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ACANTHOCEPHALA
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ACANTHOCEPHALA , a compact See also:group of cylindrical, parasitic See also:worms, with no near See also:allies in the See also:animal See also:kingdom. Its members are quite devoid of any mouth or alimentary See also:canal, but have a well-See also:developed See also:body cavity into which the eggs are dehisced and which communicates with the exterior by From See also:Cambridge Natural See also:History, vol. ii., "Worms, &c.," by permission of See also:Macmillan & Co., Ltd. A, Five specimens of Echinorhynchus acus, Rud., attached to a piece of intestinal See also:wall, B, The See also:proboscis of one still more highly magnified. means of an oviduct. The See also:size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which See also:measures from ro to 65 See also:ems. The adults live in See also:great See also:numbers in the alimentary canal of some vertebrate, usually See also:fish, the larvae are as a See also:rule encysted in the body cavity of some invertebrate, most often an See also:insect or crustacean, more rarely a small fish. The body is divisible into a proboscis and a See also:trunk with sometimes an intervening See also:neck region. The proboscis bears rings of recurved hooks arranged in See also:horizontal rows, and it is by means of these hooks that the animal attaches . itself to the tissues of its See also:host. The hooks may be of two or three shapes. Like the body, the proboscis is hollow, and its cavity' is separated from the body cavity by a septum or proboscis sheath. Traversing the cavity of the proboscis are muscle-strands inserted into the tip of the proboscis at one end and into the septum at the other. Their contraction causes the proboscis to be izvaginated into its cavity (fig. 2). But the whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall (fig. 3). 109 The skin is See also:peculiar. Externally is a thin cuticle; this covers the epidermis, which consists of a syncytium with no See also:cell limits. The syncytium is traversed by a See also:series of branching tubules containing fluid and is controlled by a few wandering, amoeboid nuclei (fig. 2). Inside the syncytium is a not very See also:regular layer of circular muscle See also:fibres, and within this again some rather scattered See also:longitudinal fibres; there is no endothelium. In their See also:minute structure the See also:muscular fibres resemble those of Nematodes. Except for the See also:absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the latter are shut off from those of the body. The canals of the proboscis open ultimately into a circular See also:vessel which runs See also:round its See also:base. From the circular canal two See also:sac-like diverticula called the From Cambridge Natural History, vol. u., "Worms, &c.," by permission of Macmillan & Co., Ltd. Echinorhynchus haeruca, Rud. (from See also:Hamann). a, The proboscis not fully ex- It, One of the spaces in the sub- b, Proboscis-sheath. [panded. cuticular See also:tissue. c, Retractor muscles of the See also:pro- i, Longitudinal muscular layer. d, Cerebral ganglion. [boscis. j, Circular muscular layer. e, Retinaculum enclosing a k, See also:Line of See also:division between the See also:nerve. sub-cuticular tissue of the f, One of the retractors of the trunk and that of the pro- g, A lemniscus. [sheath. boscis with the lemnisci. " lemnisci " depend into the cavity of the body (fig. 2). Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a scanty muscular coat. They seem to See also:act as reservoirs into which the fluid of the tense, extended proboscis can withdraw when it is retracted, and from which the fluid can be driven out when it is wished to expand the proboscis. There are no alimentary canal or specialized See also:organs for circulation or for respiration. See also:Food is imbibed through the skin from the See also:digestive juices of the host in which the Acanthocephala live. J. Kaiser has described as kidneys two organs something like minute shrubs situated dorsally to the generative ducts into which they open. At the end of each twig is a membrane pierced by pores, and a number of See also:cilia depend into the lumen of the See also:tube; these cilia maintain a See also:constant See also:motion. , The central ganglion of the See also:nervous See also:system lies in the proboscis-sheath or -septum. It supplies the proboscis with nerves and gives off behind two stout trunks which See also:supply the body (fig. 2). Each of these trunks is surrounded by muscles, and the complex retains the old name of " retinaculum." In the male at least there is also a genital ganglion. Some scattered papillae may possibly be sense-organs. The Acanthocephala are dioecious. 
