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TAPEWORMS

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Originally appearing in Volume V26, Page 412 of the 1911 Encyclopedia Britannica.
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TAPEWORMS. The Cestodes or Tapeworms form a class of purely endoparasitic Platyelnlia, characterized by their elongate shape, segmented bodies, and the absence of a digestive system. With few exceptions they are composed (I) of a minute organ of fixation (the scolex), which marks the proximal attached end of the body; (2) of a narrow neck from which (3) a number of segments varying from three to several thousands are budded off distally. These segments, or " proglottides," become detached in groups, and if kept moist retain their powers of movement and vitality for a considerable time. This fact gave rise in ancient times to the false idea that the tapeworm originated from the union of these segments; and in modern times it has led to the view that the tapeworm is not a segmented organism (the monozoic view), but is a colony composed of the scolex which arises from the embryo and of the pro-glottides, which are asexually produced buds that, upon or before attaining their full size and maturity, become separated, grow, and, in some cases, live freely for a time, just as the segments of a strobilating jelly-fish grow, separate and become sexual individuals (the polyzoic view). Whether this view is soundly based is discussed below; the fact remains, however, that a tapeworm is, with few and rare exceptions, not directly comparable at all points with a liver-fluke or indeed with any other organism. The influence of parasitism has so profoundly influenced its structure that its affinities are obscured by the development of specialized and adaptive features. In contrast to these segmented or " merozoic " Cestodes, a few primitive forms have preserved a unisegmental character and form the Monozoa or Cestodaria. We may therefore divide Cestodes into the Monozoa and the Merozoa. Order I.—MONOZOA This order comprises a few heterogeneous forms which probably constitute at least three families. Family I. Amphilinidae.—Oval or leaf-shaped animals found in the sturgeon and certain other fish. Amphilina foliacea (fig. 1) is in many ways closely allied to the Trematoda, from which, however, it is distinguished by the want of a digestive system. One end of the body (usually designated anterior) is provided with a glandular pit (fig. 1, Aa) which is regarded as a sucker or as related to the uterine opening (birth-pore). The excretory system consists of peculiar cells, each of which bears several"flames" or bunches of synchronously vibrating cilia. These cells are ,f - imbedded in the peripheral parenchyma, and lead into convoluted excretory tubes that form an anastomosis opening to the exterior by a pore at the " hinder " end of the body. The epidermis consists of pyriform cells, which send richly branched processes to the superficial cuticle. The parenchyma is made up of stellate cells the processes of which pore; c, terminal sucker; e, vaginal pore; forma reticulum. The g, male gonopore; n, o, p, nervous system. reproductive organs (From Lankester's Treatise- on Zoology, consist of the parts part iv.) shown in fig. 1, A, I and it will be seen that, in addition to the openings of the male A 4o8 and of the female (vaginal) ducts, there is a distinct uterine opening at the opposite end of the .body (b). Moreover, in Amphilina liguloidea a fourth duct (the anterior vagina) begins close to the origin of the female duct, and after running forward a short distance ends blindly (see fig. 7, C). The egg gives rise to an oval larva, one half of which is ciliated and bears gland-cells, the opposite end carrying ten hooks. The fate of the larva is unknown. Family II. Gyrocotylidae.—Leaf-shaped animals with crenate margins. One extremity carries a pedunculate rosette-organ. It is traversed by a canal from which a peculiar proboscis-like structure can be exserted. The opposite end is pointed and provided with a terminal sucker. Amphiptyches (=Gyrocotyle) urna (fig. I, B) is found in the intestine of Chimaera and Callorhynchus, and has been almost fully described by Spencer (7). The embryo is provided with ten hooks, and appears to select Lamellibranchs (Mactra) for its intermediate host. Family III. Caryophyllaeidae.