G. Lindner, in Cassel, studied certain peritrichal Infusoria (stalkless Vorticella), and connected them, probably incorrectly, with the most varied diseases of man and domestic animals, even with Sarcosporidia of pigs. It may be mentioned that according to a communication by letter from Schaudinn, Vorticella may be found in freshly evacuated fæces, but always only after the administration of a water enema. In spite of this, several other investigators mention Vorticellæ as intestinal parasites of man.
The Chilodon dentatus (Ehrenberg) recorded in 1903 by J. Guiart as a parasite of man, which may be found in all infusions, can hardly have lived in the man from whose fæces it was cultivated, but may represent a chance admixture both in the fæces and the cultivations. C. uncinatus was also found as a chance parasite of man by Manson and Sambon. According to Doflein250 (1911) certain Chilodon-like organisms have been found by Selenew in prostate secretions in gonorrhœa. Other species of the genus Chilodon are known, but only as ectoparasites (e.g., Chilodon cyprini, Moroff, 1902, from the skin and gills of diseased carp).
A number of other parasitic Ciliates are known, among which Ichthyophthirius multifiliis, destructive to fish, is important. It lives in the skin and the layers immediately below it, forming small whitish pustules which may become confluent. The pustules are most common on the head and fins, but occur also on the eyes and gills of the host. The young parasite, which is one of many formed in a cyst, is very small. At first it is free swimming, but soon attaches itself to the skin of a fish. It bores inwards and becomes surrounded by the irritated skin. There it attains a relatively large size, being 500 µ to 750 µ and occasionally more in diameter. The body has a rounded terminal mouth, short cytopharynx and a number of minute contractile vacuoles. The macronucleus is large and horseshoe-shaped; the small micronucleus is only seen in the very young animal. When full grown, the organism encysts and forces its way to the surface and bursts through, leaving a small, gaping wound behind. The cyst sinks to the bottom of the water, nuclear multiplication occurs and a number of young parasites are produced, which leave the cyst and either attack new hosts or else perish.
Opalina ranarum, parasitic in the rectum and urinary bladder of frogs and toads, shows great degradation and simplification due to parasitism, possessing no separate micronuclei, no cytostome, cytopharynx or cytopyge. It has many macronuclei, and is a large parasite. During summer and autumn nuclear multiplication followed by division of the body occurs, the process being repeated after the daughter forms have grown to the size of their parent. In spring, the Opalina divide rapidly, but do not grow much before dividing again. Finally, tiny forms, containing three to six nuclei, encyst and pass from the host with the fæces. As these latter are greedily devoured by tadpoles, the Opalina gain new hosts in which they develop.
The name Chlamydozoa was proposed by Prowazek in 1907 for a number of minute, problematic organisms (fig. 119) believed to be the causal agents of certain diseases in man and animals, such as vaccinia and variola, trachoma, inclusion blenorrhœa in infants, molluscum contagiosum, and bird epithelioma contagiosum. Other diseases possibly due to Chlamydozoa251 are hydrophobia, measles, scarlet fever, foot-and-mouth disease, the “Gelbsucht” disease of silkworms, and perhaps even typhus (Prowazek, 1913). The subject is difficult and controversial and can only be briefly discussed here. It is known that the viruses in all these diseases can pass through ordinary bacterial filters, that is, they belong to the group of “filterable viruses.” At such periods the organisms are extracellular or free. It is also known that in many of these cases the virus produces definite and characteristic reaction-products or cell-inclusions in the infected cells, during the intracellular phase of the life-history of the organism. As the organisms to be considered are problematic, it will be convenient to summarize their history:—
(1) Cell-inclusions, usually named after their discoverers, have been found in certain diseases, thus: In vaccinia Guarnieri’s bodies, in scarlet fever Mallory’s bodies, in hydrophobia Negri’s bodies, in trachoma Prowazek’s bodies occur.
