TAPE-WORMS, or CESTODA, are a group of worms forming one of the three main divisions of the Platy-helminthes, the other two being the Turbellaria (see PLANAEIANS and NEMERTINES) and Trematoda (see TREMATODA). They have been defined as follows :—" Flat worms without mouth or alimentary canal, which typically develop by alternation of generations, by budding from a generally pear-shaped nurse, with which they remain united for a lengthened period as a ribbon-like colony or ' strobila.' The individual joints of the colony, i.e., the sexual animals or ' proglottides,' increase in size and maturity as they are removed farther from their origin by the intercalation of new buds, but are not distinguished in any special way. The nurse, however, known by the name of the ' head' (seolex) is provided with four or two suckers, and usually with curved claw-like hooks. The dorsal and ventral surfaces of the head are perfectly identical, so that the arrangement of the hooks presents a strikingly radiate appearance. By means of this apparatus the worms fasten themselves on the intestinal membrane of their hosts, which (except in the case of the otherwise peculiar Archigetes) all belong to the Vertebrata. The nurses develop from little round six-hooked embryos in a more or less complicated fashion as so-called ' bladder-worms.' The latter inhabit very diverse, but usually parenchym-atous, organs of the higher and lower animals, and are thence passively transferred to the intestine of their subsequent host" (Leuckart, l, p. 270).
Historical Sketch.—Certain forms of Cestodes have been known from time immemorial. The hydatid cyst is alluded to by early medical writers, and Aristotle speaks of examining the tongue of pigs to ascertain the presence of bladder-worms. By this author and Hippocrates the Cestodes and other flat worms are spoken of as eXpuvOa 7rAaT¤iat, in opposition to the arpoyyvkai or " round worms"; the word Tenia (Gr. TaivLa) does not occur in Greek authors, but is first used by the Romans (Pliny, H. N~., xi. 33). In the treatises of the Middle Ages the tape-worm figured as Lumbricus latus, only one species being recognized. Felix Plater (23) separated Bothrio-cephalus from the other human tape-worms, and Andry (24) gave it the name Tenia A epine, mistaking the nodular generative organs for vertebrae. The appellation Bothriocephalus latus dates from Bremser, 1819 (25). Like other Entozoa, the tape-worms and bladder-worms were supposed to arise by spontaneous generation; it was found,, however, that animal forms strikingly like the Entozoa sometimes lived freely. Pallas (19), seeing that the eggs of intestinal worms are expelled from the animals in which they live, and may remain for some time unaltered in water, suggested the hypothesis that the Entozoa agree with other animals in originating from eggs which can be carried from one animal to another. He also supposed that they reached the liver and other internal organs by means of the blood-stream. Other authorities endeavoured to explain the presence of Entozoa by supposing that they were transmitted from parents to children. Von Siebold (26) in 1838 discovered the six-hooked embryos of Txnia, and came to the conclusion that they could only pass into the fully-formed animal by a kind of metamorphosis. The subject was fully discussed by Eschricht (27), who endea-voured to prove that this phenomenon was of common occurrence among the Entozoa. Shortly afterwards ap-peared Steenstrup's famous work upon the alternation of generations (28), which furnished a ready explanation of the isolated facts till then observed regarding the Cestodes. The most important advances in modern times have been due to the introduction of helminthological experiment by Küchenmeister, by means of which the demonstration has been furnished that certain bladder-worms are the larval stages of particular tape-worms. The first of these ex-periments took place in 1851, when Küchenmeister fed a dog with bladder-worms from the rabbit, and a cat with specimens from the mouse, and succeeded in rearing tape-worms in their intestines (29). Similar investigations on different species have been made by Van Beneden, Leuckart, and others. Of systematic treatises the most important are those of Rudolphi (35), Diesing (20), and Van Beneden (13), while Von Linstow, in addition to numerous scat-tered papers (30, 36), has given us an invaluable list of hosts with their respective parasites (21).
Anatomy.
In considering the anatomical peculiarities of the Cestoda it will be convenient to describe one particular species and afterwards to indicate the chief differences presented by other members of the group. For this purpose Taenia saginata, Göze(T. mcdiocanellata, Küchenmeister), may be selected as a type, as it has been perhaps more studied than any other, and is one of the species most commonly found in man; for further details, see Sommer (31).
Dimensions.—An average specimen of this tape-worm (fig. 1, A) will measure in a state of moderate contraction about 500 cm., and consist of nearly 1400 segments; of those which immediately follow the head more than 250 will be found within a length of 5 cm.; they gradually widen posteriorly, until the widest, which are situated about half-way down the chain, have a breadth of 14 mm. and a length of 6 mm.; whilst the terminal segments measure 5 mm. in breadth by 19 mm. in length.
The head (fig. 1, B) is spheroidal, 1'5 mm. in diameter, and bears on its lateral surface four equidistant suckers, which serve for the attachment of the whole worm. After death these are generally retracted, but during life they can be protruded and moved in all directions. They are a special development of the musculature of the body-wall, the radial fibres being the most conspicuous. The tape-worm now being described is abnormal, inasmuch as the front of its head is not provided with a circlet of hooks; these are well seen, however, in the other common human tape-worm (Tmnia solium), which bears a double ring of them, situated around a button-shaped muscular pad (rostellum) which forms the apex of the head (fig. 1, C). By the varying contraction of the separate parts of this organ the hooks may be moved in different direc-tions, and when the worm is attaching itself they are first extended directly forwards, and then brought back so as to force the rostellum into the tissues of the host. Each hook has a broad bifid base, to which the muscles are attached, supporting a long curved point. In Tsenia saginata, to the consideration of which we now return, the rostellum is quite rudimentary, and has been described by earlier authors as a fifth sucker or even as a mouth; it is interesting to note that during its incipient stages it bears a number of minute spines homologous with the hooks of other species. The head contains furthermore the anterior portions of the nervous and excretory systems. The latter of these consists of an annular vessel placed immediately below the rostellum, from which four canals, corresponding to the four suckers, pass backwards; two of these gradually disappear, leaving two which pursue their course down the proglottides, in connexion with which they will be again alluded to, and open at the hinder extremity of the worm by a common pore. The nervous system of the Cestodes was long sought in vain: although some early investigators described a ganglion, they were unable to give any satisfactory proof of its existence, this having been first furnished by Schneider. It seems generally to consist of a central ganglion lying within the head, from which two cords proceed backwards; these were regarded by Sommer and Landois as part of the ali-mentary system. Niemiec (6) has recently given a detailed account of its structure in several different species, and its relations have been discussed by Lang (7).