—Elongated cylindrical animals either with a single subterminal sucker at the proximal end, or with the corresponding end of the body converted into a mobile undulatory fold. Caryophyllaeus mutabilis occurs in the roach and other fresh-water fish, and passes its earlier stages of development in fresh-water Oligochaets (Tubifex). Archigetes appendiculatus lives throughout life in the coelom of Tubifex and of Limnodrilus. Archigetes and Caryophyllaeus are the only Cestodes that become fully differentiated in an invertebrate host. The former indeed is said to produce fully developed gonads, and if kept in aquaria with Tubifex, the number of infected worms steadily increases, a fact pointing to the whole cycle being passed through, without the intermediation of a vertebrate host. Conclusive evidence, however, has not yet been adduced to prove this point. The two genera agree closely in form and structure and may possibly belong to the cycle of the same or of allied species. Archigetes (3 mm. long) consists of a subcylindrical body and a caudal appendage. The former bears two terminal suckers on the flattened dorsal and ventral surfaces, the latter six hooks near the tip of the tail. The finer structure of the animal has been investigated by Mrazek (to), whose account, however, is published in the Hungarian language. It shows a close agreement with that of Caryophyllaeus. A well-developed cellular parenchyma forms a matrix in which the muscular, excretory and generative organs are imbedded. The nervous system consists of a ring below the suckers and of a large number of radially arranged tracts running forwards and backwards. Caryophyllaeus is an elongated, flattened worm provided with one extremely mobile extremity, the other being drawn out during the animal's sojourn in Tubifex into a short hexacanth tail. It becomes fully developed in its invertebrate host, but apparently cannot produce eggs until transferred into the intestine of a fish. Order II .—ME ROZOA The Merozoa, to which the ordinary tapeworms of man and domestic animals belong, includes the great majority of the Cestodes. They occur in vertebrate animals throughout the globe, thoughvarying in abundance in different districts and at different times. With few exceptions tapeworms select the small intestine for their station, and in this situation execute active movements of ex-tension and contraction. The body, or " strobila," consists of a usually minute organ of attachment (scolex or its representative) which is imbedded in the intestinal membrane, and of a series of segments that arise from the base of the scolex and increase in size distally. In one family (Ligulidae) the segmentation is only expressed in the metameric distribution of the generative organs and the worm is externally unisegmental. In the remainder the segmentation involves primarily the genitalia and includes the integument, muscles and part of the excretory system. The nervous system is, however, not segmented, and the excretory system is continuous throughout the worm. Scolex.—The scolex is biradially constructed, the proglottides flattened, quadrangular and bilaterally symmetrical. In them a ventral surface containing the usually median male and female genital apertures is generally distinguishable from the smooth yolk gland; r.s., receptaculum seminis; ut.,. uterus. E., the connections of the generative organs, lettering as above: o.d., o.d., oviducts; f., fertilizing canal. F, detached segment of T. saginata, showing ripe uterus. G, six-hooked embryo, highly magnified. dorsal surface, but in those Cestodes which possess marginal gonopores this distinction of surface is obscured. In such cases the male organs are regarded as indicating the dorsal surface, the female organs as belonging to the ventral surface. The scolex is usually a conical muscular structure. It bears adhesive organs that are either suckers or hooks, and may develop into the most varied outgrowths in order to give increased firmness of attachment to its host. Thus, starting from the two shallow pits, one dorsal and the other central, in the simplest forms. we find them becoming two elongated suckers (bothria) in the large family Bothriocephalidae (fig. 8); and by fusion of the lips they are transferred into two tubes (Solenophoridae) ; and by the closure of the lower aperture reconstituted into two suckers, the margins of which are produced and folded so as to resemble the leaf-like outgrowths of the next group. In this division (Tetraphyllidea) four suckers or bothria are developed on the scolex, but their cavities are extremely shallow and their lips extremely mobile and variable in shape. Hence they are called phyllidia (fig. 4). These organs may be raised on a short stalk, their cavity subdivided into loculi, and provided in some cases with hooks. A peculiar modification of this type of scolex occurs in the Echinobothridae, in which the axial part of the organ (the rostellum) is elongated and provided with several rows of hooks, whilst the phyllidia have partially fused. This elaborate type of scolex appears to be an adaptation to grasp the spiral intestinal valve of sharks and rays. But perhaps the most elaborate scolex is that of the Tetrarhyncha (fig. 5), which are also parasitic in Selachians. The four suckers are here united to form two pairs or fused into a single pair. Internal to the suckers are the four complex hooked proboscides. Each con- sists of an eversible hollow tentacle provided with hooklets and capable of introversion within a mem- branous sheath filled with fluid. The sheath terminates in an elongated muscular bulb. The muscles are arranged_in ten or more