(2) At first these characteristic cell-inclusions were considered to be actual parasitic organisms causing the diseases in question. The bodies received zoological names and attempts were made to work out their supposed development cycles. The supposed parasites of vaccinia and variola were referred to a so-called genus Cytoryctes, those of hydrophobia to Neuroryctes, of scarlet fever to Cyclasterium, while those of molluscum contagiosum were referred to the Coccidia. Calkins in 1904 studied in detail the cell-inclusions of vaccinia and small-pox, calling them Cytoryctes variolæ, Guarnieri. Calkins considered that in the stratified cells of the epidermis they passed through two cycles, the one cytoplasmic, the other intranuclear. The first is the vaccinia cycle, the second the pathogenic (intranuclear) variola cycle. It is hardly necessary to follow all Calkins’ stages here.
Negri (1909) described a cycle for Neuroryctes hydrophobiæ. Calkins refers both Cytoryctes variolæ and Neuroryctes hydrophobiæ to the Rhizopoda.
Siegel (1905) described quite different organisms under the name Cytorhyctes. He listed several species: C. vacciniæ; of vaccinia and small-pox, C. scarlatinæ of scarlet fever, C. luis of syphilis (this is probably the granule stage of Treponema pallidum), and C. aphtharum of foot-and-mouth disease.
(3) The afore-mentioned views were criticized, and the bodies were not considered to be living organisms but merely reaction products or cell-inclusions due to the effects of the virus on the host cells. Thus Guarnieri’s bodies were stated to consist of extruded nucleolar or plastin material, having no developmental cycle. It was further asserted that infection could be produced by lymph in which Guarnieri’s bodies had been destroyed. Similar assertions have been made regarding the Negri bodies, and others. The Cytoryctes, Neuroryctes, etc., are considered, according to these views, to be degeneration products of the nucleus or to be of a mucoid nature.
(4) More recently a positive belief has gained ground that there are true parasitic organisms causing these diseases, and that the parasites are very minute, being termed Chlamydozoa by Prowazek and Strongyloplasmata by Lipschütz.
The Chlamydozoa are characterized by (a) their very minute size, smaller than any bacteria, so that they can pass through bacterial filters; (b) they pass through intracellular stages, in the cytoplasm or the nucleus of the host cell, producing therein the reaction products or inclusions in the cell already recorded as characteristic or diagnostic of the diseases produced; (c) they pass through definite developmental cycles. Such a cycle consists essentially of growth and division. The mode of division of the Chlamydozoa resembles that of the centriole of a cell, by the formation of a dumb-bell-shaped figure. Two dots are observed connected by a fine line or strand which becomes drawn out and finally snaps across the middle. Prowazek and Aragão (1909) working on smallpox in Rio de Janeiro found that the chlamydozoal granules passed through a Berkefeld filter and that the filtrate was virulent. But if an “ultra-filter” were used, i.e., one coated with agar, then the granules were retained and the filtrate was no longer virulent. The surface of the ultra-filter was found to contain many granules.
The Chlamydozoa are parasites of epiblastic tissues (e.g., epidermal cells, nerve cells, conjunctival cells).
Fig. 119.—Chlamydozoa. Trachoma bodies in infected epithelial cells of the conjunctiva. (a) initial bodies (above) and cluster of elementary bodies (touching the nucleus); (b) cluster of granules surrounded by mantles. × 2,000 approx. (Original. From preparation by Fantham.)
The life-history of a Chlamydozoön (fig. 119), such as that of vaccinia, is, according to Prowazek, Hartmann and their school, as follows:—
1. The infection begins with elementary bodies or elementary corpuscles which live at first extracellularly. An elementary body is a minute speck of chromatin, apparently devoid of cytoplasm, which can pass through a bacterial filter. It can enter a host cell, but the entry is not a process of phagocytosis.
2. Inside the host cell the elementary body grows in size, and becomes an initial body (fig. 119, a).
3. A reaction on the part of the host cell results, for nucleolar, plastin substance is extruded from the cell-nucleus and surrounds the parasitic initial body. The latter is thus enveloped in a mantle (hence the name Chlamydozoa, from χλαμὑς, a mantle), and the characteristic cell-inclusion (Guarnieri’s body, Negri’s body, etc.) is produced. The nucleolar, mantle substance probably represents the “cytoplasm” of Cytoryctes, described by Calkins.
