The

joints are the ripe "proglottides," containing the fecundated ova. When the ova-which are microscopic in size-are liberated by the decomposition of the proglottis, they may gain access to water, or be blown about by the wind. In many ways, it is easy to understand how one of them may be swallowed by a pig. When this occurs, a "proscolex" is liberated from the ovum, and bores its way through the walls of the stomach, to become a "scolex." It now takes up its abode, generally in the muscles, in which position it was originally described as a cystic worm under the name of Cysticercus cellulosæ (fig. 99, §), constituting what is commonly known as the "measles" of the pig. In this state the scolex will continue for an indefinite period; but if a portion of "measly" pork be eaten by a man, then the scolex will develop itself into a tape-worm. scolex fixes itself to the mucous membrane of the intestine, throws off its caudal vesicle, and commences to produce "proglottides" instead, becoming thus the "strobila" of the Tania · solium, with which we originally started. The other common tape-worm of man-viz., the Tania mediocanellata is derived in an exactly similar manner from the "measles" of the ox. In like manner, the tape-worm of the cat (Tania crassicollis) is the mature form of the cystic worm of the mouse (Cysticercus fasciolaris); the tape-worm of the fox (Tania pisiformis) is derived from the cystic worms of hares and rabbits (Cysticercus pisiformis); one of the tape-worms of the dog (Tænia serrata) is the developed form of the Canurus cerebralis of the sheep, the cystic worm which causes the "staggers" in the latter animal ; another of the tape-worms of the dog (T. echinococcus) spends its larval stage in the tissues of man; and T. marginata of the dog and wolf is the mature stage of the Cysticercus tenuicollis of the ruminants and of the pig. On the other hand, the embryo of the "Russian tape-worm" (Bothriocephalus latus) is not "cystic," but is ciliated and furnished with hooklets, whilst the scolex apparently leads an independent life in water, and possesses no intermediary bearer.

Besides being liable to the attacks of various species of adult tape-worms, man is not uncommonly attacked by "scolices," or the larval forms of the tape-worms of other animals. Thus, what are professionally termed "hydatids" are really the scolices of one of the tape-worms of the dog (the Tania echinococcus). The "strobila" or adult worm (fig. 101, A) is only about a quarter of an inch in length, and is singular in consisting of only four segments, including the "head." The last segment is sexually mature, and the head is furnished with hooklets and suckers. The egg gives rise, when swallowed by

man, to a "proscolex," which bores its way through the walls of the stomach, and finds a lodgment in some solid organvery commonly the liver. The primitive scolex now consists of a spherical vesicle (fig, 101, C), with a thick laminated ex

Fig. 101-A, Sexually mature Tania echinococcus, showing the head with its hooklets and suckers, and the three succeeding proglottides, the last containing the reproductive organs (enlarged), o Ovary, v Water-vessels; B, Interior of a portion of a hydatid cyst, showing the brood-capsules and included Echinococci (from Man); C, Young Echinococcus, about six weeks old, showing the thick laminated outer capsule and the inner granular contents; D, Single Echinococcus (from Man), showing the hooklets, contained "calcareous corpuscles," and pedicle. (All the figures are enlarged.) After Spencer Cobbold and Wilson.

ternal covering enclosing a central granular mass. This mass ultimately forms a cellular membrane lining the outer laminated envelope, and from it are developed numerous secondary "scolices," each of which is attached to the parent cyst by a pedicle, and is furnished with a crown of hooklets and four suckers, but is destitute of a caudal vesicle (fig. 101, D). In fact the parent cyst may be regarded as morphologically composed of the coalescent caudal vesicles of the contained "heads" or "Echinococci." The parent cyst may grow to a great size, and is filled with a clear fluid, whilst instead of producing simple "Echinococci," it may bud off numerous "brood-capsules," each of which develops in its interior a group of Echinococcus heads (fig. 101, B), or it may produce numerous secondary cysts, which may repeat the process, and all of which may incessantly produce "brood-capsules" and

"Echinococci." The disease known as "hydatids" in the human subject is, therefore, indicated by the presence in the liver or other solid organ of strong membranous cyst, often of large dimensions, filled with a transparent watery fluid, and having attached to its interior innumerable minute secondary "scolices," or "brood-capsules," or containing daughter cysts produced by a peculiar form of gemmation. In some countries, as in Iceland, the disease is very common, and it is of a very serious character, as the growth of the tumour is apt to gravely interfere with the vital functions, or even to produce fatal results. The symptoms, of course, depend upon the position occupied by the cyst, and the importance to life of the organ affected.

CHAPTER XXV.

TREMATODA AND TURBELLARIA.

or

ORDER TREMATODA. 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 "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. The sexes are mostly united in the same individual. 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 larvæ are "cercariform," 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, &c. 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 Intestinaux Parenchymateux, along with the Taniada 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. 102, 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

Fig. 102. Trematoda. . 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.)

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 "cercariform," 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, &c., and occasionally in

man.

In Distoma lanceolatum (fig. 102, 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, &c., and has been likewise detected in man.

The only other Trematode which need be mentioned is the curious Gynecophorus (Bilharzia) hæmatobius, 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 ver miform in shape.

ORDER TURBELLARIA.-Leaf-like or vermiform Scolecids, rarely parasitic, with a mouth and alimentary canal, and sometimes a body-cavity; integument ciliated. Sexes united or distinct. The members of this order are almost all aquatic, and are all non-parasitic; thus differing entirely from the animals which compose the two preceding orders. Their external surface is always and permanently ciliated, and they never possess either suctorial discs or a circlet of cephalic hooklets. A "watervascular system" is present, opening externally by one or more apertures, or appearing to be entirely closed in the adult (Nemertida). The alimentary canal is imbedded in the parenchyma of the body (Planarida), or is freely suspended in a "perivisceral cavity" (Nemertida). The intestine is either straight or branched, and a distinct anal aperture may, or may not, be present. The nervous system consists of ganglia situated in the fore-part of the body, united to one another by transverse cords, and sending filaments backwards.

The Turbellaria are divided into two sections, termed respectively the Planarida and the Nemertida.

SUB ORDER I. PLANARIDA.-The Planarians (fig. 103) are mostly ovoid or elliptical in shape, flattened and soft-bodied. They are for the most part aquatic in their habits, occurring in fresh water, or on the sea-shore, but occasionally found in moist earth. The integument is abundantly provided with vibratile cilia, which subserve locomotion, and it also contains numerous cells, which have been compared to the "cnidæ," or nettle-cells, of the Calenterata. There is always a considerable portion of the body situated in front of the mouth, constituting the so-called "præ-oral region," or "prostomium;" and this is often modified into a singular protrusible and retractile organ, called the "proboscis," the exact use of which is not known. The mouth opens into a muscular pharynx, which is