Unisegmental Vermes, with a narrow elongated body of more or less cylindrical shape and tapering to each end. There is a well-developed cuticida, derived from a subcuticida or hypodermis; a peripharyngeal nerve-ring from which six nerves pass forzvards and six backwards; a digestive tract divided into three sections, oesophagics, mesenteron, and rectum, and two excretory tubes opening by a common anterior and ventral pore. The sexes are separate; the male itsually possesses special copidatory organs in the shape of spicules and of a bursa. Ciliated epithelium is universally absent. Free-living or parasitic. An Alternation of Generations occurs among some of the parasitic genera.
The body is of a very uniform shape. The anterior portion is filamentous, the posterior thicker in the genus Trichocephalus; so too in the female Trichosoma, though to a less degree. The head is dilated in Ichthyonema globiceps. Cuticular spines may be present, and folds as well, extending down each side of the body, to a greater distance in the female than in the male. The bursal membranes of the latter (infra, p. 681-2) must be carefully distinguished from these folds. The male is usually smaller than the female, and its tail is differently conformed, either strongly curved at the apex, or provided with a bursa, and invariably beset with a number of sensory papillae. It is sometimes forked, e. g. in Pseudalius (^Prosthe-cosacter) from the lungs of the Porpoise. The free-living genera are all small. Among the parasitic the female Dracunculus (Filaria) medinensis may attain a length of six feet and the female Eustrongylus gigas, which inhabits the kidneys of various mammals, that of three feet or more.
The greatest number of parasitic Nematoda infest in the sexual state the various classes of Vertebrata.
There is a chitinoid cuticle, which is thin and delicate in the smaller Nematoda, but in the larger becomes much thickened. It then consists of an outer thin and more resistent layer, prolonged inwards to a greater or less distance at the external apertures of the various organs, and an inner thick layer, said to be subdivisible into secondary laminae. The outer layer is frequently marked by transverse lines which give the body a ringed appearance more or less pronounced. The inner layer is fibrous, and the fibres of the different laminae decussate with one another. Cuticular processes occur in the shape of circumoral fringes, of papillae or spines, and lateral membrane-like expansions, variable in breadth and extent. A subcuticula or hypodermis lies beneath the cuticle as a more or less granular layer with sparse nuclei, most numerous posteriorly; but in larger species, such as Ascaris megalocephala from the Horse, A. lumbri-coides from Man, the subcuticula and its longitudinal thickenings contain numerous fibres which take a more or less circular direction, and have been supposed to be muscular in nature, and continuous, in part at least, with the extremities of the muscle-cells. These fibres are especially well developed in the rectum.
The subcuticula is thickened internally, as a rule in four longitudinal lines, - a median dorsal, a median ventral, and two lateral, one on either side. The latter are especially prominent, and are generally known as the 'lateral areae.' The dorsal and ventral lines are probably always present, the lateral except in Trichocephahis, and perhaps some others. Cutaneous glands are absent1. The muscles of the body-wall consist of a single layer of longitudinally disposed muscle-cells. In these cells the contractile substance is fibrillate, and forms a superficial stratum on the basal part of the cell, which is applied to the subcuticula in the Meromyarii of Schneider (e. g. Oxyuris, Strongylus), or on the base, the sides, and ends of the cells, as in other Nematoda. The body of the muscle-cells consists of a feebly granular protoplasm or medulla in which a nucleus may be found. This medulla projects freely on the inner or coelomic aspect of the cell to a variable degree, and over a variable extent of that aspect. The projection is greatest in the Coelomyarii of Schneider (e. g. Ascaris, Filarid), in which protoplasmic bands connect it to the median ventral or dorsal lines, to the lateral areae, or to both.
The ends of the muscle-cells are said to break up into processes, which are traceable into the fibres of the subcuticula. The number of cells visible in a transverse section is usually very great, but in the Meromyarian Nematoda there are only eight, two lying in each of the four quadrants limited by the median lines and lateral areae2. In addition to the body-muscles there are certain others; the exsertors and protractors of the male spicules; a paired ventral muscle which is the cause of the curvature in the tail. The radial muscles which are connected to the mesenteron and are of all, those most obviously prolonged into the fibres of the subcuticula; the dorso-ventral and latero-ventral muscles which cross the body behind the anus; the bursal muscles which pass in a similar direction, and are present only in the male; have a different structure. They are nucleated, but their medulla and contractile fibrillae are intermixed.
The nervous system, said to be absent in some of the free-living genera, consists of a fibrous ring surrounding the oesophagus, at a little distance from the anterior extremity. Six nerves originate from this ring both anteriorly and posteriorly. Of the anterior nerves one corresponds to each lateral area, two are submedian and dorsal, two submedian and ventral; of the posterior, one corresponds to each of the two median lines, the dorsal and ventral, whilst two on each side are sublateral, i. e. submedian, and are imbedded in the subcuticula. The dorsal median nerve extends to the tip of the tail; the ventral divides just in front of the anus, and its two branches pass to the sides of the rectum, in the female backwards to the lateral areae, supplying the two sensory papillae, whereas in the male the chief part of the fibres turn forwards in the lateral areae, and supply the bursal muscles. Subcutaneous, transverse, and paired fibres connect the dorsal and ventral median nerves, especially in the head and tail. The median and ventral nerve arises by two short roots; but in Plectus (sp. ?) there are two ventral nerves fused only posteriorly, and connected from place to place by transverse commissures.