CHAPTER III CESTODA INTRODUCTION-NATURE OF CESTODES-OCCURRENCE OF CESTODES -THE TAPE-WORMS OF MAN AND DOMESTIC ANIMALS-TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND DOMESTIC ANIMALS-STRUCTURE AND DEVELOPMENT OF CESTODES-TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC ANIMALS-CLASSIFICATION. THE Cestodes or Tape - worms are exclusively endoparasitic Platyhelminthes living, in the adult condition, in the alimentary canal of Vertebrates, with the exception of Archigetes (Fig. 37), which may become mature in the body-cavity of Tubifex. In relation with this wholly parasitic existence, the Cestodes exhibit certain characteristic modifications in structure and mode of development, such as the formation, by the segmentation of the "neck,” of a (usually) long chain of " proglottides " or joints, which form the "body" of the Cestode; and the entire absence of an alimentary tract, both in the larva and adult. As an adaptation to the fixed mode of life, the anterior end (head, scolex) is modified to form an adhering organ. Various adaptive forms of larvae are known. These live in the internal organs of one or more intermediate hosts, and are transferred to the final host passively during a meal. Lastly, there is the curious metamorphosis by which the adult is formed from a portion (scolex) of the larva.1 Taenia solium, from man (Fig. 39, B), or a Calliobothrium (Fig. 36), from an Elasmobranch fish, is fixed to the mucous lining of the intestine of its host by means of a radially-constructed 1 Cf. p. 5. hk te -ps hd apparatus of four suckers and a circlet of hooks (Fig. 39), which are borne by the "head" or "scolex," being that part of the worm which is directly derived from part of the larva, and which contains the central, commissural portion of the the nervous system. Firm adhesion to the host's intestine is necessary, in order to avoid the loosening action of the peristaltic movements of the intestine as the food passes along. The heads of different Cestodes exhibit a marvellous variety of suckers and hooks, from a mere muscular depression in Schistocephalus, to the compound proboscides of Tetrarhynchus 1 which is found in Elasmobranchs. The jointed body, often of enormous length (up to 20 yards in Bothriocephalus latus), is usually separated from the head by a slender neck, from which the proglottides are segmented off from behind forwards, and become more and more individualised as they recede farther away from the neck by the intercalation of younger joints. Thus in Fig. 36 the mature, distal proglottis has passed through all the stages represented by the other segments. The longitudinal muscles, the nerves, and excretory vessels which supply the proglottides are continuous throughout ut ps pe- vag FIG. 36.-Calliobothrium affine Dies., from the intestine of Torpedo. x 43. hd, Head; hk, hooks; hl, lobes of the head; ov, ovary; pe, penis; ps, penis-sheath; te, testes; ut, uterus; vag, vagina; yg, yolk-glands. (After Pintner.2) 1 For figures of various scolices see van Beneden, Mémoire sur les vers Intestinaur, 1861; Braun in Bronn's Thierreich, Cestoda (in progress), Bd. iv. Pl. xxxviii. -xlv. Arbeit. Inst. Wien, iii. 1881, p. 163; see also ibid. ix. 1890, p. 57. te -go SC and with those of the head. Each joint contains at first male genitalia comparable with those of a Trematode; then the female organs develop, and finally self-fertilisation follows. The Cestodes feed through their skin, probably by the aid of fine protoplasmic processes, which penetrate the tough investing membrane and absorb the already digested food which bathes them. When a proglottis of Calliobothrium is approaching maturity it separates from the parent, the broken ends of muscles, nerves, and excretory vessels speedily heal, and it is now capable of continued growth and of fairly active movement if it remains in the intestine of the host. According to van Beneden, it may even attain a size equal to, or exceeding, that of the whole parent or 'strobila." 