The proglottides arise by a species of budding in the narrow neck which immediately succeeds the head; they are separated from each other by grooves, which are at first so shallow and

FIG. 1.—Anatomy of Tsenia (from Leuckart). A, Portions of Txnia saginata ; X B, head of the same ; x S. C, head of T. solium, showing the crown of hooks; x 22. D, a segment of T. saginata, showing the generative organs: n., nervous system ; ex., longitudinal excretory tubes; in, transverse vessel; g.p., genital papilla; cl., cloaca: c.p., ciri us pouch; v.d., vas deferens; t.t., testes ; v., vagina; ov. ov., ovaries; sh.g., shell gland; y g., yolk gland; r.s.,receptaculum seminis; ut., uterus ; X 7. £, the connexions of the generative organs, lettering as above: o.d., o d , oviducts ; /., fertilizing canal; X 30. F, detached segment of T. saginata, showing ripe uterus ;x 2. G, six-hooked embryo, highly magnified.

indistinct that it is impossible to say with certainty where the segmentation really begins. The proglottides which have attained sexual maturity are situated some 30-40 cm. from the head, and measure 1 '5 mm. in length by 5 mm. in breadth. The segments, like the head, consist of a solid mass of tissue in which the various organs are imbedded. Like the Trematodes, the Cestodes were long thought to have no body-cavity or ccelom, and hence were called "parenchymatous" worms. Recently, however, a series of inter-cellular spaces has been described by Fraipont (8) as leading into the terminal excretory organs, and these spaces have been inter-preted both by himself and others as the homologue of a body-cavity, although this opinion has not been allowed to pass unchallenged (see Pintner, 9). The surface of the body is covered by a thin clear homogeneous cuticle, wdiich, according to some authorities, is perforated by fine closely-set pores. The hooks which have been described above, as well as the small spines and bristles found in certain species, are developments of this cuticle. This external covering cannot, according to Leuckart (1, p. 289), be regarded as homologous with the cuticle of other invertebrates, inasmuch as it is not a secretion from a special layer of subjacent cells, but is "the structureless limiting membrane of the connective tissue substance, and is comparable with the so-called basement-membrane found in the other flat-worms . . . between the muscular layer and the dermal epithelium." It is to be observed, however, that this view has by no means found universal acceptance (see Steudener, 10), and it is a priori improbable, since the Cestodes land Trematodes) would thus form an exception to the general rule by which all animals are clad with an epithelium derived from the embryonic ectoderm. The subcuticular layer is described as con-sisting of long fusiform cells (probably modified connective-tissue cells) disposed perpendicularly to the cuticle. It seems possible that they are in direct connexion with the transverse muscles of the body. The matrix of the Cestode body consists of connective tissue, the cells composing which are seldom provided with a dis-tinct membrane, and sometimes can only be separately distinguished by their nuclei. The layer of muscles (see below) separates this matrix into a central ami a cortical portion. Distributed in it, and especially in its cortical portion, are numerous calcareous corpuscles, which are generally spheroidal in form, varying up to 0 "019 mm. in diameter and concentrically laminated; they contain a large amount (often 20 per cent.) of lime salts, diffused through an organic basis, from which the salts can be removed with effervescence by the action of acids. These corpuscles have been variously inter-preted by the older authors as eggs, or as lymph or blood corpuscles, but the only theories which have been seriously maintained in modern times are—(1) that they are skeletal (Von Siebold); (2) that they are excretory (Claparede, Griesbach); or (3) that they form a reserve store of calcareous material to be used either in counteracting the acid digestive juices of the host or for the production of egg-shells (Leuckart, 1, p. 283).
The muscular system consists of three sets of fibres—longitudinal, transverse, and sagittal. The first are the best developed, and run down the inner part of the cortical layer in the form of strong bands ; the second set lie immediately below them and pass across the body in the form of two flat muscular plates, which converge towards each other as they approach the margins of the proglottis; the sagittal muscles run primitively straight from one flat surface of the body to the other, but their direction is much modified after the growth of the genital organs, between the various parts of which they lie as isolated bundles; they are the weakest of all the sets. The muscular fibres are non-striated, and when they are fully de-veloped no nucleus can be detected in them. They taper towards the extremities, sometimes branching dichotomously, and, as above mentioned, a connexion has been asserted to be visible between them and the subcuticular cells.
The excretory system in the proglottides consists of two or four longitudinal canals which lie along their two narrow margins (fig. 1, D, ex.). The origin of these in the head has been already noted, and they pass continuously down the whole worm until they open into a vesicle at the posterior extremity of the terminal segment. In the hinder part of each proglottis they are connected by a trans-verse vessel (fig. 1, D, lr.), immediately above which a valve is formed by a duplicative of the wall, so that it is impossible to inject the excretory system from behind whilst fluid can be readily forced along it from before backwards. Fraipont has drawn a distinction between ascending and descending canals. Excretory openings have been described by various observers in the anterior portion of the worm, near the suckers (Wagener, 11; Fraipont, 8; Riehm, 12), and, although their presence is denied by Pintner (9), there seems sufficient evidence to show that they are more generally present than was formerly supposed. A ramifying network of smaller vessels connected with the main trunks just described is found in the more superficial parenchyma, and this in its turn gives off still finer capillaries which terminate in ciliated funnels. According to Fraipont these open into the intercellular lacunae which are the representatives of the ccelom (see above), whilst Pintner maintains that the terminal funnels are completely closed, and are to be regarded as unicellular glands. The subject, however, is one of extreme difficulty and demands further investigation. It is worthy of notice that each of the three systems of canals above described maintains its proper diameter throughout, and that no intermediate sizes can be found. The "plasmatic vascular system" described by Sommer and Landois, and regarded by them as part of the alimentary system, consists partly of some of these delicate canals and partly of the two cords of the nervous system. The main canals open posteriorly into a pulsatile vesicle, at the end of the last proglottis; when, however, some of these have been cast off the opening may be either by a shortened transverse vesicle, as Leuckart (1) maintains to be the case in the present species, or by separate openings, one for each canal.
