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Acanthocephala AccessScience from McGraw-Hill Education Page 1 of 10 www.accessscience.com Acanthocephala Contributed by: Donald V. Moore Publication year: 2014 A distinct phylum (or class, according to some classifications) 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 Eoacanthocephala. 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 cold-blooded 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. See also: ARCHIACANTHOCEPHALA ; EOACANTHOCEPHALA ; PALAEACANTHOCEPHALA . Morphology 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 ( Fig. 1 ). Most of the larger species are from mammals; however, some mammalian forms are relatively small ( Corynosoma ) even though they parasitize seals and whales. When observed undisturbed in the intestine, the elongate body is distinctly flattened, but on removal to water or saline and after preservation the body becomes cylindrical. Living worms are translucent or milky white, although they may take on a distinctive color from the intestinal contents. The body of both sexes has three divisions: the proboscis armed with hooks, spines, or both; an unspined neck; and the posterior trunk. Proboscis The proboscis is the primary organ for attachment of Acanthocephala to the intestinal wall of the host. In some species, the proboscis is parallel with the main axis of the body, but frequently it is inclined ventrally. The proboscis may be globular, or elongate and cylindrical, in shape and is invariably armed with sharp pointed hooks, spines, or both (Fig. 1). These structures vary in shape and number and are usually arranged radially or in spiral rows. Electron microscopy studies have revealed that proboscis hooks consist of a central core of AccessScience from McGraw-Hill Education Page 2 of 10 www.accessscience.com ImageFig. 1 Variousof 1 acanthocephalan examples, adult forms drawn to same scale. ( a ) Moniliformis dubius , 4–12 in. (10–30 cm) long. ( b ) Proboscis of same. ( c ) Macracanthorhynchus hirudinaceus , 10–24 in. (25–60 cm) long. ( d ) Proboscis of same. ( e ) Oncicola canis , 0.5–0.8 in. (1–2 cm) long. ( f ) Proboscis of same. ( After A. C. Chandler and C. P. Read , Introduction to Parasitology , 10th ed ., Wiley , 1961 ) ImageFig. 2 Diagrams of 2 showing mechanics of proboscis attachment of Neoechinorhynchus emydis in intestinal wall of a turtle. ( a ) Proboscis fully introverted. ( b ) Most basal hooks in contact with host tissue. ( c–e ) Progr essive stages in extroversion of the proboscis. ( After H. J. Van Cleave, Experimental Parasitology, vol. 1, 1952 ) cytoplasm covered by hard nonliving material. In species with an elongate proboscis, the hooks seem to be in longitudinal rows with a quincuncial arrangement. In most species, the proboscis is capable of introversion into a saclike structure, the proboscis receptacle. When in this position, the tips of the hooks are all directed anteriorly and are on the inside of the introverted proboscis. As the proboscis is everted and extended, these hooks engage the mucosa of the host’s intestine, with the tips turning posteriorly as they reach their functional position on the exterior of the proboscis. The recurved points of the hooks anchor the worm firmly to the intestinal wall ( Fig. 2 ). The proboscis receptacle and neck can be retracted into the body cavity, but without inversion. Within the anterior end of the trunk are found the proboscis receptacle; musculature for retracting the proboscis, receptacle, and neck; and the lemnisci ( Fig. 3 ). The proboscis receptacle is attached to the inner surface of the AccessScience from McGraw-Hill Education Page 3 of 10 www.accessscience.com ImageFig. 3 Anteriorof 3 end of Moniliformis dubius. proboscis. Muscles known as inverters of the proboscis are attached to the anterior tip of this structure and pass through the proboscis and receptacle, emerging from the posterior of the receptacle, and continue some distance posteriorly, where they attach to the trunk wall. Usually one inverter is attached dorsally and one ventrally. Contraction of the inverter fibers introverts the proboscis into the receptacle, while further contraction of these fibers pulls the receptacle deep into the body cavity. Body wall Electron microscopy studies show that the body wall is covered by a thin mucopolysaccharide known as the epicuticle secreted onto the surface of the