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ACANTHOCEPHALA, a group of cylindrical, parasitic worms, often included with the Nematoda and Nematomorpha in a phylum Nemathelminthes, but of very doubtful affinities. There is no mouth or alimentary canal, but a retractile probos cis, contained in a muscular sac and armed with recurved hooks for attachment to the host. The body-wall consists of (I) a cuti cle; ( 2 ) a syncytial subcuticular layer containing few nuclei and traversed by branching vessels containing fluid; (3) a layer of circular muscle fibres; and (4) a slight layer of longitudinal muscle fibres. Connected with the base of the proboscis are a pair of sac-like organs called the lemnisci. The nervous system consists of a central ganglion in the proboscis-sac, giving off nerves to the proboscis and body. Excretory organs in the form of a pair of branching nephridia have been described in certain species and these organs are connected by canals with the vagina or ejaculatory duct.

The sexes are separate, the reproductive organs in both being supported by a "ligament" which runs from the proboscis-sac to the posterior end of the body. The male has paired testes and several pairs of "cement glands," connected with the ejaculatory duct, which opens into a protrusible "bursa eopulatrix." In the female, the ova are discharged in masses from the ovary into the fluid of the body-cavity, where fertilization and segmentation take place. Connected with the vagina is a structure known as the "bell," which selects the ripe eggs and passes them into the uterus, returning immature ova to the body-cavity. The genital aperture is at the posterior end of the body.

The adult worms live in the intestine of vertebrates, the ova escaping with the faeces of the host. The larval forms are found encapsuled in the body-cavity of some animal (usually a crus tacean, insect or lower vertebrate) liable to be eaten by the final host. For example, the larvae of the large Macracanthorhynchus hirudinaceus (Echinorhynchus gigas) of the pig occur in the grubs of the cockchafer and rose-beetle. Some 35 genera of Acanthocephala are recognized, and variously arranged by differ ent authors in from four to as many as eight families.

(H. A. B.)

connected, organs and host