Chapter XIV: Appendix: “Rhizopods in Poliomyelitis acuta.” (8)

The PARENCHYMA is a connective substance, the structure of which is still a matter of dispute. It consists, according to some authors, of multipolar cells, the offshoots from which anastomose with each other so that a network, permeating the entire body and encompassing all the organs, is produced. There exists also, as part of it, a homogeneous matrix, in the form of lamellæ and trabeculæ that border small cavities communicating with each other and filled with fluid. According to other authors, the parenchyma of the Trematodes consisted originally of cells, of which, however, only the cell membranes remain, while the protoplasm has been liquefied except for small residua around the nucleus. Between these cells an intercellular mass has appeared. By partial absorption of the walls, adjoining spaces unite, and the originally flat cell walls become transformed into trabeculæ. According to this view the cavities filled with fluid are _intra_-cellular, according to the former view _inter_-cellular. Pigment cells occur only in a few species.

The MUSCULAR SYSTEM of the Trematodes is composed of (1) a dermo-muscular tube, (2) the dorso-ventral or parenchymal muscles, (3) the suckers, and (4) the special muscles of certain organs.

The dermo-muscular tube, which lies fairly close to the cuticle, consists of annular, diagonal, and longitudinal fibres which surround the entire body in one or several layers, and as a rule are more strongly developed on the ventral surface as well as in the anterior part of the body. The MUSCLES OF THE PARENCHYMA are found chiefly in the lateral parts of the body and pass through the parenchyma in a dorso-ventral direction; their diverging brush-like ends are inserted on the inner surface of the cuticle (fig. 120).

The suckers are specially differentiated parts of the dermo-muscular tube. Their concave inner surface is lined by the continuation of the cuticle and their convex external surface is covered by a more dense tissue that frequently takes the form of a refractive membrane, thus separating them from the parenchymal muscles.

The principal mass of the suckers consists of muscular fibres which run in three directions--equatorial, meridional and radial. The equatorial fibres correspond to the annular muscles, the meridional fibres to the longitudinal muscles, and the radial fibres to the muscles of the parenchyma; the radial fibres are always the most strongly developed. The function of these muscles is evident from their position; the meridional fibres flatten the suctorial disc and diminish the depth of its cavity, so that the internal surface may adhere to the object to be held; if the equatorial fibres now contract, the sucker rises by elongating longitudinally, and its inner surface is drawn in by the contraction of the radial muscles. Thus the sucking disc becomes adherent. Usually also there is a sphincter at the border of the suckers, which plays its part during the act of adhesion by constricting in a circular manner that part of the mucous membrane to which it is attached. The loosening of the fixed sucker is effected by relaxation chiefly of the radial fibres, by the contraction of the meridional fibres and certain bundles of muscles situated at the base and at the periphery of the suckers. The connective and elastic tissues between the muscles of the suckers probably also take part in the process.

Of the muscles of the organs which have developed from the parenchyma muscles we may briefly mention those bundles that are attached to certain parts of the genital apparatus, to the suckers, to the hooks and claws, and also, at all events in _Fasciola hepatica_, to the spines. The sheaths used for the projection of the tentacles of the _Rhopaliadæ_ are also muscular.

The contractile elements consist of fibres of various lengths that are mostly parallel to one another, and frequently anastomose; a cortical substance finely fibrillated can usually be distinguished from an internal homogeneous mass; large nucleated cells of uniform size are always connected with them; these have been variously interpreted, but have been proved to be myoblasts, one or more of their processes constituting the muscular fibres.

The MOVEMENTS of the Trematodes consist in alterations of form and position of the body, as well as in creeping movements.

In the NERVOUS SYSTEM (fig. 121) can be distinguished a cerebral portion as well as strands (medullary strands) running from it, and peripheral nerves. The cerebral portion always consists of two large ganglia situated in the anterior end of the body which pass dorsally over the œsophagus and are connected by means of a broad and thick commissure composed of fibres only. From each ganglion three nerves run anteriorly--the inner and dorsal nerve for supplying the anterior dorsal part of the body; the median and ventral for the oral sucker; and the exterior and lateral likewise for the supply of the sucker.

