THE DIGESTIVE SYSTEM.—The mouth of the Nematoda is usually anterior and terminal, and is surrounded by from two to six projecting lips, the most common number being three. These {131}lips are well provided with sense papillae. The mouth leads into an alimentary canal, which with hardly an exception runs straight through the body to the anus without twists or loops. The anus is usually placed ventrally and is not terminal, but in _Trichina_ and _Trichocephalus_ it is at the end of the body, and in _Mermis_, where the several parts of the alimentary canal are said not to communicate, it is absent altogether. _Ichthyonema_, _Dracunculus_, _Allantonema_, _Atractonema_, and other Filariae are also aproctous. 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.