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A distinct phylum of helminths, the adults of which are parasitic in the alimentary canal of vertebrates. They are commonly known as the spiny-headed worms. The phylum comprises the orders Archiacanthocephala, Palaeacanthocephala, and Eocanthocephala. Over 500 species have been described from all classes of vertebrates, although more species occur in fish than in birds and mammals and only a relatively few species are found in amphibians and reptiles. The geographical distribution of acanthocephalans is worldwide, but genera and species do not have a uniform distribution because some species are confined to limited geographic areas. Host specificity is well established in some species, whereas others exhibit a wide range of host tolerance. The same species never occurs normally, as an adult, in coldblooded and warm-blooded definitive hosts. More species occur in fish than any other vertebrate; however, Acanthocephala have not been reported from elasmobranch fish. The fact that larval development occurs in arthropods gives support to the postulation that the ancestors of Acanthocephala were parasites of primitive arthropods during or before the Cambrian Period and became parasites of vertebrates as this group arose and utilized arthropods for food.
Adults of various species show great diversity in size, ranging in length from 0.04 in. (1 mm) in some species found in fish to over 16 in. (400 mm) in some mammalian species.
The body of both males and females has three subdivisions: the proboscis armed with hooks, spines, or both; an unspined neck; and the posterior trunk. The proboscis is the primary organ for attachment to the intestinal wall of the host. In most species the proboscis is capable of introversion into a saclike structure, the proboscis receptacle. The proboscis receptacle and neck can be retracted into the body cavity but without inversion. The body cavity, or pseudocode, contains all the internal organs, the most conspicuous of which are the reproductive organs. There is no vestige of a digestive system in any stage of the life cycle. The reproductive organs of the male consist of a pair of testes and specialized cells, the cement glands. The products of the testes and cement glands are discharged through a penis. Female Acanthocephala are unique in that the ovary exists as a distinct organ only in the very early stages of development and later breaks up to form free-floating egg balls. The eggs are fertilized as they are released from the egg balls and are retained within the ligament sacs until embry-onation is complete. The nervous system is composed of a chief ganglion or brain located within the proboscis receptacle. Two nerve trunks pass through the wall of the proboscis receptacle to innervate the trunk wall. Modified protonephridial organs are found closely adherent to the reproductive system, but in most species specialized excretory organs are completely lacking.
a class of parasitic invertebrates of the phylum Nemathelminthes that inhabit the intestine of vertebrates. The elongate body is 1–65 cm long. The proboscis is eversible into a sheath called a rhynchocoel; it has hooks for attachment to the intestine of the host. The invertebrates do not have an intestine, and feeding occurs osmotically over the entire body surface. The excretory organs are protonephridia. The nervous system consists of a brain and nerves departing from the brain. The sexes are separate. Paired ovaries break down into numerous “egg clumps,” which float in the wide primary gut and consist of developing eggs. The gonoduct of the female starts at the uterine infundibulum, which opens into the coelom and traps the mature eggs, which are then released. Development involves metamorphosis and a succession of hosts. The intermediate hosts are crustaceans and insects. Severe infestation may kill birds, fish, and mammals. There have been cases of human infestation.
REFERENCESZhizn’zhivotnykh, vol. 1. Moscow, 1968.
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1975.
A. V. IVANOV