layers, and are transversely striated. These complex organs have apparently arisen by the increase in depth and differentiation of an accessory sucker such as is borne on the phyllidia of the former group. Lastly, the scolex of the more familiar Taeniidae (Tetracotylea) carries a rostellum en- circled with hooks and four cup-shaped suckers the margins of which do not project beyond the surface of the body. It seems probable that these suckers are not the true " bothria " but are developed from accessory stickers, the bases of which have dis- appeared almost completely. In one genus (Polypocephalus) the place of a rostellum is taken by a crown of retractile tentacles. This order is almost exclusively parasitic in warm-blooded animals. The extraordinary variety of form and complication of structure exhibited by the appendages of the scolex are adaptations to fix the worm and to resist the peristaltic action of the intestine in which it lives, and are not connected directly with the absorption of food. Proglottides.—The segments into which the body is divided vary considerably in number, size and form. Taenia echinococcus has only three, Echinobothrium four, Bothriocephalus three thousand. In every species the segments develop from the scolex distally and increase in size with the maturation of the contained female genital organs. When this is reached, growth of the proglottides ceases. As a general rule the ripe proglottides are detached in chains and replaced by others which in their turn become detached, the process being repeated for a year or so until the worm weakens and is castout. In special cases, however, a proglottis may be detached before attaining full growth, and with its generative organs in an imperfectly developed condition. The minute Taenia (Davainea) proglottina (.5 to 1 mm. in length) from the common fowl detaches its four or five segments into the intestine, where they attain a length of 2 mm., and a breadth of 1.25; that is, more than twice the size of the parent. The Cestodes of Elasmobranch fish offer more convincing examples of independent growth of the pro-glottides, for these are often set free with only the male organs developed, and each attains twice the size of the parental strobila. The form of the proglottides is most generally a rhombic or trapezoidal figure. The hinder border is often drawn out into mobile processes and hollowed out around the insertion of the next a:. -SY:V76.J~iTL segment. At this neck-like zone the muscles are absent, and across it falls the line of fracture when the proglottis separates from its fellows. Structure.—The anatomy of the Cestoda differs in only two 'or three important features from that of Trematodes. In both classes the body is encased by a thick non-cellular cuticle, the deepest layer of which—the subcuticle or basal membrane (fig. 6 b)—is perforated by the branched free ends of the isolated epidermal cells, which have sunk into the body, and by the endings of gland-cells and nerve-cells (fig. 6). The mass of the body consists of richly branched stellate cells—the mesenchyma—and imbedded in this plasmic tissue are the nervous, excretory, muscular and .generative organs. C The excretory organs consist of flame-cells, richly convoluted canali- they possess, in addition to the uterus, an anterior vagina (usually culi, and a pair of longitudinal canals leading to the exterior by one present in Cestodes) and a posterior one. This last tube is probably or more pores. The muscles are composed of outer circular and the homologue of Laurer's canal (Goto, 8). The single anterior inner longitudinal layers, and of branched dorso-ventral fibres vagina is then comparable with the similarly named duct of ecto-The generative organs are of the complex hermaphroditic type parasitic Trematodes, in which group it is either single or double. The accompanying figure will assist this description. Life-histories.—The life-history of Cestodes consists of larval and adult stages, which are usually passed through in different hosts. The egg gives rise in the uterus to a six-hooked embryo, which reaches the first host in a variety of ways. It may hatch out as a ciliated organism (fig. 8, D) capable of living freely in water for at least a week (Bothriocephalus), which then, if eaten by a stickleback, throws off its ciliated envelope, and creeps by the aid of the hooks through the intestinal wall into the body-cavity of the fish. Here it develops into a larval, or rather an adolescent form. In other cases the infection of the first host is brought about by the ingestion described in Trematoda (q.v.). In these broad anatomical features both classes agree. But whilst in Trematoda a digestive sac is invariably present except in the sporocyst larval stage, the Cestodes possess no trace of this organ at any stage of their development. They obtain food entirely by osmosis through the striated cuticle, and this food consists not of blood, as in flukes, but of chyle, by which they are bathed in their favourite site, the