4. The body next breaks up into a number of smaller bodies known as initial corpuscles. These, in their turn, divide by simple division (in the manner already described) into numerous elementary bodies (fig. 119). Thus, the life-cycle is completed.
The Chlamydozoa are, then, the minute granules inside the body of the Cytoryctes variolæ or the Neuroryctes hydrophobiæ, so that the whole body of the Cytoryctes or Neuroryctes corresponds to the mantle and parasite of the Chlamydozoön. The Cytoryctes group is said to cause destruction of the host cell. The Cytoöikon group (e.g., trachoma bodies) causes proliferation of the host cell.
In September, 1913, Noguchi252 described the cultivation of the parasite of rabies in an artificial medium, similar to that used by him for the cultivation of Spirochæta recurrentis. The cultures were stated to be infective to dogs, rabbits and guinea-pigs. Levaditi, in December, 1913, stated that he had succeeded in cultivating spinal ganglia of rabid monkeys in monkey plasma.
Noguchi and Cohen (November, 1913)253 have succeeded in cultivating the so-called trachoma bodies, or at any rate bodies very closely resembling them morphologically. The medium employed was Noguchi’s ascitic fluid and rabbit kidney medium, as used for spirochætes. The coarser cultural forms stained blue with Giemsa’s solution, the finer ones stained red. Attempts to infect monkeys from the culture tubes failed.
From their behaviour on treatment with such reagents as saponin, bile and sodium taurocholate, Prowazek considers that the Chlamydozoa approach the Protozoa.
Sergentella hominis, Brumpt, 1910.
Et. and Ed. Sergent in 1908 found vermiform bodies about 40 µ long by 1 µ to 1·5 µ broad in the blood of an Algerian suffering from nausea and cold sweats, without other symptoms. The bodies were pointed at each end, with a somewhat ill-defined nucleus in the middle. Their systematic position is doubtful.
Note.—An Appendix on Protozoology will be found on pp. 733–752. This has been prepared in order to incorporate a number of new additions to knowledge made since the body of the book was printed off.
Definition: Bilaterally symmetrical animals without limbs, the form of which is leaf or tape-like, rarely cylindrical, and whose primary body cavity (segmentation cavity) is absent, the cavity being filled by a mesenchymatous tissue (parenchyma).
The mouth is either situated at the anterior end of the body, or is shifted more or less backwards on to the flat ventral surface. The alimentary canal consists of a short fore-gut, which is frequently provided with a muscular pharynx, and of a simple forked or branched mid-gut; there is neither a hind-gut nor an anus; in one class, the Cestodes, the alimentary canal has entirely disappeared except for muscular remnants in the scolex.
The INTEGUMENT OF THE BODY consists either of a ciliated epithelium of only one layer (Turbellaria), or of a cuticle and gland-like cells embedded in the parenchyma, or subcuticular layer (Cestodes, Trematodes). The dermo-muscular layer consists of annular, longitudinal, and even diagonal fibres, while the parenchyma is traversed by dorso-ventral fibres.
The central NERVOUS SYSTEM, which is embedded in the parenchyma of the body, consists of cerebral ganglia, united together in the shape of dumb-bells, and of two or more longitudinal MEDULLARY FASCICLES, often forming transverse anastomoses. Organs of sense usually occur only in the free-living species, more rarely during the free-living stages of a few parasitic species and in a few ectoparasitic forms.
[In Platyhelminthes simple eye-spots frequently occur, and in a few an auditory vesicle.]
Blood-vessels and definite RESPIRATORY ORGANS are lacking [except in Nemertinea]; the EXCRETORY APPARATUS (formerly termed water-vascular system) is typical of the entire class. It commences in the interstices of the parenchyma, with peculiar terminal cells (ciliated funnels), which will be described later (p. 219), the capillary processes of which go on uniting into larger branches, and finally form two large collecting vessels, which, sometimes separately and sometimes united, open to the exterior through one, two, or numerous pores.
