Digestive and circulatory apparatus of the Harvest Spider.

Fig. 187. Digestive and circulatory apparatus of the Harvest Spider: a, the stomach, with its lateral caeca, on which is situated the dorsal vessel; b b, vascular sinuses.

Termination of the alimentary canal of a Spider: a, stomachal dilatation of intestine.

Fig. 188. Termination of the alimentary canal of a Spider: a, stomachal dilatation of intestine; f, terminal sacculus receiving the terminations of the secerning vessels.

(962). In the Araneidae, the form and arrangement of the spiracles are somewhat different. According to Treviranus, there are four pairs on each side of the cephalothorax, situated immediately above the insertions of the legs; and in addition to these, there is one pair constantly found on the under surface of the abdomen, and four pairs of smaller apertures of less importance on its upper part.

(963). In order to understand the manner in which respiration takes place in pulmonibranchiae of the structure above described, it is necessary to suppose the existence of a vascular apparatus, by means of which the circulating fluid is continually spread over the laminae of the respiratory sacculi, and afterwards returned to the circulation in a purified condition. It is true that, owing to the extreme difficulty of tracing vessels of such small dimensions, the continuity of the entire system is rather an inference deducible from a general review of the facts ascertained, than absolutely a matter of demonstration. We will therefore briefly lay before the reader the data upon which physiologists found the opinions entertained at the present day relative to the means whereby the circulation of Arachnidans is accomplished.

(964). According to Treviranus, spiders are provided with a long contractile vessel (fig. 189, a), which runs along the mesial line of the back, and resembles in form the dorsal vessel of insects, although in structure it is widely different. In insects, it will be remembered, the dorsal vessel communicated freely with the abdominal cavity by numerous valvular apertures, and neither arteries nor veins were necessary for diffusing the blood through the system; but in the Pulmonibranchiate Arachnidans numerous vascular trunks (11) are given off from both sides of the dorsal heart and are dispersed in all directions. All the branches proceeding from the sides of the dorsal vessel are presumed to be of an arterial character, with the exception of a few large canals situated near the junction of the anterior and middle thirds of its length, and these are supposed to be veins* (branchio-cardiac vessels) destined to return the aerated blood from the pulmonibranchice (f) into the general circulation. Whoever watches the movements of the blood in one of the limbs of these creatures will perceive that, under the microscope, its motion bears little resemblance to that observable in the foot of a frog, or in animals possessed of an arterial and venous system completely developed.

So irregular, indeed, is the course of the globules, that it would be difficult to conceive them to be confined in vessels at all; the whole appearance resembles rather the diffused circulation seen in the larva of an insect, than that of a creature possessing vascular canals arranged in definite directions. The only probable way of accounting for such a phenomenon is by supposing that, in this first sketch of a vascular system, if we may be pardoned the expression, the veins are mere sinuses or wide cavities formed in the interstices of the muscles, through which the blood slowly finds a passage. From a review of the above-mentioned facts we are at liberty to deduce the following conclusions relative to the circulation of Arachnidans: - The pulmoni-branchice being apparently the only organs of respiration, the blood must be perpetually brought to these structures from all parts of the system, to receive the influences of oxygen, and again distributed through the body. Such a circulation could only be accomplished in circumscribed channels - some destined to propel it through all parts, others to collect it after its distribution and bring it to the respiratory organs, and a third set to return it in a renovated condition to the heart.

The circuit of the blood may therefore be presumed to be completed in one or other of the following modes: - The dorsal vessel, or heart, by its contraction drives the blood through numerous arterial canals to the periphery of the system; the blood so distributed gradually finds its way into capacious sinuses, through which it flows to the branchial organs, and from hence it re-enters the heart by the branchio-cardiac vessels above referred to: or else the action of the heart drives a portion of the circulating fluid into the pulmonibranchim by the same effort which supplies the rest of the system, and the blood so impelled to the respiratory organs becomes, after being purified, again mixed up with the contents of the veins which return it to the heart.

Plan illustrating the circulatory system in a Spicier.

Fig. 189. Plan illustrating the circulatory system in a Spicier: a, dorsal vessel; b, suspensory muscle; c, the ocelli; d, poison-gland; e, palpus; f, pulmonibranchial organ; g, poison-fang; h, cephalo-thorax; i, caecal appendices to the stomach; I, vascular trunks derived from the dorsal heart, running to the pulmonibranchiae; to, abdomen; n, spinnerets.

(965). In the Scorpions, the circulatory system resembles that of the Myriapoda, but it is more completely organized; the heart, which, as in the Scolopendra, is divided into compartments, is elongated at its posterior extremity into a long caudal artery, and gives off from each chamber a pair of systemic arteries, which are distributed among the viscera, and also send their principal divisions to supply the muscles of the inferior and lateral regions of the body, as well as the pulmonary sacs. At the anterior part of the abdomen, the heart assumes the character of an aorta, descending suddenly into the thorax, and dividing immediately behind the brain into a number of large vessels, that supply the head and the locomotive organs. The posterior of these form a vascular collar around the oesophagus, which gives origin to the great arterial trunk, or supra-ganglionic vessel, whereby the blood is conveyed to the posterior part of the body, as in the Myriapoda (vide § 746.) This vessel passes beneath the transverse arch of the thorax, with which it is slightly connected by fibrous tissue, and then runs backwards, gradually diminishing in size, until it reaches the terminal ganglia of the tail, where it divides into branches that accompany the nerves.