Acanthocephala

Round worms (NEMATHELMINTHES) having a body somewhat elongated, a proboscis covered with hooks and which can be retracted into a sheath, on each side of which lie lemnisci. The internal structure is complex and well developed the embryo develops larval membranes. They are parasitic in minute animals, which being taken in the drink of Vertebrates, occasions them to pa the later stages of their development in the intestines of these animals. A can tho didæ. Order of Ganoids including palæozoic fo ils having small shagreen-like scales, heterocercal tail, spines in front of some of the fins, cartilaginous skull, and no operculum. They unite the Ganoids and Sharks.