(4) A connective-tissue layer supporting (5) a layer of fairly columnar hyaline epithelium, bounded on its inner aspect by a cuticle continued from that of the pharynx. In front it passes insensibly into the pharynx, and beyond the region where the dorsal walls of the pharynx have clearly commenced, the ventral walls still retain the characters of the œsophageal walls. The œsophagus is vertically oval in front, but more nearly circular behind. Characteristic of the œsophagus is the junction of the two sympathetic nerves on its dorsal wall (fig. 16). These nerves cannot be traced far beyond their point of junction.
The Stomach.—The next section of the alimentary tract is the stomach or mesenteron (fig. 6). It is by far the largest part of the alimentary tract, commencing at about the second pair of legs and extending nearly to the hind end of the body. It tapers both in front and behind, and is narrowest in the middle, and is marked off sharply both from the œsophagus in front and the rectum behind, and is distinguished from both of these by its somewhat pinker hue. In the retracted condition of the animal it is, as pointed out by Moseley, folded in a single short dorsal loop, at about the junction of its first with its second third, and also, according to my observations, at its junction with the rectum; but in the extended condition it is nearly straight, though usually the posterior fold at the junction of the rectum is not completely removed. Its walls are always marked by plications which, as both Moseley and Grube have stated, do not in any way correspond with the segmentation of the body. In its interior I have frequently found the chitinous remains of the skins of insects, so that we are not justified in considering that the diet is purely vegetable. It lies free, and is, like the remainder of the alimentary tract, without a mesentery. The structure of the walls of the stomach has not hitherto been very satisfactorily described.
The connective tissue and muscular coats are extremely thin. There is present everywhere a peritoneal covering, and in front a fairly well-marked though very thin layer of muscles formed of an external circular and an internal longitudinal layer. In the middle and posterior parts, however, I was unable to recognize these two layers in section; although in surface view Grube found an inner layer of circular fibres and an outer layer formed of bands of longitudinal fibres, which he regards as muscular.
The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach is by far the thickest (fig. 20), and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of the stomach, where they are longest, are nearly half a millimètre in length, and only about .006 mm. in breadth. Their nuclei, as seen in fig. 20, are very elongated, and are placed about a quarter of the length from the base.
The cells are mainly filled with an immense number of highly refracting spherules, probably secretory globules, but held by Grube, from the fact of their dissolving in ether, to be fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach.
In addition to the cells just described there are present in the anterior part of the stomach a fair sprinkling of mucous cells. There are also everywhere present around the bases of the columnar cells short cells with spherical nuclei, which are somewhat irregularly scattered in the middle and posterior parts of the stomach, but form in the front part a definite layer. I have not been able to isolate these cells, and can give no account of their function.
The rectum extends from the end of the stomach to the anus. The region of junction between the stomach and the rectum is somewhat folded. The usual arrangement of the parts is shewn in fig. 6, where the hind end of the stomach is seen to be bent upon itself in a U-shaped fashion, and the rectum extending forwards under this bent portion and joining the front end of the dorsal limb of the U. The structure of the walls of the rectum is entirely different to that of the stomach, and the transition between the two is perfectly sudden. Within the peritoneal investment comes a well-developed muscular layer with a somewhat unusual arrangement of its layers, there being an external circular layer and an internal layer formed of isolated longitudinal bands. The epithelium is fairly columnar, formed of granular cells with large nuclei, and is lined by a prolongation of the external cuticle. It is raised into numerous longitudinal folds, which are visible from the surface, and give a very characteristic appearance to this part of the alimentary tract. The muscular layers do not penetrate into the epithelial folds, which are supported by a connective tissue layer.
Nervous System.
The central nervous system consists of a pair of supra-œsophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-œsophageal ganglia.
It will be convenient in the first instance to deal with the general anatomy of the nervous system and then with the histology.