[Illustration: FIG. 91.—Tarsal joint of an Ephemerid larva into which two _Gordius_ larvae (_a_, _a_) have penetrated. Magnified. (From G. Meissner.)] The number of species of _Gordius_ is large; over 100 are enumerated in the _Compendium der Helminthologie_,[214] the great majority of which inhabit insects. The life-history of _Nectonema_ is practically unknown; the adults have been found swimming near the surface of the sea at two places only: Newport, R.I., and Wood's Holl, Mass., on the south coast of New England. It has been fished close to the shore, from the end of June to the beginning of October, when the tide is going out at evening and there is no moon. This seems to indicate that it avoids the light. When first caught the worms move actively about, coiling themselves into figures of eight and then uncoiling; at the same time there is a rhythmical movement caused by waves of muscular contraction passing down each side of the body alternately; by this kind of motion they make rapid and definite progress through the water. It seems probable that the adult _Nectonema_ is preceded by one or more larval stages, and what appears to be a young form has {174}been obtained from the thoracic cavity of a prawn, _Palaemonetes_,[215] which has thus some claim to be regarded as the host of this species, but nothing is known about its early life-history. SUB-ORDER III. ACANTHOCEPHALA. The Acanthocephala, which form the third class of the Nemathelminthes, consists of but few genera; there are, however, numerous species of very different size, varying from 10 to 65 cm. long in the female _Gigantorhynchus_ (_Echinorhynchus_) _gigas_, to quite minute forms a few millimetres in length. The adult stage occurs in the alimentary canal of Vertebrates, as a rule in those which live in, or frequent water; the larvae are found in the bodies of certain Invertebrates, very frequently small Crustacea. [Illustration: FIG. 92.—Two specimens of _Echinorhynchus proteus_ Westrumb., with their anterior ends embedded in the wall of the intestine of a Pike. Magnified with a lens. (From Hamann.)]