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Acanthocephala are thorny headed parasitic worms that are found in many species of fishes, amphibians, birds, and mammals. About 500 modern species are known.
There are several morphological characteristics that distinguish acanthocephalans from other parasitic worms. Most notable among these is the presence of an anterior, protrudible proboscis that is usually covered with spiny hooks (hence the common name). Acanthocephalans have a pseudocoelom, and are probably the sister group of the Rotifers.
Acanthocephala is 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 cavity into which the eggs are dehisced and which communicates with the exterior by means of an oviduct. The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrates, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish. The body is divisible into a proboscis and a trunk with sometimes an intervening neck region. The proboscis bears rings of recurved hooks arranged in horizontal rows, and it is by means of these hooks that the animal attaches itself to the tissues of its 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 invaginated into its cavity. But the whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall.
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. 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 the "lemnisci" depend into the cavity of the body. 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. 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 "ligament" which runs from the hinder end of the proboscis sheath to the posterior end of the body. In this the two testes lie. 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. 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, more rarely by a fish. In the stomach it casts its membranes and becomes mobile, bores through the stomach walls and encysts usually in the cavity of its first and invertebrate host. By this time the embryo has all the organs of the adult perfected save only the reproductive; these develop only when the first host is swallowed by the second or final host, in which case the parasite attaches itself to the wall of the alimentary canal and
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 be 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. angustotus occurs as a larva in Asellus aquaticus, as an adult in the perch, pike and barbel: E. moniliformis has for its larval host the larvae of the beetle Blaps mucronata, for its final host certain mice, if introduced into man it lives well: E. acus is common in whiting: E. porrigeus in the fin-whale, and E. strumosus in the 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 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 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 lularia and in the leech Nephelis octcculii: tact K. 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 Hemignathiis procerus, a Sandwich Island bird.
(from an old encyclopedia)