This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Cyclop, of Anat. and Phys., art. "Arachnida." 1 Pes, a foot; palpus, a feeler.
(956). In Spiders, the organization of the mouth is altogether different. The mandibles (fig. 186, o o) are each terminated with a moveable fang (c), which ends in a sharp point, and is perforated near its extremity by a minute orifice, from which, when the Spider bites, a venomous fluid of great potency is instilled into the wound inflicted; such, indeed, is the malignity of this poisonous secretion that its effects in destroying the life of a wounded insect are almost instantaneous, and in some of the large American species even small birds fall victims to its virulence. The organ in which the poison is elaborated is represented in the figure above referred to; it is a long and slender bag, from which an attenuated duct may be traced through the body of the mandible as far as the perforated extremity of the fang.
(957). The palpi connected with the maxillae of the Spider are terminated in the female by a simple hook; but in the males of many species they exhibit a conformation slightly resembling the forceps of the Scorpion, although provided for a very different purpose. When closed (fig. 185, b), the terminal part of the palpus presents a club-like dilatation, which, however, on close inspection will be found to consist of several pieces (fig. 185, a, a, b, c, d, e), connected with each other by articulations, and capable of being opened out in the manner represented in the figure. This strange instrument was formerly imagined to be the penis of the male Spider, and was thought to contain the terminations of the seminal ducts: this supposition, however, has been proved to be erroneous; for the palpus is imperforate, and the sexual apertures of the male are situated elsewhere; but the organ in question is nevertheless apparently used in the process of impregnation, in a manner to be explained hereafter.
• (958.) Both in Scorpions and Spiders the alimentary canal is exceedingly narrow, presenting scarcely any of those dilatations met with in the digestive organs of insects. This is a natural consequence of the nature of their food; for as they live entirely upon animal juices sucked from the bodies of their victims, there could be little necessity for the presence of capacious receptacles for nutritious matter, or for any reservoirs for the accumulation of effete material.
(959). In the Scorpionidae there is no stomachal dilatation whatever: a straight intestine passes directly from the mouth to the anus, situated at the extremity of the abdom n; and the insertion of the biliary vessels forms the only distinction between its ventricular and intestinal divisions. Five delicate caeca are derived from each side of the ventricular portion, and plunge into the centre of a fatty substance in which the alimentary canal is imbedded. In Spiders, likewise, caeca are appended to the commencement of the digestive apparatus; and a slight enlargement (fig. 186, b) may be said to represent the stomach, from which a slender intestine (g) is continued to the anus. As in the Scorpion, a large quantity of fat (h) surrounds the nutrient organs and fills up a great proportion of the cavity of the abdomen. Like the fat-mass of the larvae of insects, this substance must, no doubt, be regarded as a reservoir of nutriment; and when the habits of these animals are considered, the precarious supply of food, and the frequent necessity for long-protracted fasts when a scarcity of insects deprives them of their accustomed prey, such a provision is evidently essential to their preservation.
Fig. 185. Palpus of male Spider.
(960). One peculiarity connected with the arrangement of the chy-lopoietic viscera of the Spider is the manner in which the biliary organs terminate in the intestine; for instead of entering in the usual position, namely close to the termination of the stomach, they seem to pour their secretion into the rectum, immediately in the vicinity of the anus. At this point, a kind of sacculus (figs. 186 & 188,f) joins the intestine, into which the branched tubes (fig. 188, oo,& fig. 186, s) empty themselves. This circumstance has long been a subject of interesting inquiry to the comparative physiologist. If the fluid secreted by these tubes be really bile, in what manner does it accomplish those purposes usually supposed to be effected by the biliary secretion? It would seem to be, in this case, merely an excrementitious production. Are the caeca appended to the stomach biliary organs? If so, the apparatus in question may be of a totally distinct character, and its product only furnished to be expelled from the system. In conformity with the last supposition, many anatomists have been induced to regard these vessels as being analogous to the urinary secernents of more highly organized animals, and have not scrupled to apply to them the appellation of renal vessels: but this hasty application of names we have already animadverted upon as being highly prejudicial to the interests of science; and in this instance, as in many others, to wait for the results of future investigations is far more advisable than rashly to assign a definite function to a part the real nature of which is a matter of speculation.
Fig. 186. Digestive system of the Common Spider: poison-fang; oo, the jaws, with their appended poison-glands; a a a, caecal appendages to d, the commencement of the alimentary canal, with which the muscle (e) is connected; b, stomachal dilatation; g, intestine; h h, accumulation of fat; f, sacculus receiving the terminations of the secreting-tubes (s).
(961). The respiratory system of the Pulmonary Arach-nidans is constructed upon very peculiar principles, being neither composed of gills adapted to breathe water, nor of lungs like those of other air-breathing animals, but presenting a combination of the characters of both. The pulmonibranchiae are, in fact, hollow viscera resembling bags, the walls of which are so folded and arranged in laminae, that a considerable surface is presented to the influence of oxygen. It is, indeed, highly probable that these organs are intermediate in function as well as in structure between an aquatic and air-breathing respiratory apparatus; for, as both the pedi-palp and spinning Arachnidans frequent moist situations, the dampness of the atmosphere may be favourable to the due action of the air upon the circulating fluids of these creatures. Each pulmonibranchia opens externally by a distinct orifice resembling the spiracle of an insect, and is closed in a similar manner by moveable horny lips. In the Scorpion (fig. 184) the spiracles are eight in number, placed upon the ventral aspect of the body; and just in front of the first pair of breathing-holes are two remarkable organs (represented in the figure), resembling a pair of combs, which are apparently adapted to keep the spiracular orifices free from dirt, and thus prevent any obstruction to the free ingress and egress of the air.