We believe that we have placed these facts beyond all doubt in our ' Researches on the Thorax of Articulate Animals,' presented to the Academy of Sciences of Paris in Now it is worthy of remark that what has hap pened to the arachnidans, being animals de prived of wings, is also found in the crusta ceans, which are equally destitute of these organs. Only that there exists in some of the latter, as the decapods, a vast carapace which occurs independently of the flancs, and covers them. For if the carapace is raised in a crab, the flancs or pleura are seen beneath, extending obliquely towards one another as in the thorax of a mygale, with this single difference, that in the cancer, where the carapace covers the flancs and protects them as well as the internal soft parts, the pleura or side pieces remain divaricated and are not joined at their apices as in the mygale.1 Digestive system .—The arachnidans, whose habits have been made the subject of obser vation, feed for the most part on animal matter, not in a state of decomposition or even re cently dead, but in the living state. They either boldly seize their prey, which consists of insects of greater or less size, or they attach themselves to animals much larger than themselves, and live parasitically upon their blood or some other nutritious fluid. The latter species are generally very minute : many of them, as the mites (acari), require our best optical instruments for their detection. The above differences in habits of life are accompanied with important modifications in the organs of nutrition, and especially in the oral apparatus, which we proceed to de scribe.
In the non-parasitic species, as the pulmo nary and part of the tracheary arachnidans, the mouth consists essentially, first, of two mandibube or forciples (fig. 80, a) in close ap position, endowed with little lateral motion, but rather acting vertically and provided each with a hooked claw (b), which, near its point, is perforated, and emits a poisonous fluid, secreted by a gland, hereafter to be described. In other arachnidans of the same order the mandibula are a species of pincers, one nipper of which is alone moveable, as in the scor pions. Secondly, of two maxilla (c c), each in the form of a more or less flattened and villous lobe, provided with a palp or jointed appendage (d) projecting more or less from the mouth, and terminated sometimes by pincers as in the scorpions, sometimes by a simple claw, as in the spiders, at least the females, for in the males this palp is frequently the seat of a singular apparatus (e), hereafter to be described. Thirdly, of a sternal labium (f), which, as its name implies, is inserted into the sternum, and does not give origin to any arti culated appendage or palp. With respect to the composition of the mouth in the parasitic species, such as most of the mites, and we may take as an example an argas, although it is concealed under the form of a beak, sometimes with a sharp point, yet it is essentially the same. The principal difference consists in the dart - shaped mandibles (a a), being joined toge ther so as to form a kind of lancet, the sides of which are sometimes denticulated, so as to cause them to adhere firmly to the flesh which they have penetrated.
The maxilla with their palp (b) and the inferior labium (c) are here more or less intimately blended together, so as to form a case or sheath. In some instances the maxillary palp remains free, as in the argas.
Savigny admits that in the interior of the mouth of arachnidans there exist three pharyn geal orifices, and not a single one as in crus taceans and insects. These three orifices, which are of almost imperceptible minuteness, are situated at some distance from one another, and disposed in a triangular form. He has observed this structure in spiders, scorpions, and phalangians : but he represents only two orifices in a genus allied to galeodes. Latreille denies the fact, and Treviranus, in his anato mical description of arachnidans, mentions only one pharyngeal orifice.
However this may be, Savigny confines the assumption of food in spiders to a true suction : " The mandibles," says he, " do not serve for bruising the food, but for seizing it, and for piercing and retaining it in firm contact with the maxillae ; these subject it to alternate pres sure, and express the juices which afterwards pass into the pharynx." This is a matter of daily observation when a spider seizes an insect.
The intestinal canal of the arachnidans is always short, and is never disposed in convo lutions as in certain herbivorous insects. This disposition is in accordance with their preda ceous habits, and confirms the general rule, (but which to our knowledge is not without many exceptions,) that the intestinal canal is longer in herbivorous than carnivorous animals.
In the spiders, (aranea2,) and we may take the common species ( tegenaria domestica) as an example, the alimentary canal (fig. 82 ) com municates with the mouth between the maxillae (a a) by an oesophagus, rather short and of a de licate texture (b). This terminates in four sacs (c), which M. Treviranus calls " stomach," hut which, in our opinion, merit rather the name of gizzards ; the digestive tube then continues, as a straight narrow canal (d) of moderate length, which dilates (e) and adheres, by its parietes, to a kind of epiploon filled with adi pose granules (f). Posteriorly the dilated part becomes stronger in texture, insensibly con tracts (g), then undergoes a second dilatation (h) before it opens into the rectum (i). It is near the latter part, in a kind of pouch, that the slender vessels open which M. Treviranus calls biliary vessels, and which he is, with reason, surprised to see terminating in so extraordinary a position. These vessels, in fact, which cha racterize so well by their presence the chilific stomach of insects, and are situated in these animals more or less anteriorly, always pre ceding the small intestines which have a greater or less length, terminate in the spiders in the rectum itself, and close to the anus.