In a similar manner three strands run backwards from each ganglion--one dorsal, one lateral and one ventral. The dorsal and ventral strands become united and curve backwards; the symmetrical lateral strands are connected by means of transverse commissures, the number of which vary according to the species. Such commissures also exist between the lateral and the two other strands on each side. There are ganglion cells along the entire course of the posterior cords, more particularly at the points of origin of the commissures. There also appears to be in addition a fourth anterior and posterior pair of nerves, the front pair for the oral sucker and the hind pair for the pharynx.

The peripheral nerves, which spring from the posterior strands as well as from the commissures, either pass directly to the muscular fibres or to the sensory cells that are situated at the level of the subcuticular cells, or they reach these after the formation of a plexus situated immediately beneath the dermo-muscular layer; the processes directed outwards terminate in small vesicles in the cuticle.

As to other ORGANS OF SENSE, simple eyes, two or four in number, are known in several ectoparasitic species as well as in a few free-living larval stages (Cercariæ) of endoparasitic forms. In the adult stage, however, they usually undergo complete atrophy.

The ALIMENTARY CANAL commences with an oral aperture, generally terminal or sub-terminal (ventral) at the anterior extremity, which leads into an oral cavity usually surrounded by a sucker; the œsophagus, of various lengths, is directed backwards and is generally surrounded by a muscular pharynx (fig. 122). In some cases there exists between the sucker and pharynx, pharyngeal pouches (præpharynx). Sooner or later the intestine divides into two lateral branches directed backwards, both of which end blindly (cæca) at the same level.[257] In many ectoparasites (_Monogenea_ [p. 222]) a connection exists between the genital glands and one of the intestinal branches (ductus vitello-intestinalis [fig. 123]).

[257] The following conditions represent deviations from this type: (1) In _Gasterostomum_ the oral aperture is situated in the middle of the ventral surface, and occasionally is even nearer to the posterior than to the anterior end. There is no proper oral sucker, but the pharynx is thus termed. (2) A few genera, such as _Gasterostomum_, _Aspidogaster_, _Diplozoon_, etc., have only _one_ intestinal diverticulum, which is undoubtedly to be taken as representing the primitive condition, as it is also often found in the young stages of the _Trematoda_. (3) The branches of the intestines are curved and united behind (several _Tristomidæ_ and _Monostomidæ_), while in _Polystomum integerrimum_ (in the bladder of frogs) there are several commissures between the intestinal branches, and in the _Schistosomidæ_ the united intestinal branches proceed as one channel towards the posterior end. (4) The termination of the two intestinal branches is not always on a level; they are therefore of different lengths. (5) When the œsophagus is very long the intestinal branches extend both forward and backward, so that the gut exhibits the form of an *H*. (6) In the broad and flat species the gut-forks form diverticula mostly externally but also internally; these again may branch (fig. 139). (7) In a few cases (_Nematobothrium_, _Didymozoon_) the intestine completely disappears up to the pharynx.

The oral cavity, pharyngeal pouches, pharynx, and œsophagus are lined with a continuation of the cuticle of the body; the gut cæca are lined with tall cylindrical epithelium (fig. 120). The œsophagus and intestinal branches often have also one layer of annular and longitudinal muscles; the pharynx has essentially the structure of a sucker (fig. 122).

The accessory organs of the alimentary canal consist of groups of unicellular SALIVARY GLANDS that discharge into the œsophagus in front of or behind the pharynx, or even into the pharynx itself.

The food of the Trematodes consists of mucus, epithelial cells, the intestinal contents of the hosts, and often also of blood, and this not only in those species living in the vascular system, but also in species living as ectoparasites or in the intestine or biliary passages of their hosts.

The final products of assimilation dissolved in the fluids of the body are distributed throughout the parenchyma and are thence expelled by a definite tubular system (excretory apparatus, proto-nephridia, formerly also termed the water-vascular system). This system, which is distributed throughout the entire body (fig. 124), is symmetrically developed, and, in the ectoparasitic Trematodes, it opens, right and left, at the anterior end on the dorsal surface; in all other flukes, however, it opens singly into the excretory pore (foramen caudale) at the centre of the posterior border; in those cases, however, where a sucker is present at the posterior end, as in the Amphistomata, the excretory pore is situated on the dorsal surface close in front of the sucker.

