Now if the observations of M. Treviranus are correct, and the four vessels which he de scribes are really analogous to the biliary tubes of insects, we do not hesitate to consider all the part which precedes and is intermediate to them and the four sacs, as the stomach, or chilific cavity. It would thus result, that the tegenaria domestics would be deprived of an intestine properly so called, and would pos sess no part destined to transmit along a greater or less extent the residua of the digestive pro cess. And, indeed, such residua must neces sarily be very inconsiderable in an animal which is sustained by juices, and these already animalized. We are, indeed, led to this con clusion by the structure presented by the he mipterous insects which are nourished, like the spiders, by suction, and which also have the intestines, properly so called, so short that the biliary vessels, which always accompany the posterior extremity of the stomach, are found close to the anus. We may form an idea of this disposition by casting an eye over the beautiful figures which our friend M. Leon Du four has just published in his " Anatomical and Physiological Researches on the Hemiptera." In the alimen tary canal of the t scorpions the biliary vessels d d are inserted much higher up, but this is not the only pecu liarity which the anatomy of these animals presents. Their digestive tube extends without any marked di latation straight from the mouth (a) to the anus which opens at the extremity of c the tail. It pre sents in this course a very singular struc ture: five small canals (b) go off at right angles from either side, above the place of communica tion of the bili ary vessels, and terminate by ramifying in the fatty masses which make a sort of epiploon (c.) This tru ly remarkable structure is not, however, so an omalous as might he sup posed,especially if we regard as ccecums these kind of lateral ves sels. For the alimentary canal presents a still more ramified condition in some crustaceans,— we would cite as an example the argulus studied by Jurine;* and in another animal of the same class which M. Milne Edwards and myselft have made known under the name of Nicotha, the intestinal canal sends out considerable lateral prolongations. In the leech, and es pecially the Clepsina, there exist numerous ccecums. Lastly, certain minute arachnidans (acaridee) are remarkable for analogous lateral dilatations. It is to be observed that all these beings are sustained by animal juices, and the great part, for the better gorging of the same, are fixed either momentarily or during their whole life upon the body of their victim.
We now come to speak of the epiploon and the fatty globules which it contains. The fat, or the substance which appears as such, is ex tremely abundant in the bodies of insects and arachnidans. In the latter it assumes the form of granular masses or globules of various co lours, and sometimes these are united together by a thin membrane. In the aranew the fat is especially abundant in the abdomen, of which, indeed, it determines the form. The use of this fatty apparatus cannot be mistaken, and it has been placed beyond doubt by experiment, that it supplies the place of nourishment to the animal, either when the latter passes the winter in a state of torpidity, like the hibernating ani mals, or when in particular seasons circum stances are not favourable for catching prey.
Respiratory system.—The division which has been established in the class Arachnida of Pul monaries and Trachearies indicates that there are in these animals two very different modes of respiration. In both cases the atmosphere penetrates to the interior of the body by orifices situated on different points of the body, and called stigmata. The stigmata of the Pulmo nary Arachnidans, and especially those of scor pions, are very conspicuous ; they occupy the inferior part of the abdomen, and are four in number on either side, (1, 2, 3, 4, fig. 84.)
They are in the form of narrow fissures, sur rounded as in insects with a circle of more solid substance than the rest of the integument, and to which we have given the name of pe ritrema.
In the spiders (aranece) not only do they differ in form but in number and position. Treviranus counts four pairs in the thorax above the insertion of the legs, four pairs on the upper part of the abdomen, and one pair on the lower surface ; the latter is the most constant and important, (fig. 100, d.) The stigmata of the Tracheary Aracltnidans are less easy to be distinguished, more espe cially on account of the small size of the species constituting a part of that group. We have here carefully figured them in an Acaroid species ( Ixodes Erinacei), where they are situated below the sides and on the lower part of the abdomen, (fig. 85, a,) in shape like a spherical tubercle, (fig. 86, a,) perforated by gins of which adhere to the horny circle or peritrema of the stigMa before described.
We here subjoin figures copied from those of Professor Muller of Berlin, which represent these parts in a scorpion. Fig. 87 shows one of an infinite num ber of small holes, between which in the centre we may remark a larger circular plate (b.) Each little aperture is as it were stellated at the margins (c,) by which the air penetrates the body and gets into the trachea. These tracheae are ana logous in struc ture and posi tion to those of insects;they are elastic, ramify after the man ner of vessels in the interior of the body, and penetrate to even the minutest organs.
With regard to the internal respiratory organs of the Pulmonary Arachnidans they have a very different character ; presenting the ap pearance of membranous sacs formed by la mellae applied to one another like the leaves of a book, each of these little receptacles opens into a common cavity, the membranous mar the pulmonary branchiw entire, seen in profile: a is the edge by which it adheres to the circum ference of the stigma ; b the simple membrane without folds ; c the folds or leaves. Fig. 88 shows a portion of the same pulmonary bronchia laid open: a is the horny margin of the stigma, or peritrema, to which the simple membrane b adheres; c the common cavity into which each of the spaces opens which are formed by the These organs resemble closely in their struc ture the branchial laminae, and hence Trevi ranus and Meckel compare them to branchim. Muller on the other hand maintains that they are lungs, because, he says, they can be dis tended with air. The name - of pulmonary branchim, which we have given them, seems to reconcile the two contending opinions, although we believe that the distinction between lungs and gills is in itself of very slight importance when applied to articulate animals. It is, for example, quite impossible to establish such a distinction in certain crustaceans, as the Onis cus, the Asellus, the Cymothoa, which are all provided with organs of an analogous structure, although some live in water, and others in air more or less humid. Moreover, certain crabs, as the terrestrial species called Cancer Uca, Ruricola, &c., of Linnaeus, possess branchiw which are much better adapted for respiration in air than in water. The Cancer ilkenas, so common on our coasts, is almost in the same case, since it passes a great part of its life out of the sea, and it is well known that lobsters and shrimps can live a long time out of water, provided that the air in which they are kept is humid. M. Milne Edwards and myself have demonstrated, by decisive experiments, the conditions in which the branchim in these animals act as lungs.