The reproductive organs are serially repeated in the proglottides, each of which contains a complete set of male and female organs (fig. 1, D). The male organs may be discussed first. The testes (t., t.) are very numerous and scattered throughout the greater part of the proglottis; they are round vesicles (O'lo mm. in diameter) containing spermatozoa, and attached like berries to the terminal ramifications of the vas deferens (v.d.); these gradually unite, form-ing larger and larger branches until they reach the main canal, which runs in a series of coils transversely half way across the joint a little behind its middle, and ends in a common cloaca {cl.), which receives both the male and female organs, and is con-nected with the outer world by the porus genitalis. The outer por-tion of the vas has a thickened muscular wall, and this part of it is capable of extrusion and retraction, thus forming the male intro-mittent organ or " cirrus " (c.p.). The cuticle which lines all the distal portion of the vas deferens is here thin and delicate and armed with a series of minute spines, which are directed backwards (Echeneibothrium). The cirrus in the present species is very short, but in other forms its length is sometimes considerable. The protrusion is effected by circular muscles placed around the end of the vas deferens, while the retraction is brought about by special longitudinal fibres, lying along the walls of the evaginable portion.
The female organs may be most conveniently studied by tracing them inwards from the cloaca. The vagina (v.) opens immediately posterior to the vas deferens, and like it is lined by a continuation of the external cuticle. After passing about half way across the segment it bends backwards and terminates in a small cyst, the receptaculum seminis (fig. 1, E, r.s.); this receives and stores up the male fertilizing elements, retaining them until the ova are ripe. From its posterior extremity there passes a thin-walled canal, wider than the vagina (/.), which serves to convey the spermatozoa to the ova, and hence is termed the " fertilizing canal" (Befruchtungs-canal of German authors). It unites with the common oviduct, a tube formed by the union of the two oviducts (o.d.), and the two together pass backwards into a spherical glandular structure, called from its discoverer " Mehlis's body" or the shell-gland (fig. 1, D and E, sh.g.). Within this apparatus it receives the duct of the yolk-gland {y.g.), and then passes directly forwards to open into the uterus. The ovaries (ov.) are two in number, situated one on each side of the middle line of the body; they are fan-shaped, and consist of a system of blind tubules situated on a branched efferent duct. The cells of the ovary (primitive eggs) have a sharp contour and a large nucleus; the yolk-gland (y.-g.) is very similar to the ovaries, behind and between wdiich it is situated, but is distinguished by various histological details (it is called " ovary " by Mouiez). The shell-gland, formerly regarded as the ovary, consists of closely compressed nucleated cells, and is provided with small thin ducts opening into the narrow internal cavity of the organ. The uterus (ut.), in its early stage of development, is a long straight tube, lying almost in the longitudinal axis of the proglottis, and receiving posteriorly the oviduct after it emerges from the shell-gland (fig. 1, E, ut.). From what has been said it will appear that the ova on their way down the common oviduct are impregnated as they pass the end of the fertilizing canal, and then receive in succession, first their supply of food-yolk and their shell, during their sojourn in Mehlis's body, after which they go forwards into the uterus, where they undergo the first stages of their development. The uterus assumes a very different shape as it becomes distended with eggs, which are far too numerous to be contained in a simple straight tube ; small protuberances arise from its walls, growing rapidly and bifurcating here and there, so as to produce the complicated branched appearance seen in fig. 1, F. As the uterus grows, the male, and later the female, genitalia degenerate and disappear, and in the proglottides which are ready to be liberated the only organ visible is the distended uterus. One of the most characteristic peculiarities in the sexual system just de-scribed is that there is no passage by which the ripe eggs can make their exit from the proglottis ; these are therefore extruded only on its rupture; a very different state of things obtains in the genus Bothriocephalus (see below). Self-impregnation certainly occurs, and is probably the rule; it is obvious that the contrary ease can only happen where two. individuals lie side by side within the same host. Furthermore, the cirrus has been seen protruded into the vagina of the same joint, and the emission of sperm has been witnessed (Leuckart, 1; Van Beneden, 13, p. 601).
The eggs are ovoid or spherical, and consist of the germ-cell (nucleus and protoplasm) with an albuminous enveloping substance, which is again surrounded by a thin transparent skin. The shell frequently presents one or more appendages, probably the secretion of the shell-gland drawn out into threads. The structure of the egg has been best studied in Tsenia serrata (Van Beneden, 14), where it consists of a delicate shell containing a germ-cell, with a quantity of secondary yolk ; the former divides into a "granular" cell, which segments no further, and an "embryonic" globe, which again divides into a number of cells, of which three are larger and constitute the "albuminogenous layer," whilst the remainder are smaller and form the "embryonic mass," and secrete a delicate superficial cuticle, the cell-limits being indistinct. In the embry-onic mass from three to five flattened cells form a chitinogenous layer, and give origin to a superficial homogeneous coat, a shell of radially disposed chitinoid cylinders, and an internal faintly striated lining, whilst the remaining cells become the six-hooked embryo or proscolex, a superficial layer to which the hooks belong, and a central mass of clearer cells. When the proscolex is mature the original egg-shell and the albuminogenous layer disappear, and only the chitinoid coats remain.
The proglottides are cast off by muscular action ; the fibres are not continuous between the successive segments, so that these are con-nected merely by soft connective tissue, which readily gives way: the rupture takes place through the transverse vessel above described, and, as the hinder margin of the proglottis contracts forcibly when the next one has left it, the transverse vessel gives rise to the rounded vesicle which forms the termination of the excretory system.
Life-History and Development.