The EXCRETORY SYSTEM[258] consists of several parts: (1) of the more or less numerous terminal “flame” cells or funnel cells (figs. 124, 125); (2) of the capillaries ending in them; (3) of larger vessels receiving the capillaries; and (4) of the excretory bladder. Terminal cells and capillaries may be compared to unicellular glands with long excretory ducts; the cellular body (fig. 125) is comparatively large, stretched longitudinally, more rarely transversely, and provided with numerous processes, that are lost in the parenchyma; within is a conical cavity (analogous to the secretory cavity of unicellular glands) which is continued directly into the structureless capillary; at its blind end is a bunch of cilia projecting into the cavity, and which, during life, shows a flickering motion (ciliary flame). The nucleus is situated in the protoplasm of the terminal cell at its blind end.

[258] The following description relates in the main to the _Distomata_.

The entire apparatus thus begins blindly--_i.e._, within the terminal cells, to which must be ascribed the capacity of taking up from the fluid that permeates the parenchyma the products which are first collected into their own cavities and thence excreted by means of the capillaries and vessels.

The vessels possess definite walls, consisting of a membrane and a nucleated protoplasmic layer. They unite at many points on either side, and again pass into other canals (COLLECTING TUBES), which finally, travelling towards the posterior end, discharge into the excretory bladder (fig. 124).

The form and size of the bladder vary much according to the different species, but it always possesses its own flattened epithelium, surrounded by circular and longitudinal muscles, the circular muscles forming a sphincter around the opening. Frequently also the structure of the bladder extends to the tubules discharging into it, which therefore are not to be regarded as separate “vessels,” but rather as tubular diverticula of the bladder, directed anteriorly. In some few species the diverticula also branch and the branches anastomose, so that a network of tubules ensues which receives the vessels or capillaries. In such cases there are also ciliary tracts in the tubules.

The contents of the entire apparatus usually consist of a clear or sometimes reddish fluid; in some species there are larger or smaller granules, and occasionally also concretions occur.

_Sexual Organs._--Nearly all the Trematodes are hermaphrodites, and only a few (_Schistosomidæ_, _Koellikeria_) are sexually differentiated. The sexual organs usually lie in the “central field” limited by the gut cæca; the vitellaria, on the other hand, are, as a rule, external to the gut cæca in the “lateral fields.”

The male apparatus[259] is composed of two variously formed testes (fig. 127) (globular, oval, indented, lobed, or ramified), which may lie side by side or one behind the other; from each testicle a tube (vas efferens) originates; sooner or later, both tubes as a rule unite to form the ejaculatory duct or vas deferens, which is frequently enclosed in a muscular CIRRUS SAC, or more rarely passes directly into the genital pore. The cirrus, which is the thick muscular terminal portion of the vas deferens, can be everted and protruded from the cirrus sac and serves as an organ of copulation. The walls of the muscular portion of the tube (the cirrus) are attached to the walls of the cirrus sac, and hence when the sac contracts the cirrus cannot be protruded except by evagination of its lumen. Opening into the middle portion of the vas deferens, and as a rule enclosed in the cirrus sac, is found a mass of unicellular glands (prostate), the vesicula seminalis (which is likewise within, or may also be outside the sac) being the dilated first portion of the vas.

[259] The following description relates mainly to the _Distomata_.

