of Africa. In Egypt 30% of the natives are affected by haematuria
which arises from congestion of the bladder consequent upon the
attacks of this animal. The noxious influence of Trematodes is,
moreover, not confined to their mature phase of life. The rapid
multiplication that takes place in the larval stage of nearly all
endoparasitic forms affects the tissues of the “intermediate” host
in which they live. In most cases this is a mollusc, and the larvae
bore their way into the most diverse organs, often accumulating to
such an extent as to give a distinctly orange colour to an otherwise
colourless tissue, and to cause the demolition of particular structures
e.g. the liver and gonad. Perhaps the most remarkable of these
effects is that produced by the larvae of Gasterostomum. These
organisms live in cockles, oysters and other lamellibranchs and they
so affect the gonads of these molluscs as to castrate and sterilize
their host. A different but still more interesting result is produced
by these Trematode larvae on certain lamellibranchs. The production
of pearls by oysters and mussels is common knowledge, but it
is only recently that the origin of pearls has been traced and admitted
to be due to inflammation set up by a parasite. In the case of the
pearl oyster this parasite is a cestode larva, but in the less valuable
but no less genuine pearl produced by Mytilus, &c., the nucleus is
a Trematode-larva (Jameson).
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| Fig. 2. |
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A, Fasciola hepatica, from the ventral surface; the alimentary and nervous systems only shown on the left side of the figure, the excretory only on the right; a, right main branch of the intestine; c, a diverticulum; g, lateral ganglion; n, lateral nerve; o, mouth; p, pharynx; s, ventral sucker; cs, cirrus sac; d, left anterior dorsal excretory vessel; m, main vessel; v, left anterior ventral trunk; x, excretory pore. B, anterior portion more highly magnified (from Marshall and Hurst, after Sommer); cs, cirrus sac; d, ductus ejaculatorius; f, female aperture; o, ovary; od, oviduct; p, penis; s, shell-gland; t, anterior testis; u, uterus; va, vp, vasa deferentia; vs, vesicula seminalis; y, yolk-gland; yd, its duct. C, genital sinus and neighbouring parts (from Sommer); a, ventral sucker; b, cirrus sac; c, genital pore; d, evaginated cirrus sac; e, end of vagina; f, vasa deferentia; g, vesicula seminalis; h, ductus ejaculatorius; i, accessory gland. D, a flame-cell from the excretory apparatus, highly magnified (from Fraipont). E, egg of Fasciola hepatica. (from Thomas). |
Structure.—The anatomical structure of the Trematodes is fairly uniform (Braun). The body is enveloped by a thick striated protective cuticle which is frequently raised into hooks or spines. In Dirtomum acanthocephalum the cuticle forms circlets of large and small hooks at the anterior end, somewhat as in Cestodes. The epidermis has lost its connected epithelial character and its cilia, and the isolated cells have become sunk inwards retaining their attachment to the innermost cuticular layer by slender processes. This layer also forms the attachment for the muscles, of which there are two enveloping coats, a circular and a longitudinal layer and also dorso-ventral fibres. The muscles are remarkable for two reasons. They occasionally exhibit striation and originate from large branched cells, the nucleus and unmodified part of which form conspicuous elements. The digestive system consists of a simple or bifurcated sac, opening through the mouth by means of a “pharynx bulbosus,” adapted to act primarily as a sucker, and secondarily, when drawing blood, as an aspirator. Between the blind gut and the cuticle is a reticular branched tissue which forms the chief substance of the body. This is the mesenchyma. As in other Platyelmia the elements of this tissue undergo the most varied differentiation. The main mass of it forms a spongy vacuolated matrix, but some of the cells become glandular and open by pores on the surface of the cuticle, others become “flame-cells” (fig. 2, D) and canaliculi of the excretory system as in Turbellaria, others again muscle-cells. Embedded in the matrix lies the complex genital apparatus composed usually of both male and female reproductive organs (fig. 2, B). The former consist of one pair or more of vesicular testes communicating by fine ducts with a vesicula seminalis. From this point a glandular tube runs to the genital atrium and during the last part of its course is converted into an reversible hooked “cirrus” or penis. The female organs consist of distinct ovaries and yolk-glands, the ducts of which unite in the neighbourhood of a “shell-gland” or “ootype.” Here the two elements, ovum and yolk-cells, are surrounded by a shell of operculate or of spindle-capped types. Coincidently, to allow of fertilization and the escape of excess of yolk, and of spermatozoa, other accessory ducts open at this point. Thus in ectoparasitic Trematodes, the paired vagina transmits spermatozoa to the egg: and a canal carries off yolk from this point of junction either to the gut for resorption or to the exterior for exudation. This duct (Laurer's canal) is sometimes rudimentary and ends blindly beneath the skin. The fertilized ova, provided with yolk and a shell, are next transferred to the “uterus” along which they travel to the exterior. In the endoparasitic trematodes the uterus is the only passage by which fertilization can be effected, and in cases of cross and self-impregnation this duct is physiologically a vagina. Lastly the nervous system is well developed and consists of a pair of well-marked and interconnected ganglia placed near the anterior end and dorsal to the oesophagus. From these ganglia, nerve-tracts provided with ganglion-cells are given off. Of these there are three on each side of the body; a large ventral tract, smaller lateral strands and dorsal ones. From these tracts a plexus of nerve-fibres is developed in connexion with the musculature and cuticle.
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Fig. 3.—Diagrammatic projections to show the relations of the female reproductive ducts; A, in the Malacocotylea; B, in the Heterocotylea. The ovary (a) leads into (bb) the oviduct, which is joined at (g) by the duct of the yolk-glands (h). In B it is also joined by a paired vagina (kk) and by the “vitello-intestinal duct” (Laurer's canal), f. (c) Shell-glands; (d) ootype; (ε) uterus; (g) median-vitello-duct; (i, i) intestine. |
The Trematodes are divided into three orders, primarily distinguished by the character of their suckers, viz.: Heterocotylea, Aspidocotylea and Malacocotylea.
Order 1. Heterocotylea.—Ectoparasitic Trematodes, in which a large posterior adhesive apparatus is present and is usually accompanied by a pair of suckers placed anteriorly in relation to the mouth. The large posterior organ of attachment is usually wheel-shaped and provided with hooks; but the ridges may become separated
