The Acanthocephala are entirely parasitic, vermiform in shape, and devoid of any mouth or alimentary canal; The front end of the body (fig. 119, p) is developed into a retractile proboscis, which is covered with tranverse rows of recurved hooks, and by means of which the parasite attaches itself to the wall of the intestine of its host. The integument (c c) is highly muscular, and the proboscis is contained within a strong muscular sheath, and can be retracted by special muscular bands (m m). At the base of the proboscis is placed a single nervous ganglion, and its hinder extremity is prolonged into l, the so-called "liga-mentum suspensorium," a fibrous band, which supports the generative organs. The sexes are in different individuals. The water - vascular system is in the form of subcutaneous reticulated canals which are connected with two saccular organs or "lemnisci" (b b), placed on each side of the base of the proboscis, but the vessels of this system do not appear to communicate with the exterior.
The order Acanthocephala includes only one genus, namely, Echinorhynchus, the genus Koleops being doubtfully referred here. All the Echinorhynchi inhabit in their adult condition the intestines of fishes, birds, or mammals, and they pass through a metamorphosis. The eggs are swallowed by crustaceans or insects, and give exit to free vermiform 'embryos, armed with hooks. These burrow out of the intestine of their host and encyst themselves in its tissues, not becoming finally developed till their bearers may be eaten by some vertebrate animal. Thus, the embryos of Echinorhynchus gigas of the pig inhabit the larvae of the cockchafer; whilst those of E. angustatus, of Cyprinoid fishes, live in the interior of fresh-water Crustaceans.