The female genitalia (fig. 126) consist of an ovary, usually situated in front of the testes, the form of which varies according to the species, the usually double vitellaria, the ducts and a number of auxiliary organs; the short oviduct directed towards the centre arises from the ovary, and is connected in the median line with the excretory duct of the vitelline glands. These grape-like glands possess longitudinal excretory ducts, which assume a transverse direction behind the ovary, unite together at the median line and form a single duct, often dilated into a vitelline receptacle, that unites with the oviduct. Near this point, moreover, there frequently opens a canal (Laurer’s canal) which begins on the dorsal surface, and on the inner end of which a vesicle filled with sperm (receptaculum seminis) usually occurs (fig. 126). Moreover, there are also numerous radial unicellular glands (shell glands) at or beyond the point of junction of the oviduct, vitelline ducts and Laurer’s canal. In this portion of the duct (oötype), which is usually dilated, the ovarian cells are fertilized, surrounded with yolk cells and shell material, and as ova with shells they pass into the uterus (a direct continuation of the oviduct), which, with its many convolutions, occupies a larger or smaller portion of the central field, and runs either direct to the genital pore or, forming convolutions, first runs posteriorly and then bends forward (descending and ascending limbs). In both cases the terminal part lies beside the cirrus pouch and discharges beside the male orifice either on the surface of the body or into a genital atrium. The terminal portion of the uterus, which is often of a particular structure, serves as a vagina (METRATERM).

The cirrus sac may include (1) the genital atrium (_i.e._, the common sinus, into which the vas deferens and vagina may open), or (2) a variable extent of the vas from cirrus to seminal vesicle. Thus the latter may be outside the sac. In the absence of a sac, the genital sinus may be surrounded by a pseudo-sucker, as in _Heterophyes_ (in some cases the ventral sucker itself, from its close proximity to the genital pore, serves as an accessory copulatory organ). In other cases copulatory organs are formed by hooks projecting into the lumen of the terminal portion of the vas.

The GENITAL PORE, which is the opening from the genital sinus on to the surface, is generally situated at or near to the median line on the ventral surface and in the anterior region of the body; in most of the _Distomata_ it is in front of the ventral sucker, in other cases, _e.g._, in the _Cryptocotylinæ_, it is behind.[260]

[260] The typical position of the genitalia is subject to many deviations, which are of importance in the differentiation of the genera and families. The following are some few of these deviations: (1) The genital pore remains on the ventral surface, but is situated beside or behind the ventral sucker, or it becomes marginal, and is then found in front of or beside the oral sucker, or at a lateral edge, or, finally, in the centre of the posterior border; the ducts also correspondingly alter their direction. (2) The ovary usually lies in front of the testes, not rarely, however, behind them or between them. (3) The three genital glands mostly lie together close in front of, or behind, the centre of the body; they may be moved far back, and may incidentally become separated one from the other. (4) The vitellarium may be single, in which case it then may lie in the central field. (5) A few forms possess but one, others several or numerous testes. Amongst the ectoparasitic trematodes there are also species with but one testis; but they mostly have several. As a rule, their uterus is short, but the oötype well developed. Special canals (vagina), single or double, are used for copulation, not the uterus. The vitelline ducts also communicate with the intestine through the canalis vitello-intestinalis (fig. 123).

The spermatozoa do not differ essentially in their structure from those of other animals; the ovarian or egg cells are cells without integument and contain a large nucleus and a little protoplasm; the vitellaria also produce nucleated cells, in the plasm of which there are numerous yellow yolk granules; the yolk cells detach themselves, like the ovarian cells, from the ovarium, and pass into the oviduct to surround each ovarian cell in the oötype. They disintegrate sooner or later in the completely formed egg and are utilized as food by the developing embryo.

DEVELOPMENT OF THE TREMATODES.

(1) _Copulation._--Observation has demonstrated that the one or two vaginæ occurring in the ectoparasitic Trematodes are utilized as female organs of copulation, and that the copulation is cross; it is also known that Laurer’s canal, which was formerly generally regarded as the vagina, has only quite exceptionally, if at all, served the digenetic Trematodes as such--it appears to be homologous with the canalis vitello-intestinalis of the _Monogenea_[261]--but the terminal portion of the uterus, termed the metraterm, is used for copulation. Cross-copulation occurs as well as auto-copulation and auto-fecundation. The spermatozoa subsequently pass through the entire uterus, which is still quite short at the time the male organs are matured; the maturation of which, as usually is the case in hermaphrodites, precedes that of the female organs. It is only later with the onset of egg formation that the uterus is fully developed. Copulation, however, takes place also in the case of fully grown forms with completely developed uteri.

[261] _Monogenea_: Trematoda in which the anterior sucker, if present, is double. Development without an intermediate host.

(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).