Nearly all the Platyhelminthes are HERMAPHRODITIC, and in nearly all there are, in addition to the ovaries producing ova, other glands attached to the female genital apparatus, namely, the vitellaria or yolk glands, which provide a substance termed yolk, which serves as nourishment for the embryo. The fully formed eggs have shells and are “compound,” i.e., composed of the egg or ovarian cell, which is surrounded by numerous yolk cells or their products of disintegration. The two sexual openings usually lie close together, frequently in the fundus of a genital atrium; they are rarely separated from one another. Shell glands also usually occur (p. 221).
Reproduction is sexual, often, however, combined with asexual methods of propagation (segmentation, budding). The Platyhelminthes live partly free in fresh or salt water, exceptionally also on land. The greater part, however, live as parasites on or in animals.
Classification of the Platyhelminthes.
Class I.—Turbellaria (or Eddy Worms). Flat worms for the most part, free living, and always covered with a ciliated epithelium.
Order 1.—Rhabdocœlida, gut unbranched.
Order 2.—Tricladida, gut with three main branches.
Order 3.—Polycladida, a central gut with lateral cæca. Development direct or through metamorphosis. They live in fresh and salt water or on land; very seldom as parasites.
Class II.—Trematoda (Sucking Worms254). [Usually known as Flukes.—F. V. T.] Flat worms, living as ecto- or endoparasites, that are only ciliated in the larval condition, and in their adult state are covered with a cuticle, the matrix cells of which lie in the parenchyma. They have either one, a few, or several suckers,255 and frequently also possess chitinous fixation and adhesive organs. The intestine is single, but generally bifurcated, and not uncommonly there are transverse anastomoses between the forks or diverticula on them. Excretory organs double, with two orifices at the anterior extremity or a single one at the posterior end. Development takes place by a metamorphosis or alternation of generations (p. 283). These worms are almost always hermaphroditic, with two or more female and one male sexual orifice. They live, almost without exception, as parasites on vertebrate animals, but the intermediate generations are passed in molluscs.
Class III.—Cestoda (Tapeworms). Endoparasitic flat worms without an alimentary canal. The larval stages are rarely ciliated, but are usually provided with six spines; the adult worm is covered with a cuticle, the matrix cells of which are embedded in the parenchyma. The body consists of a single segment (Cestodaria) or a chain of segments, in which case it consists of the scolex and the segments containing the sexual organs (proglottides) (Cestodes s. str.). The scolex is provided with various adhesive and fixation organs, and there are calcareous corpuscles in the parenchyma. Excretory organs symmetrical, opening at the posterior end. These worms are always hermaphroditic, and then possess one or two female and one male sexual orifice. During development a larval intermediate stage (“measle”) occurs and almost always in a different host to that in which the adult sexual worm lives. The adult stage is parasitic in vertebrate animals; but the larval stage may occur in invertebrates.
These worms are usually leaf- or tongue-shaped, but also barrel-shaped or conical; they vary from 0·1 mm. to almost 1 m.256 in length; most of them, however, are small (5 mm. to 15 mm.). The surface on which the orifice of the uterus and the male sexual opening are situated is termed the ventral surface; the oral aperture, which also acts as anus, is always at the anterior end in the sub-order Prostomata (p. 230), but in the sub-order Gasterostomata it is ventral.
Suckers are always present and occur in varying numbers and positions at the anterior extremities as well as on the ventral surface, and occasionally on the lateral margin and on the dorsum; the beginning of the intestine (mouth) is always surrounded by a sucker in the Prostomata.
In or near the suckers there may be chitinous hooks, claws or claspers, or the surface of the body is more or less covered with spines, scales or prickles; in one genus (Rhopalias) there are projectile tentacles beset with spines on the sides of the anterior part of the body.