The six-hooked embryo (fig. 1, G) may be conveyed to the inter-mediate host in several ways, but the commonest is for it to be taken into the alimentary canal along with food to which it may have adhered, or with water in which it was swimming; the infection may take place either by means of free eggs or by whole proglot-tides. In the latter case the soft tissues are first digested by the gastric juice of the host, and in either case the egg-shell is dissolved or rendered so brittle that the embryo readily escapes by the movements of its hooks. The proscolex, after spending a longer or shorter time free in the stomach or intestine, proceeds to perforate the wall of these organs by means of active burrowing motions. Although the embryo of a Tsenia has only once been captured in its progress through the wall of the alimentary canal (Eaum, 15, p. 28), there can be no doubt that this is the route by which it usually proceeds, and that its next locality is a blood-vessel, probably some small branch of the portal vein, in the blood of which it has been found by more than one observer. This would explain the frequency with which the next stage is found in the liver. There seems, however, reason to believe that many embryos penetrate the intestinal wall completely and reach the body-cavity, in which they can wander freely. When the six-hooked embryo has reached its resting-place, which in addition to the liver may be lung, muscles, brain, connective tissue, or eye, it at once commences its further development, and in a few days becomes visible to the naked eye. Like any other foreign body, it causes a proliferation of cells, which in due time form a sheath of con-nective tissue, with a cellular lining, and smooth like a serous cavity within ; this covering, however, is not found when the parasite is situated in the brain or the eye. The embryo now grows in size, generally becoming somewhat elongated, and the hooks drop off. Sometimes they can be found lying detached in the connective-tissue sheath. The central cells enlarge and become clear, and in 'all the Tseni® they liquefy, forming a quantity of fluid which fills the centre of the bladder-worm. At this stage the larvae constitute the so-called "measles" of beef (that of pork being due to Cysticercus cellulosse, the immature stage of Tsenia solium); they are ovoid vesicles lying between the muscle fibres and varying in length from 4 to 8 mm. and being usually about 3 mm. in diameter (fig. 2, A). At a point on one side of the bladder there appears a small thickening, the meniscus or rudiment of the future head ; this is soon followed by an invagination of the cuticle (fig. 2, B). When the rudimentary head thus formed is about 0 2 mm. in length (the bladder being 1 '5 mm. in diameter) the forma-tion of muscles in its walls commences. At four equidistant points near the bottom of the invagination the suckers are developed, and at the lowest part of the cavity the rostellum (and the double circlet of hooks in the armed species) is formed. Surrounding the head-rudiment on that surface which is towards the cavity of the bladder is a thin layer, known as the "receptacle." This is best seen in Cysticercus cellulose, in which there is also a very characteristic bend or kink in the pedicle of invagination, which in most other bladder-worms remains straight (see fig. 2, C). The rudiments of the vascular system of the bladder have already appeared before the formation of the head, and now they extend into it as four longitudinal vessels, which become connected at the bottom of the invagination by a circular vessel. About this time, too, the cal-careous corpuscles above described make their appearance. When the hooks and suckers are fully formed, the head undergoes a process of evagination, so that wdiat was previously a hollow cavity becomes a solid cylinder, and the hooks, which were below the suckers, come to lie above them. Mature bladder-worms vary in size from Cysticercus fasciolaris, the size of a pea, to Cysticercus tenuicollis, six inches or more in length.
The development into the adult tape-worm takes place only after the Cysticercus has been swallowed by the permanent or definitive host. The course of this metamorphosis has been followed experimentally, and it has been found that first the bladder and next the neck of the worm are dissolved by the gastric juice. The head only is left ; in the moist warmth of the intestine its suckers and rostellum exhibit very lively motions, which serve to bring about its attachment to the intestinal wall. It gradually increases in length, and the formation of segments speedily commences.
The life history of the Cestodes is generally summed up as con-sisting of three different forms:—(1) the proscolex, or six hooked embryo, which gives rise to the bladder-worm ; (2) the scolex, which develops the chain or strobila by a process of budding ; and (3) the proglottis, or sexual animal, which produces eggs, Each of these three forms has certain claims to be regarded as a zoological "individual.'' Van Beneden (13, Vers Int., p. 251) has laid great stress upon the correspondence between a Trematode and a ripe proglottis, and it has been since nointed out that a proglottis may
FIG. 2.—Development of Taenia (from Leuckart). A, Cysticercus bovis in beef; nat. size. B, mvaginated head of a Cysticercus before the formation of the suckers; X 25. C, invaginated head of Cysticercus celluloses, showing the bent neck and receptacle r; x 30. D, stages in the development of the brood-capsules in Fchinococcus: a, the thickening of the parenchyma of the bladder; b, subsequent formation of a cavity in it; c, development of the suckers; d, a capsule with one head inverted into its cavity; e, a capsule with two heads; x 00.
of the tape-worms furnishes an admirable instance of the alterna-tion of generations. The individuality of the proglottides, however, although maintained by authorities so eminent as Leuckart, is by no means universally accepted; a distinction has been drawn be-tween their formation and true budding seen in other animal colonies, such as the Polyzoa; and Biehm, in a recent work (12), has pointed out that the casting-olf of these sexual segments is in some respects comparable to the detachment of the hectocotylized arm of a Cephalopod, and the formation of new joints to the de-velopment of an Oligochaetous worm from a few segments; further-more, certain organs—for example, the nervous and excretory systems—are continuous throughout the whole chain, and an isolated proglottis is unable to maintain its existence for more than a very limited period. According to this view, alternation of generations only occurs in Cestodes in forms such as Gcenurus, where there is a proliferation of heads in the wall of the* bladder.
The Cestode larva; corresponding to the stage which has just been described present considerable variations both in form and struc-ture, and upon these classifications of the group have been based, and generic names have been given to the different forms; Von Lin-stow (36) has, however, pointed out the undesirableness of this, as they are all parts of the life-history of one genus, Tsenia. The most, recent arrangement is that of Villot (16), which is as follows:—
I. The caudal bladder arises from the proscolex by simple growth
and structural modification, without the formation of any new
parts. A larger or smaller quantity of fluid is present, and also
a connective-tissue sheath ; the host is a vertebrate. Cysticercus
(true bladder-worms), Piestocystis, Cceuurus, Echinocoecus.
II. The caudal bladder arises from the proscolex by budding, that
is, by the formation of new parts; there is no connective-tissue
sheath, and the host is an invertebrate. Cysticercoidei, Leuckart.
(a) Caudal bladder formed by endogenous budding; the head is
surrounded, not only by the body of the worm and the caudal
bladder, but also by the blastogen (proscolex). Polycercus (from the
earthworm), Monocercus (from the black slug, Arion). (b) Caudal
bladder formed by exogenous budding; the head is only surrounded
by the body of the worm and the caudal bladder. Cercocystis (from
Tenebrio), Staphylocystis, Urocystis, Cryptocystis.
