiana (Indian immigrants), and Cochin China.

Gastrodiscus ægyptiacus, Cobbold, 1876, and G. secundus, Looss, 1907, occur in the horse; G. minor, Leiper, 1913, in the pig in Nigeria and Uganda.

Family. Fasciolidæ, Raill., 1895.

Sub-family. Fasciolinæ, Odhner, 1910.

Genus. Fasciola, L., 1758.

Fasciola hepatica, L., 1758.

Syn.: Distomum hepaticum, Retz., 1786; Fasciola Humana, Gmel., 1789; Distomum caviæ, Sons., 1890; Cladocœlium hepaticum, Stoss., 1892.

Length 20 to 30 mm., breadth 8 to 13 mm., cephalic cone 4 to 5 mm. in length and sharply differentiated from the body by a shoulder on each side. Spines in alternating transverse rows and extending on the ventral surface to the posterior border of the testes, and on the dorsal surface not quite so far. The spines are smaller on the cephalic cone than on the posterior part of the body, where they are discernible with the naked eye. The suckers are hemispherical, and near each other; the oral sucker is about 1 mm. and the ventral sucker about 1·6 mm. in diameter. The pharynx, which includes almost the entire œsophagus, measures 0·7 mm. in length and 0·4 mm. in breadth. The intestine bifurcates at the limit of the cephalic cone and the branches are even here furnished with diverticula directed outwardly. The ovary is ramified and situated in front of the transverse vitelline duct, usually on the right side; the shell gland lies near the ovary in the median line; posterior to the transverse vitelline ducts are the greatly ramified testes, which occupy the greater portion of the posterior part of the body, with the exception of the lateral and posterior border; the long vasa efferentia only unite as they enter the cirrus pouch. The vitellaria occupy the sides of the posterior part of the body, commencing at the level of the ventral sucker and uniting behind the testes. The ova are yellowish-brown, oval, operculated, 130 µ to 145 µ in length, 70 µ to 90 µ in breadth (average size 132 µ by 70 µ).

The Liver Fluke inhabits the bile-ducts of numerous herbivorous mammals (sheep, ox, goat, horse, ass, rabbit,268 guinea-pig, squirrel, beaver, deer, roe, antelope, camel, kangaroo, and others), and is distributed over the whole of Europe, though not to an equal extent. It is further known in North Africa, in North and South America, as well as in Australia; it is also found in Asia, as it has been reported from Japan, China, and Tonkin (Gaide, two cases in man). In some districts of Germany it is very frequent, and the slaughter-house statistics of various places show that it is of daily occurrence. Fasciola magna occurs in herbivora in America.

[The following records show the enormous loss caused in sheep by this parasite. In 1812, in the Midi, principally in the Departments of the Rhône, Herault, and Gard, the disease was rampant; 300,000 sheep perished in the Arles territory, and 90,000 in the Arrondissements of Nîmes and Montpellier. In 1829 and 1830, in the Department of the Meuse and near localities, not only sheep but oxen died in enormous numbers; for instance, in the Arrondissement of Verdun out of 50,000 sheep 20,000 died, and out of 20,000 cattle 2,200 died. In England, in 1830, 2,000,000 sheep were carried off; whilst in 1862 60 per cent. of the sheep died in Ireland; and in 1879 over 300,000 were lost in England; whilst as late as 1891 one owner in the same country lost over 10,000 sheep (Live Stock Journal, October 30, 1891).—F. V. T.]

The disease usually commences towards the end of summer with an enlargement of the liver, induced by the invasion of numerous young flukes; in the autumn and winter the animals suffer from the consequences of disordered biliary secretion; they become feverish, emaciated, and anæmic, and lose their appetite. In consequence of the consecutive atrophy of the liver, œdema and ascites set in, and many animals succumb to this “liver rot.” On examination the liver is found to be shrunken, the bile-ducts are enormously dilated and in parts saccular and full of flukes. Should the animals survive this stage, spontaneous recovery ensues in consequence of the flukes commencing to leave the liver in the spring, but the liver remains changed and its sale is prohibited269 when the changes are extensive.270

[The following stages may be noticed in sheep suffering from fascioliasis. Gerlach recognized four stages, based on the varied relations that the flukes contract with the liver of their host. These periods are sometimes very marked, but at others, owing to subsequent infections, the features become merged and so obliterated. But when a single infestation occurs they are very marked.