[262] [Recent work (_e.g._, Goldschmidt, _Zool. Anzeiger_, xxxiv, p. 482) has shown that the older views regarding the formation of the egg must be modified. In certain species, at any rate, the shell material is formed by the yellow droplets of the yolk glands and not by the so-called shell gland (Mehli’s gland) secretion, which is clear and watery. The function of this secretion accordingly still requires explanation; according to Looss it serves as a covering secretion for the egg-shell proper. It appears also that other granules, the yolk granules as distinct from the shell drop granules, are not always used up during the development of the embryo and hence do not function as yolk, so these also when they exist, and frequently they are wanting, must serve some other purpose, possibly that of imbibing water for the use of the embryo.--J. W. W. S.]

(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).

[263] _Holostomata_: Prostomata with (in addition to the oral and ventral suckers) a third fixation apparatus, generally on a separate part of the body.

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 MIRACIDIA[264] (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 REDIÆ[265] (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.

[264] [Also known as ciliated embryos.--F. V. T.]

[265] [In _Fasciola hepatica_ in the summer months the rediæ give rise to daughter rediæ, which then give rise to cercariæ.--J. W. W. S.]

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.

Thus certain species of ducks and geese become infected with _Echinostoma echinatum_ by devouring certain water-snails (_Limnæus_, _Paludina_) in which the encysted cercariæ occur. Oxen become infected with _Paramphistomum cervi_ (= _Amphistomum conicum_) by swallowing with water, cysts of this species which occur at the bottom of puddles and pits. Sheep are infected with _Fasciola hepatica_ by eating grass to which the encysted cercariæ of the liver-fluke are attached; our song-birds infect themselves or their young with _Urogonimus macrostomus_ by tearing off pieces containing the corresponding sporocysts which are full of encysted cercariæ from snails (_Succinea amphibia_), which act as the first intermediate hosts, and eating, or offering their young these pieces.

(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 CERCARIÆ[266] 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.

[266] The cercaria is the characteristic larval stage of the Trematodes, and corresponds to a cysticercus or cysticercoid, though there is the important difference that the cercaria has an enteric cavity. According to some observers the enteron is represented by the frontal sucker of some Cestodes, and by the rostellum of the majority of others.

The sporocyst and redia are regarded as intercalated stages, _viz._, as cercariæ exhibiting _pædogenesis_, _i.e._, development of young by a parthenogenetic process from individuals (_i.e._, cercariæ) not yet adult.

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.

Anterior sucker single and median, present. Eggs few. The
(specialized) terminal portion of the uterus serves as a vagina.
Development indirect, _i.e._, an intermediate host is required.

Sub-order. *Prostomata*, Odhner, 1905.

Mouth surrounded by the anterior sucker.

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

Gut forked, two suckers, the posterior sucker (acetabulum) terminal
or ventro-terminal behind the genitalia, or at most embraced by the
vitellaria. Skin with no spines. Excretory bladder a simple sac
opening dorsally near hind end. Testes in front of ovary. Genital
pore, median in anterior third of body. Thick flukes, almost circular
in cross section.

Family. *Paramphistomidæ*, Fischoeder, 1901.

Amphistomata: Body not divided into a conical anterior portion and
disc-like caudal portion. Ventral pouch absent.

Sub-family. *Paramphistominæ*, Fisch., 1901.

Paramphistomidæ: Oral sucker without evaginations. Not in man.

Sub-family. *Cladorchiinæ*, Fisch., 1901.

Paramphistomidæ: Oral sucker with evaginations; testes, two, deeply
cleft (fig. 137). Genera: _Watsonius_, _Cladorchis_, etc.

Family. *Gastrodisciidæ*, Stiles and Goldberger, 1910.

Amphistomata: With body divided into a conical cephalic and disc-like
caudal portion (fig. 138). Posterior sucker ventro-terminal. Oral
sucker with evaginations. Genera: _Gastrodiscus_ and _Homalogaster_.

Group. *Distomata*, Retzius, 1782.

Gut forked, two suckers, the posterior sucker (acetabulum) ventral.
It is always separated from the hind end by at least a part of the
genitalia.

Family. *Fasciolidæ*, Railliet, 1895.

