Fig. [185].—Diagram showing the anatomy of the cephalothorax of a Spider. The right alimentary diverticulum has been removed. a, Aorta; c, left diverticulum with secondary caeca; e, endosternite; oes, oesophagus, descending to the mouth; s, sucking stomach; sh, dorsal shield of sucking stomach.
We now reach the sucking stomach, which occupies the centre of the cephalothorax. It is placed directly over a skeletal plate, the “endosternite” (Fig. [185], e), to which its lower surface is connected by powerful muscles, while its upper wall is protected by a hard plate or “buckler,” which is similarly attached to the roof of the cephalothorax in the region of the “fovea media.” The walls of the stomach are not themselves muscular, but by the contraction of the muscles above mentioned its cavity is enlarged, and fluids from the pharynx are pumped up into it.
The canal thus far is lined by chitin, like the exterior of the body, and forms a sort of complicated mouth-apparatus.
The Mesenteron lies partly in the cephalothorax and partly in the abdomen. The thoracic portion, shortly behind the sucking stomach, sends forward on either side a large branch or “diverticulum,” from each of which five secondary branches or “caeca” are given off (Fig. [185]). Of these the anterior pair sometimes join, thus forming a complete ring; but usually, though adjacent, they remain distinct. The other four pairs of caeca curve downwards, protruding into the coxae of the legs, where they often terminate, but sometimes (Epeira) they continue their curve until they meet, though they never fuse, under the nerve-mass. Behind the origin of the diverticula the mesenteron continues as a widish tube, and shortly passes through the pedicle and enters the abdomen, where, curving slightly upwards, it proceeds along the middle line till it ends in the proctodaeum.
In the abdomen it is surrounded by a large gland, the so-called liver, and is dilated at one spot (Fig. [186]) to receive the ducts from this gland. The fluid elaborated by this large abdominal gland has been shown to have more affinity with pancreatic juice than with bile.
The Proctodaeum consists of a short rectum, from the dorsal side of which protrudes a large sac, the “stercoral pocket.” At its origin, the rectum receives the openings of two lateral tubes which reach it after ramifying in the substance of the liver. These have been called “Malpighian tubules,” but their function is unknown. Loman[[263]] has shown that they open into the mid-gut and not into the rectum, and there is reason to believe that true Malpighian tubules homologous to those of Insecta are absent in Arachnida, where their place seems to be taken by the coxal glands, which are considered to be the true excretory organs. In most spiders they open near the third coxae. Like the stomodaeum, the proctodaeum has a chitinous lining.
Vascular System.—The earlier investigations on the circulation of the blood in Spiders were made by direct observations of the movements of the blood corpuscles through the more or less transparent integuments of the newly-hatched young. Claparède’s[[264]] results were arrived at by this method. It is invaluable for demonstrating roughly the course taken by the blood, but in these immature spiders the blood-system has not attained its full complexity, and other methods of research have shown the spider to possess a much more elaborate vascular system than was at first suspected.
The tubular heart lies along the middle line in the anterior two-thirds of the abdomen, sometimes close up against the dorsal wall, but occasionally at some little distance from it, buried in the substance of the liver. It is a muscular tube with three pairs of lateral openings or “ostia,” each furnished with a simple valve which allows the entrance, but prevents the exit, of the blood. It is contained in a bag, the “pericardium,” into which the ostia open. Both heart and pericardium are kept in place by a complicated system of connective tissue strands, by which they are anchored to the dorsal wall of the abdomen. Eight arteries leave the heart, the principal one, or “aorta,” plunging downward and passing through the pedicle to supply the cephalothorax. Besides this, there is a caudal artery at the posterior end, and three pairs of abdominal arteries, which proceed from the under surface of the heart, and the ramifications of which supply, in a very complete manner, the various organs of the abdomen. The heart is not divided up into compartments. The anterior aorta passes through the pedicle, above the intestine, and presently forks into two main branches, which run along either side of the sucking stomach, near the front of which they bend suddenly downwards and end in a “patte d’oie,” as Causard[[265]] expresses it—a bundle of arteries which proceed to the limbs (Fig. [185]). Where the downward curve begins, a considerable artery, the mandibulo-cephalic, runs forward to supply the chelicerae and the head region. We have omitted certain minor branches from the main trunks which supply the thoracic muscles. The nerve-mass receives fine vessels from the “patte d’oie.”
