The alimentary canal is divisible into three parts—(i.) the oesophagus, (ii.) the intestine, and (iii.) the rectum. The suctorial oesophagus is a very muscular, thick-walled tube, lined with cuticle continuous with that which covers the body, and like it cast from time to time. Its lumen is usually much reduced, and is almost invariably triangular or triradiate in section (Fig. 62). In many genera the hinder end of the oesophagus is swollen into a muscular bulb, which is armed with teeth in Heterakis, Oxyuris, Pelodera, Leptodera, etc. Other species, such as Tylenchus, Aphelenchus, Dorylaimus, are armed with a spear, which in Onyx,[[162]] a genus recently described and allied to the last named, is borne on a special bulb. The use of the spear is to pierce the tissue upon the juices of which the animal lives. A gland lies embedded in the thick walls of the oesophagus, and opens into its lumen by a fine tube. This was first described by Schneider[[163]] in A. megalocephala, and more recently it has been found by Hamann[[164]] in a number of Ascaridae and Strongylidae from the Adriatic, and also in Lecanocephalus.
With a few exceptions, such as Mermis, where it is blind, the oesophagus opens posteriorly into the intestine. This is a somewhat flattened tube, whose shape and position are often altered by the development of the generative organs. Its wall consists of a single layer of columnar cells, with large nuclei coated internally and externally by a layer of cuticle. The inner layer of cuticle is usually perforated by very numerous minute pores. In some species the intestine is degenerate, in Mermis it is a closed tube opening neither into the oesophagus nor into the rectum; in Trichina spiralis and in the larva of Tylenchus tritici it consists of a single row of cells perforated by a duct, but in the adult of the last named there are many cells in a transverse section.
Fig. 65.—A longitudinal section through the body of Strongylus filaria Rud. (From O. Augstein.[[165]]) A portion of the body, on each side of the excretory pore, is seen in optical section. a, Mouth; b, oesophagus; c, intestine; d, excretory canal; e, excretory pore, and the opening of the poison glands, i; f, circumoesophageal nerve-ring; g, ventral nerve; h, dorsal nerve; i, unicellular poison glands; k, ovary, with the ova separate; l, oviduct; m, uterus, the first egg in the uterus is surrounded by spermatozoa; n, opening of uterus; o, inner end of ovary with the ova undifferentiated.
In some genera, Leptodera and Pelodera, the lumen of the intestine at any one level is bounded by two horseshoe-shaped cells, but by far the commonest arrangement is a tube formed of fairly numerous columnar cells crowded with granules and with large nuclei.
The rectum is usually short; its cuticular lining, like that of the oesophagus, is cast at intervals. At its anterior end there is usually a sphincter muscle, and its walls are divaricated by muscular strands which run from it to the body-wall. The anus is a transverse slit, which in the male Strongylidae is surrounded by a funnel-shaped membrane.
The food of Nematodes seems to be almost entirely fluid, and consists, at any rate in the parasitic forms, of the elaborated juices of their hosts. Little is known about the nutriment of the free-living forms.
The Excretory System.—The excretory organs are peculiar, and, like many other Nematode structures, do not fall readily into line with what is known of similar organs in other animals. They consist of two canals embedded in the lateral thickenings of the sub-cuticular tissue. The canals end blindly behind, but near the anterior end of the body they bend inwards, and after uniting, open by a common pore situated in the middle ventral line, a little way behind the mouth. The lateral canals are in some cases continued in front of the transverse branch, and they then end blindly in the head. The walls of these canals consist of an internal, structureless, refractive layer surrounded by a granular layer with nuclei. They contain a fluid, but nothing is known of its composition.
An interesting divergence from the usual form of excretory organ has been described by Hamann[[166]] in the genus Lecanocephalus. Here there is only one canal, the right; anteriorly this bends towards the ventral surface and opens by a small median pore close behind the nerve-ring. Posteriorly the canal does not extend much beyond the middle of the body, where it forms a coiled mass, and diminishing in size, opens into the body-cavity. The same author also states that both canals in Dochmius have a similar internal opening; these observations, if confirmed,[[167]] show a conformity to the ordinary structure of excretory organs which was not supposed to exist in the lateral canals of the Nematoda.
The Reproductive Organs.—With the exception of the genera Angiostomum, Pelodytes, and of Rhabdonema nigrovenosum, which are physiologically hermaphrodite and self-impregnating, the Nematodes have separate sexes. The males are, as a rule, smaller than the females, and may usually be distinguished by the posterior end of the body being curved towards the ventral surface; a genital bursa, and one or more spicules are often found in this sex. Further, the position of the genital opening differs; in the male the vas deferens opens on the ventral surface of the rectum close to the anus, but the oviduct in the female opens in the ventral middle line, usually near the middle of the body, but sometimes close behind the excretory pore, or in some Strongylidae just in front of the anus. The tail of the male bears very numerous papillae, which are of considerable systematic importance.