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Invertebrate Zoology

Kingdom:  Animalia, Phylum: Acanthocephala



Phylum:  Acanthocephala -- Spiny-

     headed worms


    Class:  Archiacanthocephala

    Class:  Eoacanthocephala

    Class:  Palaeacanthocephala

Bibliography      Citations

Sample Examinations

Invertebrate Classification


CLICK on underlined file names and included illustrations to enlarge: 


          The Phylum Acanthocephala derives its name from "spiny-headed."  All species are parasites in the digestive tract of vertebrates with intermediate hosts that are generally arthropods.  The anterior end of the animal has a probosis with spiny hooks.


          A representative genus is Macracanthorhynchus with characteristics noted as follows:


          General Body Features.-- A retractable proboscis is present.  There is a genital pore at the posterior end, which in the female is very simple and in the male is an inflatable fan-like affair or bursa that is used for copulation.  The female is larger than the male.


          Body Wall.-- A heavy cuticle is present.  Underneath the cuticle and secreting it lies the epidermis.  All cell membranes are lost between the cells, a condition that is known as a syncytium.  In the syncytium nuclei are scattered through an undivided mass of cytoplasm.  Both circular and longitudinal muscles are well developed and lie under the epidermis.  The muscle cells consist of tapering hollow cylinders .



          Pseudocoelom.-- This structure differs from a true coelom in not being derived entirely from the mesoderm.  In a true coelom the body cavity is lined on all sides by mesoderm.  However, it serves the same purpose as a true coelom.  It takes up practically all of the interior of the animal.  Inside the pseudocoelom there is a fluid, which maintains turgor.


          Food & Digestion.-- There is no mouth or any sign of a digestive tract, and thus the animals are similar to the Cestoda.  Predigested food is absorbed through the body wall.


          Respiration.-- The animals respire by diffusion and it is anaerobic for considerable periods.


          Excretion.-- Flame cells, which are not scattered, accomplishes excretion.  They are gathered into two large tufts projecting out into the pseudocoelom and emptying out through the reproductive tract (= urogenital system).


          Nervous System.-- A simple nervous system is present consisting of a single ganglion, which lies on the outer wall of the proboscis sheath.  Various nerves run out to different parts of the body particularly to the muscles.  This is then much simplier than that of the Turbellaria.


          Reproduction.-- Extending the whole length of the body and dividing up the pseudocoelom in sections are suspensory ligaments.  The male has two testes attached to a suspensory ligament, and the vas eferens and vas deferens are present.  There are a series of 6-8 cement glands on the sides of the vas deferens, which lead back to the copulatory bursa that serves an adhesive funtion during copulation.



          The female has a series of ovaries attached to the suspensory ligaments.  Floating ovaries are present, which are little patches of ovary tissue that break off of the main ovary and bear the maturing ova.  The eggs when mature detach from the ovaries and they too float in the fluid of the pseudocoelom.  Eggs are fertilized in the pseudocoelom by the sperm that enter the cavity.  Eggs with developing embryos bear 2-3 heavy shell layers.  At the opening of the female genital tract is a remarkable structure, the uterine bell, which is a selective apparatus.



          Fluid of the pseudocoelom is sucked into the mouth of the apparatus.  A selection of mature eggs is made at the base and immature eggs are passed back into the pseudocoelom.  The mature eggs are laid outside following selection that is made on the basis of size and shape of the egg.


          Lemnisci.-- These are glandular structures, which occur at the anterior end of the animal.  They are part of the lacunar system that runs all through the epidermis.  They may serve as reservoirs for fluid, which moves around the lacunar system.


          Cell Constancy.-- There is a definite number of cells per each organ, thus the number of epidermal, muscular, nerve cells, etc. is constant per species.  This results in a cessation of cell division in the late embryonic period of development, except for the ovaries and testes.  All further increase in size is due to the enlargement of existing cells.  Various parts of the body consist of definite numbers of cells (except the gonads).  Along with this there is a tremendous increase in the size of the nuclei.  A syncytium frequently results.


          Life History. -- Macracanthorhynchus is a parasite of pigs.  The eggs pass out with the host's faeces.  June beeetle larvae consume the eggs and the embryo leaves the egg and bores through to the body cavity of the beetle.  The hog must devour the beetle to complete the life cycle.  Different species of parasite have different intermediate hosts of aquatic insects and crustaceans, etc.


          Importance.-- There is relatively little economic importance although infection by Macracanthorhynchus may lower the general health of pigs if present in large numbers.




Please see following plates for Example Structures of the Acanthocephala:


Plate 29 = Phylum: Acanthocephala: Macracanthorhynchus sp. male

Plate 30 = Phylum: Acanthocephala: Macracanthorhynchus sp. -- Cross-sections

Plate 31 = Phylum: Acanthocephala, Macracanthorhynchus sp. -- Dissected Female specimen






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