Two similar ventral nerves have been also met with in the young Ascaris (Joseph). Ganglion cells are found in the oesophageal ring, chiefly at the origins of the nerves; in the circumoral plexus into which the anterior nerves pass, in the anterior lateral nerves, in the ventral median nerve, and in the bursal nerves of the male. In the larger A scar ides at least (A. megalocephala, A. lumbricoides\ there is a well-marked ganglion at the spot where the ventral nerve bifurcates, and in the male a circumanal ganglionic ring (Rohde). Organs of special sense are represented by sensory cutaneous papillae, and by eyes. The former are widely distributed, and occur as oral papillae placed on the lips, or on the cuticle round the mouth, on the neck, at the sides of the anus, and on the ventral aspect of the tail in the male. These papillae project in part, and are in part imbedded in the cuticle. A single nerve-fibre has been traced into a papilla, and the granular mass which occupies its centre is probably a sensory nerve-bulb. Eyes are confined entirely to the free-living genera, and are situated anteriorly in the region of the nerve-ring. They consist, so far as is known, of a mass of pigment, black, brown, or blue, which may or may not inclose lens-like bodies.

1Long pear shaped glands, open by a common duct on the ventral aspect, and near the tip of the tail in the free-living marine genus Enoplus. They secrete a tenacious substance, by which the animal anchors itself. They are perhaps present in some fresh-water forms.

2Schneider supposed that in certain Nematoda, e. g. Trichina, Trichocephalus, the muscles of the body-wall were 'either not divided, or only divided in a longitudinal direction,' i. e. by the lateral areae and median lines. Such Nematoda he classed as Holomyarii. There are, however, numerous muscle-cells in all so-called Holomyarian genera which have been carefully re-examined, e. g. in those named above and in Mermis. See Butschli, 'Giebt es Holomyarii?' Z. W. Z. xxiii. 1873.

There is a coelome, which in most cases does not form a large cavity; but whatever space there may be, it is largest round the oesophagus and in the tail. It is traversed by fibres which may be muscular in nature, and appear to be in connection with the fibres of the subcuticula. The coelomic fluid is clear and coagulable, but it is doubtful whether or no it contains any corpuscles. It is often reddish in colour, but this occurs only when the worm feeds on blood, e. g. Syngamus trachealis. The red colour is due to discharged oxy-haemoglobin, and gradually disappears if the animal is isolated and starved.

The digestive tract appears to undergo regressive changes in Sphae-ndaria, and its several parts are said not to communicate in Mermis. The mouth is either anterior and terminal, or else it opens, as in Strongylus, Dockmius. Sclerostomumy etc, into a more or less spheroidal cavity, which may be smooth-walled, or covered with small pointed teeth variable in number and size. When terminal it is usually surrounded by small projecting lips formed by the cuticula and sub-cuticula, in number two, three, four, or six, but most commonly three. There are no muscles to these lips, but the hold of the animal is maintained by suction through the oesophagus. In Sclerostomum, and, according to Luckart, also in Dochmius, two long glands, probably poisonous in nature, open into the oral capsule. The digestive tract itself is divisible into three sections:• an oesophagus with muscular walls lined by a cuticle which is shed with the cuticle of the body, a mesenteron, and a rectum, the later lined by a cuticle shed in the same manner as that of the oesophagus. The oesophageal walls have an external cuticle, with radial muscles passing from it to the internal lining cuticle.

Granular remnants of the original cells with nuclei are to be found here and there between the muscle-fibres, most plentifully in the young. Longitudinal muscle-fibres may also be present. The oesophagus sometimes retains very clearly its cellular origin. This is especially the case in Trichina and Trichocephalus. In Mermis muscular elements are entirely wanting. The cavity of the oesophagus is usually triangular in cross section, and may even be reduced to a three-legged slit. Its hinder-part is dilated into a muscular bulb or gizzard, frequently armed with teeth in Heterakis, in Oxynris and its allies. There are often two such bulbs in the Anguillulidae. A solid oesophageal spine is present in Tylenchus Tritici (=Anguillula scandens) and the embryo Mermis; a hollow tubular spine in the free-living Dorylaimus. The mesenteron is composed of a single layer of columnar or flattish cells, covered both internally and externally by chitinoid membranes, which may be resolved into small plates, adapted to the ends of the individual cells, the internal plates being usually perforated by vertical pores.

In Trichina spiralis it consists of a single row of cells, one behind the other, perforated by a canal; so, too, the larval Tylenchus Tritici. It consists in Leptodera, Pelodera, and Pseu-dalius injlexus (?), of two rows of cells alternating from side to side, whilst the genus Strongylus constitutes a transition to the usual structure, in which there are many rows of cells in a transverse section of its walls. A muscular coat is, as a rule, absent; but in a few instances, e. g. Oxyuris vermicularis, the hinder region is covered by a network of circular fibres; and there is sometimes a strong sphincter-muscle developed at the junction of the mesenteron with the rectum. These muscles, as well as the mesenterial or radial bands, which support the mesenteron in situ in some forms, e. g. Eustrongylus, are probably connected, like the muscular coat of the rectum, with the subcuticular fibres. The mesenteron is, properly speaking, cylindrical, or flattened dorso-ventrally, but it becomes deformed by the pressure of the generative organs as they mature. The rectum is usually short, but it is of great proportional length in Trichina; and it is said not to communicate with the mesenteron in Ichthyonema globiceps.