The body of adult Trematodes is covered by a homogeneous layer of varying thickness, which either lies directly over the external layer (basement membrane) of the parenchyma, or over the muscles embedded in the parenchyma. This investing membrane (cuticle) arises from pear-shaped or spindle-shaped cells arranged singly or in groups (which lie between or internal to the diagonal muscles), and is connected with them by processes; these cells one may regard as epithelial cells which have sunk down, or possibly as parenchymatous cells. An epithelium of one layer is also found on the body of young stages, but it disappears during growth, and only occasionally do its nuclei persist until adult life. In its place we then find the cuticle, which, moreover, extends into all the body openings more or less deeply.
It is thus a debatable point whether the “investing layer” of flukes is a cuticle—that is, consists of modified epithelial cells—or whether it is a basement membrane, i.e., compressed and modified connective tissue cells; in this latter case the true epidermis and cuticle have been cast off. In the former case the epidermal cells are the pear-shaped cells referred to above. According to recent authors it consists of two parts, an outer true cuticle and an inner basement membrane. There are also unicellular cuticular glands, lying isolated or in groups, which are termed cephalic, abdominal, or dorsal glands according to the position of their orifice.
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).
Fig. 120.—Half of a transverse section through Fasciola hepatica, L. 25/1. Cu., Cuticle with scales; under the cuticle are circular muscles, and adjoining them the longitudinal and diagonal muscles; internal to the latter are the matrix cells of the cuticle; I., gut; the other similarly contoured cavities are gut diverticula that have been transversely or obliquely sectioned; F.v.s., vitellaria; Ex.v., excretory vessels; T., testes; Md., median plane; the fibres passing from the ventral to the dorsal surface are the muscles of the parenchyma. The parenchyma itself is omitted.
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.
Fig. 121.—Harmostomum leptostomum, Olss., an immature specimen from Helix hortensis. Nervous system, according to Bettendorf. A.s., ventral sucker; C.g., cerebral ganglion; Ex.p., excretory pore; G.p., genital pore; O.s., oral sucker; M.d., dorsal medullary nerve; M.l., lateral medullary nerve; N.ph., pharyngeal nerve; M.v., ventral medullary nerve. Magnified.
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.
Fig. 122.—Median section through the anterior part of Fasciola hepatica: the oral sucker, pharyngeal pouches, pharynx, œsophagus, cuticle with spines, and the body parenchyma.
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]).
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.
Fig. 123.—Polystomum integerrimum, a monogenetic fluke from the urinary bladder of the frog. i., intestine; h., large hooks of the sucking disc; h.k., smaller hooklets; l.c.v., longitudinal vitelline ducts; o., oral orifice; Oot., oötype; ov., ovary; s.p., suckers of the disc; tr.c.v., transverse vitelline ducts; Ut., uterus with ova; v., entrance to the vagina; v.d.e., vas deferens; v.d.i., ductus vitello-intestinalis; the vitellaria and testes are not shown. Magnified. (After Zeller.)
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 SYSTEM258 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.
Fig. 125.—Terminal flame cell of the excretory system. n., nucleus of cell; c., bundle of cilia forming the “flame”; p., processes of cell extending into parenchyma; d., excretory capillary. (Stephens.)
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.
Fig. 126.—Diagram of female genitalia. Ov., ovary; ovd., oviduct; L.c., Laurer’s canal; Rec. sem., receptaculum seminis; Vit. R., vitellarian reservoir; t.v.d., transverse vitelline duct; Oo., oötype; Sh. gl., shell gland; Rec. ut., receptaculum uterinum; ut., uterus. (The various parts are not to the same scale.) (Stephens.)
Fig. 127.—Diagram of male and part of female genitalia. ut., uterus; vag., vagina; ♀, opening of vagina; g.s., genital sinus; g.p., genital pore; ♂, opening of ejaculatory duct or vas deferens; c.s., cirrus sac; c., cirrus; p.p., pars prostatica; s.v., seminal vesicle; e.j., ejaculatory duct or vas deferens; v.e., vas efferens; t., testis. (Stephens.)
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 apparatus259 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.
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
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.
(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 Monogenea261—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.