small intestine The second point of difference between tapeworms and Trematodes lies in the absence of a definitely demonstrable " brain." The concentration of nervous matter and ganglionic substance at the oral end of Trematodes is equivalent to the " brain " of the Planarians, but the similar thickening in the scolex of Cestodes is by no means so certainly to be called by that name. It appears to be primarily related to the organs of attachment and to have attained greater elaboration than the rest of the nervous system because the proximal end is the most specialized and most stimulated portion of the worm. Those Cestodes which possess no very distinct organ of attachment (such, for example, as Gyrocotyle) have no distinct ganglionic thickening more pronounced at one end of the body than at the other; and as these are forms which have retained more primitive features than the rest, and show closer affinity to the Trematodes, it seems highly probable that the complicated nervous thickening found in the scolex, and often compared with the " brain " of other Platyelmia, is a structure sui generis developed within the limits of the sub-class. In the opinion of several zoologists it marks the tail-end and not the head-end of the worm. The third important contrast in structural features has also been acquired by the Cestoda Merozoa, namely, the repetition of certain organs in a metameric fashion. The Monozoa are unsegmented; the Ligulidce have segmented gonads and gonopores without any trace of somatic metamerization except secondary excretory pores in addition to the usual terminal one; the remaining Cestodes are unisegmental only in their larval stage, and all of them show in their later stages repetition of the reproductive organs and of the musculature. In addition, some show duplication of the gonads and of their ducts, so that we find both transverse and longitudinal repetition of these organs, without corresponding multiplication of the nervous ganglia mesenehyma, or excretory opening. The last structural peculiarity of the group is the absence of the functions of regulation and reparation which are so highly developed in the more primitive Planarians. This statement is suite consistent with the continuous production of new segments at the neck of the scolex, for such a process is analogous to the development of the segments in a Chaetopod, which is a perfectly distinct phenomenon from the regeneration of new segments to supply the place of a head or tail-end or some other portion that has been lesioned. The replacement of detached mature proglottides at the distal end of the Cestode-body by others is not regeneration, for the replacing set has already developed, and in certain cases they can complete their development quite independently after being detached from the parent. More convincing evidence of the absence of true regeneration, however, is the argument from malformation and the phenomenon known as " pseudo-scolex. It has long been known that proglottides of the same species often exhibit sporadic malformation from the normal shape, and the evidence goes to show that the variation was due to arrested growth or some unusual stress or pressure which, acting upon the young strobila, produced a deformation, and that the proglottides so affected could not regain their normal form. The power of reparation, so conspicuous a feature of Turbellarians, is slight or absent in Cestodes. Moreover. injury to the scolex, or amputation of that organ, reveals the concomitant absence of a regulative mechanism such as that which generally controls the form. and fitness of regenerated organs. In such an event, a Cestode cannot replace the injured or severed portion. The first two or three proglottides merely become deformed and produce an appearance known as the pseudo-scolex. The absence of these functions of regeneration and of regulation affords, therefore, corroborative evidence of the highly specialized nature of the Cestode organization. Reproduction.—The reproductive organs are usually repeated in each proglottis, and in some families two complete sets of such organs occur in each segment; in a few cases, parts only of the system are duplicated. The structure of these organs is seen in figs. 3, 6 and 7, and, _as we have said, agrees closely with that of Trematodes. The chief difference between the reproductive organs of the two classes is the presence in Cestodes of a separate vagina and uterus, each of which opens in some families to the exterior by an independent pore. The vagina of Cestodes is undoubtedly comparable with the so-called " uterus " of Trematodes, but the nature of the Cestode uterus is not so clear. It has been compared with the canal of Laurer of Trematodes (the vitello-intestinal duct of the ectoparasitic flukes), but if we take the more primitive Cestodes, and especially Asnphilina, into consideration we find that B of proglottides or of eggs which are disseminated along with the faeces of the final host and subsequently eaten by herbivorous