NEMATOMORPHA (Gordiacea or "hairworms"), a group of elongate, thread-like, unsegmented worms, probably related to the Nematoda (q.v.), but differing from them in certain respects. The adults occur mainly in fresh water (occasionally in wet earth), and are sometimes found in tangled masses. The species vary in length from a few inches to two or three feet, but none are thicker than whipcord. They are usually of a brownish colour.
The body-wall consists of (I) a stout cuticle, with an outer layer (smooth or with shagreen-like "areoles," sometimes bearing spines or bristles of various kinds) and an inner layer of obliquely crossed fibres; (2) a single subcuticular layer of cells; and (3) a layer of longitudinal muscle-fibres. Most of the space within the body-wall is filled with parenchymatous tissue, in which the in ternal organs are embedded. There is an alimentary canal, open ing posteriorly into a cloaca together with the genital ducts. A mouth is usually absent in the adult. No circulatory or excretory organs are known. There is a single ventral nerve-cord, with an terior and posterior ganglia.
The sexes are separate, the males being usually smaller than the females and, in some genera, distinguishable by having the tail bifurcate. The tail of the female is usually undivided, but in Paragordius it has three prongs. The gonads are paired in both sexes, and are continuous with their ducts. In the gravid female
the ovaries give off thin-walled lateral pockets, which ultimately break down and discharge the ova into spaces in the parenchyma. The eggs, held together by a cement-like substance, are laid in strings or masses, usually on plants or stones under water. The first larval stage is a minute creature armed with spines and having a boring organ anteriorly, by means of which it enters a host (usually an aquatic animal). In species which attack insects such as grasshoppers, the second larval stage may apparently be reached without change of host. In others it may develop in a second host, usually a beetle, which preys upon the first. The sec ond larval stage is elongate and worm-like, and develops directly into the adult form. Pigment is developed, and the larval cuticle shed, just bef ore emergence from the host. This usually takes place on contact with water, the worm bursting its way out through a soft place in the body-wall.
The group includes the genera Gordius, Paragordius, Chordodes and Parachordodes, to which a fifth, Gordionus, has been added by G. W. Muller. It is still uncertain whether N ectonenta (a pelagic marine form provided with rows of bristles) is more closely related to this group or to the Nematoda. (H. A. B.)