Leaf-like internal (sometimes external) parasites, provided with one or more ventral suckers; a mouth and alimentary canal, but no anus. No body-cavity. Integument of the adult not ciliated. Sexes generally united. This order includes a group of animals, which, like the preceding, are parasitic, and are commonly known as "suctorial worms" or "Flukes." They inhabit various situations in different animals - mostly in birds and fishes - and they are usually flattened or roundish in shape. The body is provided with one or more suctorial pores for adhesion. An intestinal canal, with one exception, is always present, but this is simply hollowed out of the substance of the body, and does not lie in a free space, or "perivisceral cavity." The intestinal canal is often much branched, and possesses but a single external opening, which serves alike as an oral and an anal aperture, and is usually placed at the bottom of an anterior suctorial disc. A "water-vascular" system is always present, and consists of two lateral vessels which generally open on the surface by a common excretory pore. The nervous system consists of two pharyngeal ganglia. With few exceptions, the sexes are united in the same individual; and the young may be developed directly into the adult, or may pass through a complicated metamorphosis, which varies in different cases, and does not admit of description here. In many cases, the larvae are "cercariiform," or "tailed;" and one of the early stages of their existence is often spent in the interior of fresh-water molluscs, from which they are finally transferred to their definitive host.

The "Flukes" inhabit, in their adult condition, the most varied situations. Most are internal parasites, living in the intestines or hepatic ducts of mammals, birds, or batrachians, the vitreous humour or lens of the eye, the blood-vessels, etc. A few are external parasites, living on the skin and gills of fishes, crustaceans, and other animals.

From the absence of a perivisceral cavity, the Trematoda were formed by Cuvier into a separate division of Entozoa, under the name of Vers Intes-tinaux Parenchymateux, along with the Taeniada and Acanthocephala, in which no alimentary canal is present. By Owen, for the same reason, they are included in a distinct class, under the name of Sterelmintha.

The Distoma (Fasciola) hepaticum (fig. 116, 1) may be taken as the type of the Trematoda. It is the common "Liver-fluke" of the sheep, and inhabits the gall-bladder or biliary ducts, giving rise to the disease known as the "rot." In form it is ovate, and flattened on its two sides, and it presents two suctorial discs, the anterior of which is perforated by the aperture of the mouth, whilst the posterior is impervious. Between the suckers is the "genital pore," at which the efferent ducts of the reproductive organs open on the exterior. A branched water-vascular system is present, and opens posteriorly by a small aperture. The alimentary canal bifurcates shortly behind the mouth, the two divisions thus produced giving off numerous lateral diverticula, and terminating posteriorly in blind extremities. The nervous system consists of two cephalic ganglia, giving off filaments both forwards and backwards. The embryo of Distoma, on its discharge from the egg, is inversely conical in shape, and is covered with cilia ; but it appears soon to lose its cilia, and to become "cercarii-form," abandoning its free aquatic life, and entering into the body of some fresh-water mollusc. When its host is eaten by some mammal the larva passes into its mature stage of development. The adult Distoma hepaticum is found in the sheep, ox, horse, ass, hare, deer, etc, and occasionally in man.

Trematoda And Turbellaria Order Trematoda 148Fig. 116.   Trematoda. 1. Distoma hepaticum, the

Fig. 116. - Trematoda. 1. Distoma hepaticum, the "Liver-fluke," showing the branched alimentary canal. 2. Anterior extremity of Distoma lanceolatum. a Anterior sucker; b Posterior sucker; c Generative pore; d (Esophagus; e Alimentary canal. (After Owen.)

In Distoma lanceolatum (fig. 116, 2) the intestine has not the ramose, complex character of that of D. hepaticum. On the other hand, the alimentary canal, after its bifurcation, is continued on each side of the body to the posterior extremity without giving off any branches on the way, and it terminates simply in blind extremities. It occurs in the liver of the ox, sheep, pig, etc, and has been likewise detected in man.

The only other Trematode which need be mentioned is the curious Gynaecophorus (Bilharzia) haematobius, which occurs abundantly in the interior of the blood-vessels of the human subject in certain regions (Egypt, South Africa, Mauritius), and has also been found in a similar situation in monkeys. The sexes are distinct in this form, the male being about half an inch, whilst the female is nearly an inch in length, and both being vermiform in shape.