1911 Encyclopædia Britannica/Nematomorpha
NEMATOMORPHA. This zoological group includes Gordian worms which are found swimming in an undulatory manner or coiling round water-weeds in ponds and puddles, or knotted together in an apparently inextricable coil. They may be several inches in length and are no thicker than a piece of whip-cord. The male is distinguishable from the female by the presence of a fork at the posterior end of the body. The body is covered by a cuticle which is sculptured and the various markings are of systematic importance: it is secreted by a hypodermis which also includes nerve-cells and some gland-cells. In the adult aquatic stage the alimentary canal shows signs of degeneration, and it seems probable that in this stage Gordian worms take no food. The mouth is terminal or subterminal; there is a weak. sucking pharynx situated behind the brain, and a long intestine lying along the medio-ventral body-cavity; it ends in a cloaca which receives the vasa deferentia in the male. There is a single unsegmented nerve-cord which runs along the ventral middle line and enlarges posteriorly into a caudal ganglion and anteriorly in a ganglion, the brain, which is not supra-oesophageal. The peripheral nervous system is minutely described by T. H. Montgomery. There is a median eye on the head.
The Nematomorpha are nearly solid,—quite so at each end, and only in the middle region of the body are there any body-cavities, the space within the body being usually filled up with parenchyma. There are four closed spaces of the nature of body-cavities, two lateral and a dorso-median and a ventromedian. Into the former the ovaries project, though the lumen of the lateral body-cavity is quite shut off from the lumina of the ovaries or uteri. In the adult male the lateral body-cavities are absent. A curious duct with lateral branches termed the supra-intestinal organ lies above the intestine in the female. There are two series of ovaries extending through a large part of the body and accompanied by two uteri; the latter open by two oviducts which debouch into an atrium which also receives the intestine and a single receptaculum seminis, and is continued backward as the cloaca; this opens posteriorly. The ovaries are epithelial sacs which open into the uteri. The paired testes extend through the greater part of the body and end in two vasa deferentia which unite with the intestine to form a cloaca.
From Cambridge Natural History, vol. ii., “Worms,” &c., by permission of Macmillan & Co., Ltd.
Fig. 3.—Tarsal joint of an Ephemerid larva into which two Gordius larvae, (a, a) have penetrated. Magnified.
The eggs are laid in the spring as a rule, and after about a week they give rise to a minute, ringed larva with a protrusible boring apparatus consisting of three chitinous rods. By the aid of this the larva makes its way into the soft body of some insect larva, Ephemerids, Chironomids, or even of Molluscs, and encysts in the muscles or fat body. The insect, which may have become an imago with the Gordian larva still in it, is then eaten by a carnivorous insect or by a fish, and the contained Gordian larva becomes elongate and mature in its second host. After a year or more this larva emerges into the water and commences to reproduce.
The unexpected occurrence of these worms in pools and puddles, often in great numbers, has given rise to myths about showers of worms. They occasionally make their way into the human stomach with the drinking-water and are vomited; but this is a case of pseudo-parasitism—they are no true parasite of man.
There are a considerable number of species divided among the four genera: Gordius, Paragordius, Chordodes and Parachordodes; the last, a genus of Camerano's, is looked upon with some doubt by Montgomery. A free swimming marine form with longitudinal rows of bristles, known as Nectonema A. E. Verrill, may also come here, but at present its life-history is unknown. The Nematomorpha form an isolated group; at first sight they seem to be connected with the Nematoda, but in reality their only common feature is the tubular genitalia opening into a cloaca, and it seems at present impossible to connect them with the Annelida. Until more is known it seems wisest to look upon them as an isolated assemblage of animals with no near affinities to any of the great phyla.
Literature.—L. Camerano, “Monografía dei Gordii,” Mem. Acc. Torino, xlvii. (1897), contains literature; O. von Linstow, Arch. mikr. Anat., li. (1898); T. H. Montgomery, Bull. Mus. Harvard, xxxii. (1898); Amer. Natural., xxxiii. (1899); Zool. Jahrb. Anat., xviii. (1903) p. 387; F. Vejdovsky, Zeitschr. wiss. Zool., lvii. (1894); A. Villot, Arch. Zool. exp. ii. (1887); C. R. Ac. Sci., cviii. (1889); H. B. Ward, Bull. Mus. Harvard, xxiii. (1892). (A. E. S.)