In addition to the larva of Sialis lutaria, the first larval stage has also been found in the larva of Ephemera, Tanypus, Corethra, and Chironomus; the second in Carabus hortensis Fabr., Procerus (Carabus) coriaceus Linn., Calathus fuscipes Goeze, Molops elatus Fabr., several species of Pterostichus, and a number of other beetles. It is probable that its normal hosts are S. lutaria and Pt. niger, but it is clear that it often comes to rest in other insects. The view that the Gordiidae have no special hosts, but may either pass the whole of their life-history within one and the same animal, or, on the other hand, may inhabit animals belonging to very different groups, is held by Villot, who has paid great attention to the subject. He finds the first larval form encysted in the walls of the alimentary canal in fishes, such as Leuciscus phoxinus, the minnow, Cobitis barbatula, the loach, and Petromyzon planeri, the lamprey; in the larvae of Diptera, Ephemera, and beetles, in Planorbis (a water snail), in Enchytraeus (an Oligochaet); the second larval form in all kinds of insects, spiders, Crustacea, fish, frogs, birds (Otis), and in man, and these various habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes spéciaux." On the other hand, as von Linstow points out, it is contrary to our knowledge of parasites that a single species should develop equally well in the body of warm and cold-blooded Vertebrates and of Insects, and the explanation of the presence of the larvae in these various forms may either be that they belong to different species of Gordius or, more probably, that they are accidentally present, having passed into their hosts with drinking water.
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 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.
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.)