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Encyclopedia Britannica - Main :: A10-ADA |
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ACANTHOCEPHALA , a compact group of cylindrical, parasitic worms, with no near allies in the animal kingdom. Its members are quite devoid of any mouth or alimentary canal, but have a well-developed body
From Cambridge Natural History, vol. ii., "Worms, &c.," by permission of Macmillan & Co., Ltd.A, Five specimens of Echinorhynchus acus, Rud., attached to a piece of intestinal wall
B, The proboscis of one still more highly magnified. means of an oviduct. The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from ro to 65 ems. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body
insect
horizontal
wall
109 The skin is peculiar. Externally is a thin cuticle; this covers the epidermis, which consists of a syncytium with no cell limits. The syncytium is traversed by a series of branching tubules containing fluid and is controlled by a few wandering, amoeboid nuclei (fig. 2). Inside the syncytium is a not very regular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is no endothelium. In their minute structure the muscular fibres resemble those of Nematodes. Except for the 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 vessel which runs round its base. From the circular canal two sac-like diverticula called theFrom Cambridge Natural History, vol. u., "Worms, &c.," by permission of Macmillan & Co., Ltd.Echinorhynchus haeruca, Rud. (from Hamann). a, The proboscis not fully ex- It, One of the spaces in the sub- b, Proboscis-sheath. [panded. cuticular tissue .c, Retractor muscles of the pro- i, Longitudinal muscular layer. d, Cerebral ganglion. [boscis. j, Circular muscular layer. e, Retinaculum enclosing a k, Line of division between the nerve. sub-cuticular tissue of thef, 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 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 organs for circulation or for respiration. Food is imbibed through the skin from the 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 cilia depend into the lumen of the tube; these cilia maintain a constant motion. , The central ganglion of the nervous system lies in the proboscis-sheath or -septum. It supplies the proboscis with nerves and gives off behind two stout trunks which 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 atleast there is also a genital ganglion. Some scattered papillae may possibly be sense-organs. The Acanthocephala are dioecious. There is a " stay " called the "ligament " which runs from the hinder end of the proboscis-sheath to the posterior end of the body. In this the two testes lie (fig. 3). Each opens in a vas deferens which bears three diverticula or vesiculae seminales, and three pairs of cement glands also are found which pour their secretions through a duct into the 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 float in its fluid. Here the eggs are fertilized and here they segment so that the young embryos are formed within their mother's body. The embryos escape into the uterus through the " bell," a 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 " 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 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
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 fourth must he added. (i.) Fam. Echinorhynchidae. This is by far the largest family and contains the commonest species; the larva of Echinorhynchus proteus lives in Gammarus pulex and in small fish, the adult is common in many fresh-water fish: E. polymorphus, larval host the crayfish, adult host the duck: E. angustatus occurs as a larva in Asellus aquaticus, as an adult in the perch
(ii.) Fam. Gigantorhynchidae. A small family of large forms with a ringed and flattened body. Gigantorhynchus gigas lives normally in the pig, but is not uncommon in man in South Russia, its larval host is the grub of Melolontha vulgaris, Cetonis auratus, and in America probably of Lachnosterna arcuata: G. echinodiscus lives in the intestine of 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 capsule. B, A section through the same; a, the invaginated proboscis; b, proboscis sheath; c, beginning of the neck; d, lemniscus. Highly magnified (both from Hamann). king vulture, Sarcorhampus papa, and G. taeniodes in Dicholopus cristatus, a cariama. (iii.) Fam. Neorhynchidae. Sexually mature whilst still in the larval stage. Neorhynchus clavaeceps in Cyprinus carpio has its larval form in the larva of Sialis lutaria and in the leech Nephelis octocula: N. agilis is found in Mugil auratus and M. cephalus. (iv.) Apororhynchidae. With no proboscis. This family contains the single species Apororhynchus hemignathi, found near the anus of Hemignathus procerus, a Sandwich Island bird. (A. E. S.) End of Article: ACANTHOCEPHALA If you wish, you can link directly to this article.
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