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Rh 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).

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.

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