Of these the most important are the first group, and regarding some of them a few words must be added. In the great majority of species only one tape-worm head is produced, and such bladder-worms constituted the genus Cysticercus of the older helmintho-logists. In certain cases, however, notably in the worm which produces the " staggers " of sheep, numerous heads are formed in the wall of each bladder ; such larvae formed the genus Ccenurus, but apart from their polycephalous condition there are no structural peculiarities calling for special notice. The third variety of bladder-worm (Echinococcus) is characterized by the fact that the tape-worm heads are not directly developed in the wall of the bladder itself, but from "brood capsules " which lie in numbers on the inner wall of the bladder.
Development of the Echinococcus.—The smallest bladder yet seen was reared by Leuckart in the pig, and consisted of a minute proto-plasmic mass surrounded by a structureless cuticle. This cuticle thickens by deposition of new layers as growth proceeds, and the lamination of the cuticle is one of the characteristic peculiarities of the Echinococcus, another being the absence of an excretory system. At certain points in the parenchyma lining the cyst small warts are noticed (fig. 2, D,£t), which enlarge and become hollow; then the cavity enlarges in a direction opposite to the point of origin, and at the extremity of this hollow suckers and hooks are formed as in the case of Oysiicercus described above (6, c). No sooner has the devel-opment of the first of these reached a certain degree of completeness than others are formed in similar fashion. The first part of the invagination takes place, by which the future head comes to lie within the brood-capsule and the pedicle is no longer hollow but solid («) ; the suckers and hooks are, however, still invaginated, and remain so for a considerable period. Seeing that the interior of the brood-capsule is lined with cuticle, it corresponds to the outside of the parent cyst, and hence is probably the representative of a previous invagination. If this be so then the development of Echinococcus would be quite comparable with that of Cysticercus, the only difference being that, instead of the head being an inva-gination of the wall of the cyst itself, it is a secondary invagina-tion, the primary being the brood-capsule. This does not, however, exhaust the peculiarities of the Echinococcus; the form just de-scribed, with a simple cyst and brood-capsules, is common in cattle, and hence goes by the name of Echinococcus velerinorum; but cases are frequent, and are the most common in the human subject, in which the cyst contains daughter-vesicles, differing from those just described in being sterile—giving rise to no heads. These daughter-bladders may originate in three different ways :
(1) from little granular heaps, which are seen between the different layers of the cuticle, and which are probably derived primarily from the parenchymal layer,—since new layers of cuticle are continually formed internally, these bladders gradually make their way out-wards, until they come to lie externally to the mother-vesicle (Echinococcus exogena, Kuhn ; E. scolecipariens, Kiichenmeister);
(2) from brood-capsules ; (3) from Echinococcus-henás ; these last two modes of development give rise to vesicles, which are within the mother-vesicle, and produce a form which has been variously called Echinococcus endógena, Kuhn, E. altricipariens, Kiichenmeister, and E. hydatidosus. A very remarkable form is Echinococcus multilocularis, which consists of a number of very small vesicles embedded in a common soft stroma; it is found exclusively in man, and for long was regarded as a form of alveolar cancer. The mode of its development is unknown (for further information, see Virchow, 17). Compound bladders occur in man and the ox, whilst other ruminants, swine, and monkeys usually harbour the simple or exogenous forms. The organs most often affected are liver and lungs. The adult tape-worm (T. echinococcus) is found in the intestine of the dog, jackal, and wolf, occurring in consider-able numbers between the villi. Its length (fig. 3, A) is at most 5 mm. and it consists of only three or four segments; the head has four suckers and a double circlet of hooks.
Pathological Effects.
The pathological effects of Cestodes fall naturally into two categories—(1) those due to the adult worm, and (2) those due to the larvae or bladder worms.
(1) Those of the first group are in general slight, being confined to the abstraction of a certain amount of nutriment, and to a more or less acute feeling of irritation, sometimes amounting even to colic-like pains, in the intestine. There have indeed been many authorities who have maintained that they were beneficial; Jordens went so far as to describe them as the good angels and unfailing helpers of children, and Schimper records that the Abys-sinians consider that they prevent constipation, and only regard them as disadvantageous when they grow too long. Notwithstanding all this, however, there are not a few cases on record in which anaemia and neurotic, or even mental, diseases have been caused by the malnutrition and irritation which they occasion.
(2) The effects of Cestode larvae may again be divided into two subdivisions, (a) That due to the invasion and wandering of a large brood of six-hooked embryos has been most successfully studied in cases in which animals have been fed for experimental purposes with fragments of ripe tape-worms; in such instances a train of symptoms has been observed to which the name " acute cestodic tuberculosis " has been given. It is characterized by loss of appetite, fatigue, ruffling of the hair, and fever; on post-mortem examination it has been found that the lymphatic system is in a state of inflammation, while the muscles present the appearance which has already been described. (6) The effects of formed bladder-worms may be summed up as dependent upon the pressure of the growing cyst and the consequent absorption of the sur-rounding tissues of the host, so that the importance of the results depends almost entirely upon the organ which is affected. Bladder-worms in the brain are, of course, the most frequently fatal, especially when, as is not unfre-quently the case, they exert pressure upon the ganglia at its base. Kiichenmeister has collected a considerable number of occurrences of cystic worms in the brain; among these sixteen were not accompanied by pathological symptoms during life; in six others these were slight; twenty-four were cases of epilepsy, six of cramp, forty-two of paralysis, and twenty-three of mental disturbances of varying intensity. Cysticerci in the brain vary greatly in size and form according to the precise situation which they occupy; in its ventricles they have been found as large as a pigeon's egg. In the meshes of the arachnoid the bladder sometimes grows into a remarkably branched structure, which has been called Cysticercus racemosus by Zenker (3). Another peculiar form from the same organ has been described by Koberle (4); it is characterized by the great length of its head-process (2 cm.), which is coiled up into a regular spiral of sometimes three turns ; it has received the name Cysticercus turbinatus, though its specific distinctness is doubtful. The occurrence of Cysticerci in the eye is of special interest, because of the opportunity it affords of observing, by means of the ophthalmoscope, the development of the worm in its natural environment. It seems generally to lie at first below the retina, and is visible as a bluish-white sharply defined body; subse-quently the retina is destroyed by the pressure, and the worm falls forward into the vitreous body; sometimes the head may be seen protruding first through the opening; in the chambers of the eye the Cysticercus is almost always free, that is, without a capsule, and swimming in the fluid, so that its form and motions may be readily and accurately observed. A large number of cases of this affection have been recorded, principally by Von Graefe in Berlin (5), and in some the bladder has been successfully removed by operation.