Large flat forms, genital pore _in front_ of ventral sucker, the
latter powerful. Vitellariæ of numerous follicles, united by
branching vitellarian ducts, at the sides of the body meeting
posteriorly and extending ventrally and dorsally. Cirrus and vagina
without spines. No crown of strong spines around sucker. Testes much
branched. Uterus not well developed. Excretory bladder much branched.
Eggs large.

Sub-family. *Fasciolinæ*, Odhner, 1910.

Large or median forms, gut much branched. Body has a shoulder
separating head from body. Receptaculum seminis absent. Ovary
branched, ventral sucker in anterior part of body. Genus: _Fasciola_.

Sub-family. *Fasciolopsinæ*, Odhner, 1910.

Shoulder absent. Receptaculum seminis present. Ovary branched, gut
takes a zig-zag course with kinks on it, ventral sucker in anterior
part of body. Genus: _Fasciolopsis_.

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

Ovary in front of testes. Small to medium flukes, very transparent,
tapering anteriorly. Vitellaria moderately developed not extending
in front of sucker. Cirrus absent. Seminal vesicle a twisted tube
free in parenchyma. Testes near hind end one behind the other, lobed
or branched, but not dendritically. Excretory bladder *Y*-shaped,
the two limbs short, the stem *S*-shaped passing between the testes.
Receptaculum seminis well developed. Laurer’s canal present. Uterine
coils transverse, numerous. Eggs small.

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

_Opisthorchiidæ_ in which the excretory pore is terminal. Excretory
bladder long, dorsal to testes. Uterine coils not overlapping gut
forks. Genera: _Opisthorchis_, _Paropisthorchis_, _Clonorchis_,
_Amphimerus_, etc.

Sub-family. *Metorchiinæ*, Lühe, 1909.

_Opisthorchiidæ_ in which the excretory pore is ventral. Excretory
bladder short, ventral to testes. Uterine coils partly overlapping
gut forks and extend anteriorly beyond the sucker. Vitellaria
compressed on the sides of the body. Genus: _Metorchis_.

Family. *Dicrocœliidæ*, Odhner, 1910.

Ovary _behind_ testes. Testes behind the ventral sucker, between it
and the ovary. Body thin and transparent. Cirrus sac encloses the
pars prostatica and seminal vesicle. Skin smooth. Gut forks do not
reach posterior end. Receptaculum seminis and Laurer’s canal present.
Vitellaria, moderate, lateral in mid-body slightly overlapping the
gut. Uterus with an ascending and descending branch and numerous
transverse coils extending to hind end. Eggs dark brown, 25 µ to
60 µ. Excretory bladder tubular in posterior third or half of body.
Parasitic in bile-ducts of mammals and birds. Genus: _Dicrocœlium_.

Family. *Heterophyiidæ*, Odhner, 1914.

Ovary _in front_ of testes. Genital pore _behind_ or on a level with
ventral sucker. Genital pore surrounded by a pseudo-sucker (_i.e._,
its muscle is not sharply separated from but blends with the body
muscles). Cirrus sac absent, consequently vesicula seminalis and pars
prostatica lie free. Vagina and ejaculatory duct unite into a common
duct before opening. Small and very small forms. Body covered with
scales. Genera: _Heterophyes_, _Metagonimus_, etc.

Family. *Troglotremidæ*, Odhner, 1914.

More or less flattened Distomes of compact form, 2 to 13 mm. long.
Ventral surface flat or somewhat hollowed, dorsal surface _arched_.
Skin completely covered with pointed spines. Musculature weakly
developed also in the suckers in those forms that inhabit cysts. Gut
with pharynx and a not very long œsophagus and cæca, which end more
or less shortly before the hind end. Excretory bladder *Y*-shaped or
tubular. Pars prostatica and seminal vesicle always distinct. Testes
elongated, symmetrically placed in or behind the middle of the body.
Ovary directly in front of the testes, right-sided, generally much
lobed. Receptaculum seminis and Laurer’s canal present. Vitellaria
generally well developed, exclusively or for the most part confined
to _the dorsal surface_, leaving only a median band unoccupied.
Uterus either very long, coiling here and there, or shorter and more
convoluted. Eggs in first case small 17 µ to 25 µ, in the second much
larger 63 µ to 85 µ or even 120 µ (?) long. Parasitic in carnivora or
birds, generally occurring in pairs in cyst-like cavities. Genera:
_Paragonimus_, _Pholeter_, _Collyriclum_, _Troglotrema_.