1 These considerations led Leuckart, von Siebold, P. J. van Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is really a colony composed of two generations-the head and neck derived from the larva, and the proglottides produced by the segmentation of the neck. This view of the colonial nature of jointed Cestodes was generally FIG. 37. Archigetes adopted from 1851 to 1880. During the sieboldii (appendiculatus), from the last fifteen years, however, the varied intercoelom of Tubifex pretations of the facts of the ontogeny of this app, Persistent larval group have led some authors to adopt the appendage; go, genimonozootic view (that a Cestode is one indital pore; hk, persistent larval hooks; vidual), others are still of the older opinion, ov, Ovary; sc, and Hatschek (Lehrbuch, p. 349) and Lang yg yolk-glands. take up intermediate positions. Lang considers (After Leuckart.) that the formation of the joints of a tapeworm from a small fixed "scolex," is not only largely comparable with the strobilation of a scyphistoma and the consequent formation of a pile of medusae, as in the life-history of Aurelia, but rivulorum. x 40. sucker; te, testes; 1 The mature proglottis of Calliobothrium eschrichti is 8-9 mm. long, whereas the strobila only measures 4-5 mm. in length. Species of Phylliobothrium, Anthobothrium, and Tetrarhynchus show a similar but not an equal contrast between the size of the parent and proglottis (P. J. van Beneden, "Les Vers Cestoides," Nouv. Mém. de l'Acad. Roy. d. Belgique, tom. xxv. 1850). 2 The difficult question of the nature of the Cestode body and Cestode larvae is adequately discussed by Braun, loc. cit. p. 1167. A that both processes have arisen from the seas. Occurrence of Cestodes.-The distribution of Cestodes and their larvae is analogous to that of the digenetic Trematodes, although the absence of an alimentary canal limits the habitat of the mature worms to certain sites, such as the blood-vessels, the lymphatic and coelomic spaces, and the digestive system, where their body may be bathed by a nutritive. fluid. Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes, and particularly of Elasmobranchs, are distinguished by certain structural and developmental features; those of birds by B fc... -nl tpEl exo- FIG. 38.-Scolex polymorphus Rud. (larva of Calliobothrium filicolle Zschokke), from the muscles of Apogon, a Mediterranean fish; also found in many Invertebrates (e.g. Sepia). A, Inverted scolex, with calcareous bodies; B, everted older larva. br, Brain; exo, terminal excretory aperture; fc.flame-cells; for.sec, secondary excretory pores; hk, hooks of the adult Cestode; inrag, pit at the bottom of which the head is developed; msc, anterior sucker; nl, lateral nerve; sc, suckers; tl, tp, lateral and main excretory vessels. (After Monticelli.) The others; those of mammals, by a third set of characters. young stages of the Cestodes of Sharks and Rays occur encysted in the body-cavity, or in the pyloric appendages, of Teleosteans, which probably swallow them along with those invertebrate animals upon which they prey. The larvae of the Cestodes of carnivorous mammals or piscivorous birds, live respectively in herbivores and fishes, but how the latter are infected we know in very few instances. Cestode larvae are known to occur in many Invertebrates, and occasionally are taken free swimming in the sea, presumably crossing from one host to the next. phores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs especially Cephalopods, Earthworms, and other Annelids, are the intermediate hosts of these larvae (see Fig. 38), the fate of which, however, has been determined in but few cases. Cteno Occurrence of Cestodes in Man.'-Tape-worms, either in the adult or larval stages (bladder-worms), have, from ancient times, been known to occur in man, and in the animals that serve him as food. Until comparatively recent times, however, the true nature of these parasites, and particularly of "hydatids" (cystic larvae), was unrecognised. Up to the seventeenth century the larvae were regarded as abscesses or diseased growths of the affected organs, and it was only at the close of that century that their animal nature was even suggested. Even at the beginning of the nineteenth century, three modes of origin of Cestodes-by" generatio aequivoca" from the tissues of the body, or by the union of previously distinct proglottides, or again by metamorphosis of free-living worms drunk with water by cattle or birds (as Linnaeus suggested)—were still variously held, at a time when Malpighi, Pallas, and Goeze had recognised the true connexion between the cystic and segmented states of Taenia crassicollis (the cat tape-worm), and when Goeze had seen the eggs of Taeniae, and Abildgaard had even conducted the first helminthological experiments (conversion of the larval Schistocephalus, Fig. 40, into the adult form). Generally speaking, "a tape-worm" in Western Europe will prove to be Taenia saginata Goeze (the beef tape-worm, Fig. 39, A), 1 Leuckart, Die Parasiten d. Menschen [English trans. by W. E. Hoyle]; Blanchard, Traité de Zoologie médicale, 1893. 2 For a full account of the history of this subject see Leuckart, Parasiten d. Menschen, p. 28; Braun, loc. cit. Bd. iv. p. 929 et seq.; Huxley, Collected Essays, vol. viii. p. 229. |