The special symptoms of the Echinococcus vary, like those of other bladder-worms, with its situation and size : when it grows within cavities with more or less firm limits compression of adjoining vessels and glandular passages often results, producing oedema, varicose veins, congestion of various organs, or even dyspnoea, if the parasite occur in the thorax. The liver is its most frequent seat, and next the lung; but there is scarcely any organ of the body in which it has not been found, even the bones being sometimes affected. Since the expanding cyst grows in the direction of least resistance, it has a tendency to pass
which the first two are the most reliable. Turpentine may be given in half-ounce doses along with castor oil, or made up into an emulsion with yolk of egg ; while .the male fern is usually administered in the form of liquid extract (half a drachm to one drachm). Careful search should be made in the evacuations for the head or scolex, without the expulsion of which there is no certain evidence that the parasite has been removed from the body.



towards the surface of organs, and sometimes a cure is effected spontaneously by its rupturing into the alimentary canal or into some other passage leading to the exterior. Cases in which the cyst opens into the- blood-vessels are

FIG. 3.—Various Forms of Tape-Worms. A, Tie-iiia echinococcus; X 12 (from Leuckart). B, Archigetes sieboldi; x 60(from Leuckart). C,Eehinobothrium typus; x 10 (from Van Beneden). D, Caryophyllseus mutabilis; x about 5 (from Carus).
almost always suddenly fatal. When the Echinococcus occurs near the surface of the body, it may be evacuated by puncture and a cure effected with but little risk.
Systematic Arrangement of the Cestocla. The following classification of the Cestodes, based mainly on that of Van Beneden, exhibits the present state of our knowledge of the group :—
Class Cestada.
Family I. Amphilinidee.—Body oval, flattened, with a sucker at the anterior extremity; testes vesicular, vas deferens opening posteriorly; ovary (germarium) single, yolk glands double, vagina opening near the vas deferens, uterus opening anteriorly; embryo ciliated in front and with ten hooks. Examples : Amphilina, Wagener (see below), Amphiptyches, Wagener.
Family II. Caryophylltvidee.—Body unsegmented, flat, extended ; head expanded, bilobed, and without hooks ; a single set of sexual organs in the hinder portion ; development probably a simplified .metamorphosis. Example: Curyophyllinus niutabilis, from the intestine of Cyprinoid fishes (fig. 3, D).
Family III. Pseudophyllidiv.—Head provided with two sucking grooves ; proglottides not always well defined; a uterine aperture always present in addition to the openings of the vas deferens and vagina ; embryo always (?) with a ciliated coat, and egg-shell with an operculum. Examples : Bothrio-cephalus (see below), Trieenophorus (= Tricuspidaria), Soleno-phorus, Schislocephalus, Ligula, Archigetes, and perhaps Duthiersia (see below).
Family IV. Diphyllidse.—Neck and two suckers armed with hooks. Example : Eehinobothrium, two species known from Selachians, one immature from a mollusc (fig. 8, C).
Family V. Tetrarhynchidsn.—Head provided with four suckers and four protractile probóscides armed with hooks ; sexual openings marginal. Example : Tetrarhynchus (see below), about forty species known, many only described from im-mature forms.
Family VI. Tetraphyllid.se. —Head with four very mobile and distinct suckers, which are often armed with hooks or chitinous rods ; body segmented, proglottides cast off when mature ; sexual openings marginal.
Subfamily i. Phyllobothrinse. —Suckers without hooks or spines. Examples : Echeneibothrium, Phyllobothrium, Anthoboih-rium, a few species of each, all from Elasmobranch fishes.
Subfamily ii. Phyllacanthinm.—Suckers each with two to four hooks. Examples : C'alliobothrium, Onchobothrium, Acan-thobothrium, two or three species of each genus known from Selachians.
Family VII. Teeniachc.—Head furnished with four suckers and often with a single or double circlet of hooks ; proglottides well-defined and cast off when mature ; no uterine aperture. Example : Tenia (see below).
It seems advisable to add a few details regarding some of the forms alluded to in the above synopsis.
Amphilina foliacea, described as a Trematode by Rudolphi, is found in the body-cavity of the sturgeon. A number of unicellular glands open into the sucker, and are surrounded by the muscles of that organ; the nervous system consists of two ganglia, with a commissure, and two lateral nerves ; the male organs resemble those of Bothriocephalus, the female those of the Trematodes ; the family is generally regarded as furnishing a connecting link be-tween the Ccstoda and Trcmatoda ; see Salensky (18) and Lang (7).
ex. v.o. _ __. ex.

D
FIG. 4.—Bothriocephalidre. A, A segment of Bothriocephalus latus, showing the generative organs from the ventral surface; ex., excretory vessels; c, cirrus; __., cirrus pouch ; v.d,, vas deferens ; r.o., vaginal opening ; v , vagina ; sh.g., shell-gland ; od., oviduct; ov., ovary ; y.g., yolk-gland ; y.d., its duct; ut., uterus; _._., uterine opening; the testes are not visible from this side; x 23 (from Sommer and Landois). _, C, marginal and lateral views of the anterior part of B. cordatus, showing the cephalic grooves; x 5 (from Leuckart). D, Ciliated embryo of B. latus ; x 60 (from Leuckart).