There is a " stay " called the "See also:ligament " which runs from the hinder end of the proboscis-sheath to the posterior end of the body. In this the two testes See also:lie (fig. 3). Each opens in a vas deferens which bears three diverticula or vesiculae seminales, and three pairs of See also:cement glands also are found which pour their secretions through a duct into the See also:vasa deferentia. The latter unite and end in a penis which opens posteriorly. The ovaries arise like the testes as rounded bodies in the ligament. From these masses of ova dehisce into the body cavity and See also:float in its fluid. Here the eggs are fertilized and here they segment so that the See also:young embryos are formed within their See also:mother's body. The embryos See also:escape into the uterus through the " See also:bell," a See also:funnel-like opening continuous with the uterus. Just at the junction of the " bell " and the uterus there is a second small opening situated dorsally. m The " bell " swallows the matured embryos and passes them on into the uterus, and thus out of the body via the oviduct, which opens at one end into the uterus and at the other on to the exterior at the posterior end of the body. But should the " bell " See also:swallow any of the ova, or even one of the younger embryos, these are passed back into the body cavity through the second and dorsal opening. The embryo thus passes from the body of the See also:female into the alimentary canal of the host and leaves this with the faeces. It is then, if lucky, eaten by some crustacean, or insect, more rarely by a fish. In the See also:stomach it casts its membranes and becomes See also:mobile, bores through the stomach walls and encysts usually in the body-cavity of its first and invertebrate host. By this See also:time the embryo has all the organs of the adult perfected See also:save only the reproductive; these develop only when the first host is swallowed by the second or final host, in which See also:case the See also:parasite attaches itself to the wall of the alimentary canal and becomes adult. A curious feature shared by both larva and adult is the large size of many of the cells, e.g. the nerve cells and the bell. O. Hamann has divided the group into three families, to which a See also:fourth must he added. (i.) Fam. Echinorhynchidae. This is by far the largest See also:family and contains the commonest See also:species; the larva of Echinorhynchus See also:proteus lives in Gammarus pulex and in small fish, the adult is See also:common in many fresh-See also:water fish: E. polymorphus, larval host the See also:crayfish, adult host the See also:duck: E. angustatus occurs as a larva in Asellus aquaticus, as an adult in the See also:perch, See also:pike and See also:barbel: E. moniliformis has for its larval host the larvae of the See also:beetle Blaps mucronata, for its final host certain mice, if introduced into See also:man it lives well: E. acus is common in See also:whiting: E. porrigeus in the fin-See also:whale, and E. strumosus in the See also:seal. A species named E. hominis has been described from a boy. (ii.) Fam. Gigantorhynchidae. A small family of large forms with a ringed and flattened body. Gigantorhynchus gigas lives normally in the See also:pig, but is not uncommon in man in See also:South See also:Russia, its larval host is the See also:grub of Melolontha vulgaris, Cetonis auratus, and in See also:America probably of Lachnosterna arcuata: G. echinodiscus lives in the See also:intestine of See also:ant-eaters: G. spira in that of the
From Cambridge Natural History, vol. ii., "Worms &c.," by permission of Macmillan & Co., Ltd.
A, The larva of Echinorhynchus proteus from the body cavity of Phoxinus laevis, with the proboscis retracted and the whole still enclosed in a See also:capsule.
B, A See also:section through the same; a, the invaginated proboscis; b, proboscis sheath; c, beginning of the neck; d, lemniscus. Highly magnified (both from Hamann).
See also: With no proboscis. This family contains the single species Apororhynchus hemignathi, found near the anus of Hemignathus procerus, a See also:Sandwich See also:Island See also:bird. (A. E. Additional information and CommentsThere are no comments yet for this article.
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