The first forty-one somites of the body including the clitellum, dissected to display so far as possible the reproductive system as well as the portions of the digestive, and circulatory systems contained in this part of the body.
The integument has been divided down the middle dorsal line and fastened out on either side. The digestive tract occupies the centre of the preparation. It consists of a buccal cavity, not seen here; of a pharynx, oesophagus, crop, gizzard, and intestine or stomach, and a short rectum. The pharynx occupies the first five somites of the body. It has a rough exterior owing to the number of muscular bundles which pass between it and the body walls and have been severed in dissection. The oesophagus is narrow and extends through ten somites. About a quarter of an inch from its posterior end there is very visible on the right side, one of the three calcigerous or oesophageal glands, - the glands of Morren. The crop occupies a large space in the sixteenth and seventeenth somites. It has thin walls and dark-coloured contents. The gizzard comes next and lies in the seventeenth and eighteenth somites. It is smaller than the crop and has light-coloured muscular walls; and is followed by the intestine or stomach. The first portion of this tube is distinctly sacculated laterally, a feature which becomes less and less marked posteriorly.
Its walls have a darkish appearance due to the modified and pigmented 'chloragogen' cells of the peritoneum which coat it and form the tissue, often miscalled hepatic.
The dorsal blood-vessel is clearly seen on the dorsal surface of the crop and intestine in the middle line. It has a distinctly moniliform appearance which is more marked in some other terrestrial Oligochaeta. On either side of the anterior part of the oesophagus may be seen four of the five or six pairs of 'hearts' which connect the dorsal vessel with the supra-neural vessel, in the somites behind the pharynx. The large pendulous vesiculae seminales lie anteriorly to the crop and to the outer side of the oesophagus. The two pairs of spermathecae may be seen, especially on the left side, in the line of the outer row of setae. Remnants of the septa may also be discerned on the internal surface of the body-walls, and in most of the posterior somites the nephridia or excretory organs are left in situ on either side of the intestine.
The stomodaeum in Aeolosoma is restricted to the first somite of the body; in other Oligochaeta it extends backwards through a variable number of somites, and is differentiated into a buccal cavity and a musculo-glandular pharynx. The muscular tissue is most developed on the dorsal aspect of the pharynx, and is irregular in arrangement. The pharyngeal glands appear to correspond to the septal glands of the Enchytraeidae: they are large in Megascolex Moseleyi and Pontodrilus.
The oesophagus and stomach-intestine jointly represent the archenteron. The oesophagus of Perichaeta has three sets of glands attached to it. The gizzard is absent in Pontodrilus and all aquatic Oligochaeta except some Naidomorpha. It is in some instances placed more anteriorly in the body, e. g. in Urochaeta, Antaeus, etc, than it is in Lumbricus. Digaster possesses two, Moniligaster five, gizzards. The second gizzard in the former is separated by a whole somite from the first: the last four gizzards of Moniligaster follow one another closely, but are six somites behind the first. In these cases a longer or shorter tube intervenes between the gizzards, as is the case also between the gizzard and the sacculated region of the intestine. It is difficult to say whether the last-named tubular region should be considered as intestine or oesophagus. Glands are attached to it, e. g. three pairs in Urochaeta, one pair in Microchaeta, which appear to bear a lime-secreting function. They may be regarded as either homologues or analogues of the glands of Morren, according to the view which is taken as to the oesophageal or intestinal nature of the tube from which they originate. Calcigerous glands are entirely absent in some instances, e. g. in Megascolex, Typhaeus, Pontodrilus, etc.
A pair of ventrally placed caeca open into the commencement of the sacculated intestine of Perichaeta, whilst Typhaeus and Megascolex possess a series of dorsal paired glands, five in the former, fifteen to sixteen in the latter, opening into it more posteriorly.
The walls of the digestive tract consist of the following layers from without inwards: (1) an epithelial peritoneum; (2) a longitudinal, (3) a circular, layer of muscle-cells; (4) a layer of capillary bloodvessels, and (5) the epithelium. The peritoneal cells in the region of the stomach and the stomachal vessels are pyriform and branched at their attached ends. They are coloured in all instances - green, black, or in the Earthworm brownish-yellow: hence chloragogen cells. They have usually a well-defined membrane, and contain concretions. They appear to be set free into the coelome, and then degenerate; and their products, soluble and insoluble, are probably excreted through the nephridia. The longitudinal layer of muscle-cells is thin, but is better developed in the gizzard where the circular layer attains an extraordinary development. The capillaries are well developed everywhere, but more especially in the pharynx and oesophagus. The lining epithelium is columnar. In the Earthworm it secretes a cuticle in the pharynx, oesophagus, crop and gizzard, but in the stomach is ciliated. In many of the lower Oligochaeta it is ciliated throughout the whole tract. The cells in the oesophagus from the 11th to the 13th somite are glandular. To this region belong the calcigerous glands.
The first pair of these glands in the Earthworm is a pair of hollow diverticula, whilst the second and third pair consist solely of enlarged gland-cells (Claparede). They secrete a milky fluid which owes its milkiness to the presence of Calcium carbonate in the form of rhombohedra (especially in the first pair), or of minute spherules. The Calcium carbonate has a chemical action, not a mechanical, as supposed by Claparede, and partly neutralises the acids of the humus swallowed by the worm, partly acts on the humus itself.