Bothriocephalus latus (32) is the most conspicuous example of the family Pseudophyllidie, and is, moreover, noteworthy as being the largest tape-worm found in man ; its length often reaches 8 to 9 metres, and its extreme breadth 10 to 12 mm. The head bears two grooves, which correspond in position with the flat sides of the body. There are two (more correctly three) genital openings, which are situated, not on the margin but on the flat side of the body, on that surface which is usually called the ventral. The most anterior of these is the male aperture (fig. 4, A, c.), and im-mediately behind it is that of the vagina (v.o.), so close that on superficial examination the two often seem to coincide. This vaginal opening, like that of the Tseniadie, serves for the intromission of the penis and for the fertilization of the ova, but not for the exit of the ripe eggs; this being provided for by a special aperture at the other end of the uterus from that at which the eggs enter it. This uterine opening (_._.) is situated at a short distance behind the other two. The result of this arrangement is that the eggs can be evacuated without any injury to the proglottis, and consequently their discharge commences before its separation from the parent worm and may continue for a long period. The uterus (ut.) itself, owing to its disposition in folded coils, when full of eggs, presents an irregular, round, lobular appearance, which has been compared to a flower or heraldic lily. The yolk-gland (y.g.) is widely disseminated in the lateral areas of the segments, and its ducts (y.d.) form a series of branching tubules, first described by Eschricht (27) under the name "yellow ducts." The excretory organs (ex.) differ from those of the Tseniadm in that the canals exhibit a reticu-late arrangement. The embryo (fig. 4, D) as it leaves the egg is covered with a ciliated mantle, which corresponds to the firm egg-shell and associated membrane of the cystic tape-worms, and perhaps also to the ciliated envelope of certain Trematode larva? (see TIIEMATODA). This ciliated organism swims freely about in the water, but after a time the six-hooked proscolex escapes from it. The next stage in its life-history is not yet known, but it has been recently shown by Braun of Uorpat (33) that at a subsequent stage it inhabits the pike and burbot, and develops into the sexual adult when transferred to the intestine of the human subject. The geographical distribution of Bothriocephalus is limited; it has been recorded with certainty in but few places outside Europe; while within that continent the coasts of the Baltic and Switzerland are the principal localities; it is widely distributed in Russia, and has been recorded from Poland, Denmark, Germany, as well as from France and Britain, though it is possible that the cases occurring in these latter countries have been due to importation.
The genus Ligula has the segmentation obscure or indistinguish-able. About six species are known. One is found encapsuled in a monkey, one in the common seal, others in reptiles and teleosteans. Archigetes sieboldi (fig. 3, B) occurs in the body-cavity of an Oligochaetous worm (Tubifex rivulorum); it is about 3 mm. long, and consists of an oval body (scolex), to which is attached a cylin-drical tail (proscolex), bearing at the posterior extremity three pairs of hooks; both these parts are capable of motion. The scolex has eight longitudinal excretory canals, and a terminal vesicle; the ven'trally situated genital aperture is the common exit of the vas deferens, the vagina, and a uterus separate from the latter; the development is direct, and it attains sexual maturity without a change of host. Duthiersia, Perrier (34), contains two species, both from the intestines of varanian lizards. The genus is characterized by the presence of two large compressed frilled suckers, separated by a septum and perforated at their bases. The proglottides have three genital apertures resembling those of Bothriocephalus.

FIG. 5.—Tetrarhynchus. A, General view of the worm; x 4. B, head showing the suckers, probóscides, and excretory canals; X 25. C, portion of a pro-boscis showing the two forms of hooks ; highly magnified. (All from Pintner.)
The genus Tetrarhynchus was, a few years ago, made the subject of an elaborate memoir by Pintner (9), who investigated T. longi-collis, V. Ben. The head, in which its most striking anatomical peculiarities are situated, really includes both the head and neck of previous authors (fig. 5, A); it is some 9'94 mm. long, but only 0'75 mm. in diameter, and bears at its anterior end two obliquely placed oval disks (fig. 5, B), each of which is perforated towards the apex by two round holes through which the four probóscides pro-trude. Each of these dkks, moreover, shows traces of a division into two, a fact which indicates that it is formed by the fusion of two suckers corresponding to those commonly found in tape-worms. The flattening in this genus seems to be in a direction at right angles to that in which it usually takes place. The probóscides, which are the most characteristic organs of the genus, are four in number, and protrude from or can be retracted into the anterior surface of the head. Each consists of three parts:—(1) the toothed portion is the most anterior; it is shaped like a long narrow glove-finger, like which it is invaginable; on its external surface it bears rows of hooks, closely set in diagonal lines (fig. 5, C); there are two forms of these: those which are directed outwards are large triangular hooks, with apices pointing backwards, whilst those situated on that surface of the proboscis which is turned towards the other probóscides are fine, delicate, and curved; between the hooks are fine chitinous hairs; (2) the membranous sheath is firmly attached wdiere the general surface of the body passes over into the toothed portion around the orifice of the invagination ; it consists of a thick homogeneous transparent skin, apparently an excretion of cells lining the cavity of the pro-boscis; (3) the muscular portion is the most posterior of all, and is composed of six layers, remarkable as containing striped muscular fibres ; throughout all these three portions of the proboscis there extends a retractor muscle. The action of these various structures is not thoroughly understood, but it is probable that the proboscis is protruded by the action of the last-named muscular sheath, whilst it is retracted, after the relaxation of this, partly by the retractor muscle and partly by the pressure of the surrounding medium. The family Tseniadm is usually described as containing only the one genus Taenia, but, owing to the number and variety of its species, of which more than 350 have been described, it has been subdivided into groups, regarded by different authors as genera or subgenera. The subjoined arrangement is mainly that of Leuckart. It labours under the disadvantage that its chief divisions are based upon the bladder-worm or larval stage, which is only known in the case of comparatively few species.
I. Cystici (cystic tape-worms).—Head rarely unarmed; usually
provided with a rostellum and with one or more rows of hooks ; proglottides longish oval when mature ; uterus with median stem and lateral branches; the larva has a caudal bladder containing fluid.
1. Cystotmnia, Leuckart.—The head arises in the wall of the
embryonic bladder.
a. Taenia saginata, Goze.—Without hooks ( = T. medio-
canellata, Küchenmeister, = genus Taeniarhynchus, Weinland).
b. Taenia solium, Rudolphi. — Head with a double
circlet of hooks.
c. Tivnia acantholrias, Weinland.—Head with a triple
circlet of hooks ( = genus Acanthotrias, Weinland).
2. Echinococcifer, Weinland.—The heads arise in special
brood-capsules. Taenia echinococcus, V. Siebold.
II. Cystoidei (ordinary tape-worms).—The larva has no distended
caudal bladder containing fluid.
1. Uymenolcpis, Weinland.—Proboscis with a single row of
small hooks. Taenia nana, V. Siebold, T. flavopunctata, Weinland.
