The whole of the rest of the body is formed by the entoblast. Within the latter a circular split arises which separates a single layer of outermost cells from an axial strand of many cells (Fig. 98, B). The split is the future body-cavity; the axial strand forms the proboscis, its sheath, the cerebral ganglion, muscles, etc., and the ligament with the contained generative organs; the outermost layer of cells forms the muscular lining to the skin. It is interesting to note that these cells destined to become muscle-fibres are at first arranged as a single layer of cubical epithelial cells lining the body-cavity; most of them become circular muscle-fibres, but a few are pushed inwards so as to lie next the body-cavity, and these become the longitudinal fibres. CLASSIFICATION.—Until recently the Acanthocephala were supposed to include but one genus, _Echinorhynchus_, with several hundred species, but Hamann[219] has pointed out that these species {182}present differences which enabled him to divide the group into three families, each with a corresponding genus. To these I have ventured to add a fourth family, to include a remarkable species, _Arhynchus hemignathi_, described below. The characters of the first three families in the account given below are taken from Hamann's paper. [Illustration: FIG. 100.—Fully formed larva of _Echinorhynchus proteus_ from the body-cavity of _Phoxinus laevis_. (From Hamann.) Highly magnified. _a_, Proboscis; _b_, bulla; _c_, neck; _d_, trunk; _e_, _e_, lemnisci.]