or omnivorous mammals, insects, worms, molluscs or fish. Man himself, as well as other mammals, is the intermediate host of the dangerous parasite, Taenia echinococcus, in countries where cleanliness is neglected; the pig is the host of Taenia solium, and other cases may be seen from the table at the end of this article. The transition of the larva from the intermediate to the final host is accomplished by the habits of carnivorous animals. The Elasmobranchs swallow infected molluscs or fish; pike and trout devour smaller fry; birds pick up sticklebacks, insects and worms which contain Cestode larvae; and man lays himself open to infection by eating the uncooked or partially prepared flesh of many animals. The peculiar feature of the larval history of Cestodes is the development in most cases of a cyst or hydatid on the inner wall of which the scolex is formed by invagination. The cyst is filled with a toxic fluid and may bud off new or daughter scolices. In this way bladders as large as an orange and containing secondary bladders, a/1#/H////iAV If/IlrllygiG geagee/(/rigre A each with a scolex, may arise from a single embryo. We have, in fact, a form of larval multiplication that recalls the development of digenetic Trematodes. The eggs of Cestodes consist of oval or spherical shells (lb in. ae.a middle layer of cells, and a central spherical hexacanth body closely enveloped by the middle coat. In a few genera the place of the chitinoid coat is taken by a ciliary investment and in most families the structure of the layers is characteristic. Arrived in the intestine of the intermediate host, the hooked embryo is set free and works its way to some distant site. Here it undergoes a change into a cystic or " metacestode " state. A cavity appears in its centre and it acquires a pyriform shape. The thicker portion develops a terminal muscular rostellum and two or four suckers, the thinner end (" tail ") is vesicular, more or less elongated, and contains the six embryonic hooks. By a process of infolding, the thicker end is partially invaginated, the middle portion or "hind-body" and the organism may now present a superficial likeness to a cercaria. An excretory system develops, opening at D diameter), containing a fertilized ovum surrounded usually by many yolk-cells. The shell is thick, and operculate in some forms; thin, and provided with filaments, in others; in the latter cases it may contain only a few yolk-granules suspended in an albumen-like substance. The development of the six-hooked embryo or " oncho- sphere " takes place in the uterus. The ovum first divides into (a) a granular cell, and (b) a cell full of refringent spherules. The former divides into (c) small cells or micromeres, and (d) large cells or megameres. (c) forms the body of the embryo, (b) and (d) enclose it and form a covering. The embryo undergoes differentiation into an outer layer of cells that produce a chitinoid coat, the base of the tail; nervous and muscular systems arise; and finally the rostellum and suckers become completely enclosed in the sac formed by the lateral extension of the " hind-body." When swallowed by the final host such a " cysticercoid " larva evaginates its scolex, throws off its hooked vesicular tail, and begins to bud off proglottides at its free end (fig. to). Such is the general history of Cestodes whose intermediate host is an Invertebrate. In most other cases the tail is not distinguish-able, and the body of the larva is separable only into a scolex invaginated with a bladder (=hind-body and tail). This form of larva is known as a cysticeecus. In some genera a " urocyst " is formed, the tail of which gives rise to a new cyst and a fresh scolex. The most remarkable feature of this cystic development is the formation in many genera of several internal buds within a common cyst, each of which forms an independent inverted scolex (Coenurus. Polycercus) ; or these internal vesicles may bud off a large number of scolices on their external surface (Staphylocystis). Morphology of the Cestodes.—With regard to the vexed questions of the morphological nature and of the affinities of the Cestodes, divergent views are still held. One view, the monozoic, regards the whole development as a prolonged metamorphosis; another, the polyzoic view, considers that not only is the Cestode a colony, the proglottides being produced asexually, but that the scolex which buds off these individuals is itself a bud produced by the spherical embryo or onchosphere. On this view, therefore, at least two asexual generations (embryo and scolex) alternate with a sexual one (proglottides) ; and in the case of Staphylocystis the cyst contains two asexually produced generations, so that in such forms three stages (embryo, primary scolex-buds, secondary scolices) intervene between the proglottis of a Cestode and that of its off-spring. The polyzoic view is ably championed by Braun (2) and (3). The more valuable point of view is undoubtedly the monozoic one In accordance with this we can