Family. *Echinostomidæ*, Looss, 1902.

_More or less elongated flukes, small or very large, much flattened
anteriorly, less so posteriorly, or even round. Suckers near one
another, the anterior small and weak, the posterior large and
powerful directed obliquely backwards. Surrounding the oral sucker
dorsally and laterally but not ventrally is a fold or “collar”
bearing a row or rows of pointed spines which are continued round
laterally on to the ventral corners, the number being constant
for each species, the corner spines large or specialized, skin
anteriorly scaled or spiny. Alimentary canal consists of a pharynx,
epithelial “pseudo-œsophagus” and gut cæca reaching to posterior
end. Testes behind one another in hind body. Ovary on right side
or median directly in front of the testes. Vitellaria lateral,
usually extending to the hind end and not beyond the ventral sucker
anteriorly. Genital pore just in front of ventral sucker. Uterus
in transverse loops. Genital sinus absent or present. Receptaculum
seminis and Laurer’s canal present. Eggs thin shelled and large,
bright yellow, 65 µ to 120 µ long. Excretory bladder *Y*-shaped.
Parasitic in gut of vertebrates, especially birds._

Sub-family. *Echinostominæ*, Looss, 1899.

_Cirrus sac usually reaching to centre of ventral sucker, but not
beyond. Cirrus long, usually without spines, coiled when retracted.
Seminal vesicle tubular, twisted. On the head a ventral uniting ridge
between the angles of the collar. Dorsal circlet of spines, single or
double, not interrupted unless the collar itself is dorsally divided.
Genera_: Echinostoma, etc.

Sub-family. *Himasthlinæ*, Odhner, 1910.

Cirrus sac reaching far beyond ventral sucker. Cirrus armed with
strong rose-thorn-shaped hooks. Vesicula seminalis tubular not
coiled. Cervical collar not continued across ventral aspect. Spines
on collar in one row. Body armed with fine needle-shaped spines.

Family. *Schistosomidæ*, Looss, 1899.

Sexes separate. Genital pore behind the ventral sucker. Ventral
sucker elevated above the surface. Pharynx absent. Gut forks reunite
to form a single stem. In ♂ four or more testicular follicles. In ♀ a
single ovary, just in front of the union of the gut forks. Vitellaria
on either side of the united gut stem.

THE TREMATODES OBSERVED IN MAN.

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

Sub-family. *Cladorchiinæ*, Fisch., 1901.

Genus. *Watsonius*, Stiles and Goldberger, 1910.

_Cladorchinæ_.--Body pyriform. Ventral pouch absent. Acetabulum
ventral or (?) ventro-subterminal, very large, margins projecting,
aperture small. Genital pore in front of bifurcation of gut, not
surrounded by a sucker; ductus hermaphroditicus apparently absent.
Excretory pore at posterior end of excretory vesicle, behind Laurer’s
canal. Oral sucker with a pair of irregularly globular suctorial
pouches; œsophagus thickened distally; cæca long, not wavy; end in
acetabular region.

_Male Organs_.--Testes two lobed, smaller than acetabulum;
longitudinally, nearly or quite coinciding; transversely they abut or
slightly overlap; preovarial in equatorial and caudal thirds. Pars
musculosa not largely developed; cirrus pouch absent.

_Female Organs_.--Ovary and shell gland post-testicular. Vitellaria
extend from gut fork to slightly beyond gut ending; uterus
intercæcal, partly post-testicular. Laurer’s canal in front of
excretory vesicle.

_Type Species_.--_Watsonius watsoni_, Conyngham, 1904.

*Watsonius watsoni*, Stiles and Goldberger, 1910.

Syn.: _Amphistomum watsoni_, Conyngham, 1904; _Cladorchis watsoni_,
Shipley, 1905.