2. Dipylidium, Leuckart.—Head with several rows of hooks,
each with a discoidal base; a right and left set of genital organs in each joint, the uterus, however, being single and common to the two. Taenia cucumerina, Rudolphi (= T. elliptica, Batsch). Hamann (2) has recently proposed a new genus, Ptychovhysa, for Taenia lineata, Göze, which is defined by the following characters:—(1) the porus genitalis is on the surface and not on the margin of the joints ; (2) the vaginal opening is anterior to that of the cirrus ; (3) at a certain period the uterus is convoluted ; (4) there is a peculiar shell-gland. In many of these characters the species shows a resemblance to the Bothriocephalidse.
Taenia saginata. T. solium. T. nana. T. flavopunctata. T. madagascariensis. T. cucumerina. Bothriocephalus latus. B. cristatus. B. cordatus.
Occurrence in Man.—The Cestodes which in the adult state infest man, with their corresponding larva? and temporary hosts, are as follows :—
Cysticercus bovis. Ox.
C. cellulosae. Pig, man.
(?)
(?)
cucumerinm.
(?)
(?) (?)
C. T
Trichodectes canis. Pike, burbot.
(?)
Fish(?)
Other species, however, inhabit the human body in their larval condition ; a list of them, with the corresponding adult forms and permanent hosts, is subjoined : —
Cysticercus cellulosae. Taenia solium. Man.
C. acayithotrias. T. acanthotrias (incog.) (?)
C. tenuicollis. T. marginata. Dog, wolf.
Echinococcus. T. echinococcus. Dog.
Phytogeny.—There can be no doubt that the Cestodes and Tre-matodes are intimately related and have sprung from a common ancestor ; there are so many structural peculiarities in which they agree (compare TREMATODES), and they are connected by so many intermediate forms, that their affinity can admit of no doubt. According to Leuckart, the original ancestor of both was probably allied to the Planarians, while Huxley (22, pp. 213, 676) points out that it is at all events possible that they have no connexion with free forms but have always been anenterous, and in fact are nothing but " gigantic morula?, so to speak, which have never passed through the gastrula stage."
Bibliography.—(1) Leuckart, Die Parasiten des Menschen, Leipsie, 1863-76 ; 2d ed.. 1879-86; Eng tränst, The Parasites of Man, 8vo, Edinburgh, 1886, (2) Hamann, Zeitschr.f. wiss. Zool.,x\ii., 1885. (3) Zenker, in Ziemssen's Cyclop, of Prac. Med., iii. (4) Köberle', Des Cysiicerques des Tmnias de VHomme, Paris, 1861. (5) Graefe, Archiv f Ophthalmologie, xii-, 1874. (6) Nlemiec, Ree. Zool. Suisse, ii., 1885. (7) Lang, Mitth. Zool. Stat. Neapel, ii., 1881. (8) Fraipont, Archives de Biol., i., 1880, ii., 1881. (9) Pintner, Arb. Zool. Inst.,ü\., Vienna, 1881. (10) Steudener, Abhandl. naturf. Gesellsch., xiii., Halle, 1877. (11) Wagener, Nova Acta Cms. Leop.-Carol. Acad.. xxiv., Suppl., 1854. (12) Kiehm, Correspbl. d. nalurw. Ver. f. Sachsen u. Thüringen, 1882. (13)P. ,1. Van Beneden, " Vers Cestoides," Mem. Acad. Bruxelles. xxv., 1851, and " Verslntestinaux," Comptes Rendas, Paiis, Sappl., ii., 1861. (14) Ed. Van Beneden, Archives de Bio!., ii., 1881. (15) Raum, Beitr. zurEntwiekelnngs-gesch. der Cijsticercen, Dorpat, 18S3. (16) Viilot, Rev. Sei. Nat. Montpellier, Sepr. 1882, 1883. (17) Virchow, Verhandl. Wurzb.phys.-med. Gesellsch., vi., 1856. (18) Salensky, Zeitsehr. f. wiss. Zool., xxiv., 1874. (19) Pallas, Neue nordische Beiträge, i., ii., 1781. (20) Diesing.Systema Helminthum,8vo, Vienna, 1S50. (21) Linstow, Comp, der Helminthologie, 8vo, Hanover, 1878. (22) Huxley, Anal. Invertebr. Anim., 8vo, London, 1877. (23) Plater, Opus Praxeos Mediae, 1601. (24) Audry. An Account of the Breeding of Worms in Human Bodies, 8vo, London, 1701 (transl.). (25) Bremser, lieber lebende Wurmer im lebenden Menschen, 4to, Vienna, 1819. (26) Burdach's Physiologie, ii., Leipsic, 1838. (27) Eschricht, Nova
Acta Cxs. Leop.-Carol. Acad., xix.. Suppl., 1841. (28) Steenstrup, On the Alterna-
tion of Generations, Ray Society, 8vo. London, 1845. (29) Küchenmeister,
Animal Parasites. Sydenham Society, 8vo, London, 1857,- new German ed.,
Leipsic, 1880-81. (30) Linstow, Archiv f. Naturgesch , 1872 sq. (31) Sommer,
Zeitschr. f. wiss. Zoo!., xxiv., 1874. (32) Sommer and Landois, op. cit., xxii.,
1872. (33) Braun, Zur Entwiekelungsgesch. des breiten Bandwurmes, Würz-
burg, 1883. (34) Perrier, Arch, de' Zool. Exper.. ii., 1873 (35) Rudolphi,
Entozoorum Hist Nat , 8vo, Amsterdam, 1808. (36) Linstow, Arch. /. Natur-
gesch., 1886, p. 113. For a concise account of the comparative anatomy and
copious bibliography, see Jackson, in Rolleston's Forms of Animal Life, 2d ed.,
Oxford, 1887. (W. E. HO.)


Footnotes

These figures refer to the bibliography, pp. 55, 56.

At the moment of going to press, Grassi (Zeitschr. f. Parasitenk., ii. 11) makes the important communication that Tsenia murina (=T. nana) may develop without an intermediate host.

The method of treatment for the removal of these tape-worms from the human body consists in the administration, first of purgatives, and thereafter of one or other of the following anthelmintics:—tur-pentine, male fern (Lastrea Filix-mas), pomegranate, or kousso,—of which the first two are the most reliable. Turpentine may be given in half-ounce doses along with castor oil, or made up into an emulsion with yolk of egg ; while .the male fern is usually administered in the form of liquid extract (half a drachm to one drachm). Careful search should be made in the evacuations for the head or scolex, without the expulsion of which there is no certain evidence that the parasite has been removed from the body.