regard the development as an adaptive one and the scolex as invaginated for protective purposes inside the cyst, which is itself an organ comparable to an amnion. On this view, multiple scolices are, therefore, not buds, but an example of the unlocalized organization of the embryo such as occurs in other groups of animals, and is demonstrated by experiment. The evolution of the cysticercoid, cysticercus and other forms of larvae is a varied adaptive phenomenon. With regard to the adult worm we have to remember that its two extremities, scolex and terminal proglottis, are different from the intervening region. The terminal or first-formed proglottis is sterile, and contains the primitive and (except in a few genera) the only excretory pore. The excretory tubes, the nervous system, and the parenchyma and integument are continuous from one end of the worm to the other. The repetition of the genitalia is the real mark of the Cestodes, and we can trace the independence of the somatic from the gonidial metamerism in such forms as Triaenophorus and others In fact, the whole history of the Platyelmia is marked by a great specialization of the reproductive evolutionary history, accompanied by a simple somatic line of evolution. We therefore regard the body of a Cestode as a single organism within which the gonads have become segmented, and the segmentation of the body as a secondary phenomenon associated with diffuse osmotic feeding in the narrow intestinal canal. The origin of the repetition of the gonads has yet to be investigated. The Effects of Cestodes on their Hosts (Shipley and Fearnsides [4)-i. By their presence. This depends largely on the station adopted by the parasite. Cysticercus cellulosae may be comparatively innocuous in a muscle or subcutaneous tissue, but most hurtful in the eye or brain. Of all parasites the one which by its mere presence is the most dangerous is the larva of Taenia echinococcus. Its bulk alone (equal to that of an orange) causes serious disturbances, and its choice of the liver, kidneys, lungs, cranial cavity and other deep-seated recesses, gives rise to profound alterations. 2. By their migrations. The migration of the Cestode-larvae through the walls of the intestine into the blood of their host is the cause of grave disturbances, due largely to the perforation of the tissues, inflammation of the vessels and peritoneum, and other effects of these immigrants. 3. By feeding in their host. The loss of nutrient fluid caused by the presence of intestinal Cestodes is probably slight, indeed, the sharper appetite that accompanies their presence may be the means of fully compensating for it. The tapeworm, Taenia saginata, throws off eleven proglottides a day during its mature stage, and if this rate of increase were maintained for a year the total weight of its progeny would be about 55o grammes. The broad worm, Dibothriocephalus latus, is similarly estimated to discharge 15 to 20 metres of proglottides, weighing 140 grammes. The loss of substance represented by this growth is probably only of serious account when the host is a young growing animal that needs all available nourishment. 4. By producing Toxins. It is generally admitted that Cestodes, both adult and larval, contain toxins of great virulence, though in what way and in what organs these substances are produced is uncertain. Injection of the fluid-extract of such worms into the blood or coelom of their host causes grave disturbance. Thus Echinococci contains a leucomaine which sets up an urticaria; Cysticercus tenuicollis occasions anaemia and death if injected into rabbits; and the cystic fluid of the common Coenurus serialis is said to be used by Kirghizes to poison wolves. But the evidence in favour of the view that tapeworms normally excrete toxin into the body of their host in such amount as to occasion disease is not generally accepted as conclusive. This evidence is, however, strengthened by the results of recent work on changes in the blood of patients suffering from helminthiasis. The occurrence of the broad tapeworm in man is often associated with anaemia of a most severe type. The coloured constituents of the blood are most affected. New elements appear in addition to degenerative changes in the normal red corpuscles. Large nucleated red blood-cells make their appearance. The white blood-cells, or leucocytes, undergo other changes. In hydatid disease there is, as a rule, a marked increase in the number of those white corpuscles which possess a specially staining affinity with the dye eosin, and are therefore known as eosinophile cells. This change, which is called cosinophilia, indicates the production of a noxious substance in the blood. The fact of this increased leucocytic activity during the early stages, or the whole course of infection by Cestodes, is indirect proof that these parasites do normally discharge toxic substances into their hosts. Classification of the Cestoda Merozoa
End of Article: TAPEWORMS
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