worm. Beneath the epicuticle is the thin cuticle with an outer membrane and a homogeneous layer perforated by numerous pores of canals, which pass through the striped layer beneath. Many of the canals are filled with an electron-dense material. Below the striped layer is the felt layer composed of many fibrous strands extending in various directions. Mitochondria and numerous vesicles are present in the felt layer. Some vesicles seem to be connected to the canals of the striped layer. The radial layer beneath the felt layer contains fewer fibrous strands, large thin-walled lacunar channels with mitochondria arranged around the channels. Lipids and glycogen are present in the radial layer. Circular and longitudinal muscle layers are found beneath the radial layer separated by plasma membranes. Some investigators consider all body-wall layers beneath the stripped layer as hypodermis. Giant nuclei The wall of the trunk has an external cuticula beneath which lies the thick syncytial hypodermis, or subcuticula. The hypodermis contains a relatively small number of giant nuclei, which may be round, oval, ameboid, or elongate with a number of lateral branches ( Fig. 4 ). Lacunae Within the hypodermis is a series of intercommunicating spaces, the lacunar system. Usually the lacunar system is composed of two longitudinal vessels either dorsal and ventral or ventral and lateral. In some species, only the AccessScience from McGraw-Hill Education Page 4 of 10 www.accessscience.com ImageFig. 4 Crossof 4 sections showing body plans of acanthocephalans. ( a ) Palaeacanthocephalan. ( b ) Archiacanthocephalan. ( After L. H. Hyman, The Invertebrates, vol. 3, McGraw-Hill, 1951 ) ImageFig. 5 Lacunarof 5 system of Moniliformis , dorsal view, showing regular circular branches. ( After L. H. Hyman, The Invertebrates, vol. 3, McGraw-Hill, 1951 ) dorsal vessel is present. The longitudinal vessels are connected by a series of smaller vessels, the lacunae, which ramify throughout the body. In species exhibiting pseudosegmentation ( Moniliformis ), the regularly spaced lateral lacunae and the body musculature divide the wall into transverse folds ( Fig. 5 ). These folds have no effect on the arrangement of internal organs. Pseudocoele The body cavity, or pseudocoele, contains all the internal organs, the most conspicuous of which are the reproductive organs enclosed in axial connective-tissue ligament sacs. These ligament sacs are hollow tubes that extend most of the length of the cavity of the trunk. They are single in both males and females of the Palaeacanthocephala, but are divided into dorsal and ventral sacs that communicate anteriorly in females of the other orders. There is no vestige of a digestive system in any stage of the life cycle. AccessScience from McGraw-Hill Education Page 5 of 10 www.accessscience.com Reproductive system The reproductive organs of the male consist of a pair of testes and specialized cells, the cement glands. In most species, there is a saclike structure behind the cement glands, Saefftigen’s pouch, through which run the sperm ducts and the ducts from the cement glands ( Fig. 6 a ). The products of the testes and cement glands are discharged through a penis, which is surrounded by a posteriorly located bell-shaped copulatory bursa, which is usually introverted into the posterior extremity of the trunk. At copulation, the bursa is extended and applied to the posterior extremity of the female, where it is held firmly in place by the secretions of the cement glands. This material hardens, forming a copulatory cap on the female, which is an internal cast of the bursa of the male, and remains attached to the posterior extremity of the female for some time following copulation. 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 embryonation is complete. The genital orifice is posterior. A vagina provided with a strong sphincter extends anteriorly from the orifice and a saccular uterus is anterior to the vagina. The anterior end of the uterus is surrounded by a series of guard cells, the selective apparatus. From this extends a funnel-like structure, the uterine bell, the broad anterior end of which opens into the body cavity or one of the ligament sacs holding the developing eggs (Fig. 6 b ). During embryonation in the body cavity, the eggs acquire a series of membranes and a hard outer shell.
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