_Body_, 8 to 10 mm. long, by 4 to 5 mm. broad, by 4 mm. thick; tapers anteriorly to 2·5 mm. Caudal extremity bluntly rounded, venter surrounded by an elevated ridge, surface with transverse ridges best defined ventrally. Genital pore median about one-quarter of body length from anterior end at level of suctorial pouches. Acetabulum 1 mm. in diameter, margin projecting, aperture small. Mouth in a groove with digitate papillæ. Oral sucker very large, one-fifth of length of body, with a pair of irregularly globular pouches. Œsophagus somewhat longer than sucker. Excretory pore at the level of the acetabular aperture. The vesicle extends from the plane of the transverse vitelline ducts to centre of acetabulum.

_Male Organs_.--Testes deeply notched adjoining one another. Vesicula seminalis much coiled and dilated, pars musculosa not coiled. Pars prostatica (?) dilated, ejaculatory duct long and narrow, opening on a papilla; genital atrium papillated.

_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.

_Eggs._--123 µ to 133 µ long by 75 µ to 80 µ broad.

_Habitat._--Jejunum and duodenum of man, German West Africa. The parasite has only been found once in man. The patient, a negro from German West Africa, died at Zola, Northern Nigeria. The symptoms were persistent watery diarrhœa without blood or mucus. The parasites were also passed in the stools. It occurs also in monkeys.

Family. *Gastrodisciidæ*.

Genus. *Gastrodiscus*, Lkt., 1877.

Acetabulum small, caudal and ventral margin raised, aperture
relatively large. Genital pore without sucker. Excretory pore
post-vesicular, posterior to opening of Laurer’s canal. Œsophagus
with muscular thickening; cæca not wavy, long, end post-equatorial
and post-testicular.

_Male Genitalia._--Testes two, branched pre-ovarial.

_Female genitalia._--Ovary and shell gland post-testicular.
Vitellaria extracæcal; uterus intercæcal; Laurer’s canal entirely
prevesicular.

_Type._--_Gastrodiscus ægyptiacus_, Cobbold, 1876.

*Gastrodiscus hominis*, Lewis and McConnell, 1876.[267]

Syn.: _Amphistomum hominis_, Lew. and McConn.

[267] Leiper places this species in a new genus _Gastrodiscoides_. Genus _Gastrodiscoides_, Leiper, 1913, distinguished from _Gastrodiscus_ by: (1) large genital cone; (2) position of genital orifice; (3) disc without papillæ; (4) testes one behind the other.

_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.

_Genital Pore._--About the middle of the conical anterior portion. (It appears to be surrounded by a muscular sucker.) Leiper (1913) describes the ducts as discharging at the tip of a large fleshy papilla, the surface of which bears cuticular bosses.

_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.

The ventral sucker is situated at the level of the junction of the
cone with the body, _viz._, at the level of the “shoulder,” and is
large and powerful. The cuticle is covered with strong spines; the
gut cæca run in the mid-line to the hind end, and are provided with
numerous long lateral and fewer and shorter median branches. The
ovary lies on one side in front of the transverse vitelline duct;
the testes lie obliquely one behind the other. The uterus, in the
shape of a rosette, lies in front of the genitalia. Laurer’s canal
is present; the vesicula seminalis lies in the cirrus pouch; the ova
are large, not very numerous, and only develop after they have been
deposited. Parasites of the biliary ducts of herbivorous animals.

*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.

[268] [There does not seem to be any direct evidence of either rabbits or hares normally being invaded by this fluke.--F. V. T.]

The liver fluke, however, is by no means a harmless parasite, for it produces in domestic animals, more especially in sheep, a disease of the liver that appears epidemically in certain years and districts, and commits great ravages amongst the flocks.

[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 prohibited[269] when the changes are extensive.[270]

[269] [This is not the case in Great Britain; fluky sheep are sent to market, there being no danger to man from eating the flesh.--F. V. T.]

[270] As an example, this occurred in Berlin in the case of 19,034 oxen, 15,542 sheep, 1,704 pigs, and 160 calves in the period of 1883–1893; during which time 719,157 oxen, 1,519,003 sheep, 2,258,110 pigs, and 567,964 calves were slaughtered. As a matter of fact, however, the number of infected beasts was really larger.