Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/314

ARACHNIDA [SPIDERS. mammal, but just as little to the gills of a fish. We woxild propose, therefore, to distinguish these modified and localised tracheal sacs by the name of sac-trachece, the ordinary ones being designated tube-tracheae. In some spiders, Theraphosides (including the giant spiders of the family better known as Mygalides), the respiratory organs are solely sac-trachece ; these are in this case four in number, and placed in two pairs on the lower side of the abdomen. Their external orifices are protected by spiracular plates (as before noticed) ; in other spiders there is, close in front of each of the fore- pair of sac- trachefe, an orifice leading to tube-trachese. In some few spiders a kind of supernumerary spiracular slit or opening is visible near to the ordinary one, but not always similarly placed. A pair of others are also visible close behind the sexual aperture in Cambridgea fasciata (L. Koch) (fig. 27, t, t). These openings probably lead to other tube tracheae. We have also noticed slit-like openings similar to those above mentioned in some undescribed species of Drassides, and some also exist in the well-known European spider, Argyroneda aquatica. 1 Tracheal openings also exist, according to Siebold, near the spinners. The sac-tracheae of spiders are of the same construction as the organs described as pulmo-branchiae in the family Scorpionides. The following short description is abstracted from E. Simon, I.e., following Emile Blan chard : "The respiratory apparatus is con tained in sacs beneath the stigmata ; these sacs are globular and flattened, formed by a silvery white membrane. Each of them con tains a body not fastened to the walls of the sac, but lodged as in a chamber ; this body is composed of sixty or seventy plates (lamellae) lying one upon another like the leaves of a book. Each one of these apparent plates is in fact itself a flattened sac, with an opening on its lower side communicating with the outer air admitted through the spiracular orifices. The walls of these numerous sacciform lamellae consist of two membranes of different composition, the inner one hard, the other soft, and between them is a very fine strong network analogous to the spiral thread of the tracheae of insects." The inspiration and expiration of air in these sac-tracheae are effected in a very simple manner. To the outer wall of the sac a broad and strong ligament is attached ; this ligament ascends across the abdomen, and is fixed to the walls of the heart. Each move ment of this organ, diastole or systole, strains or slackens the ligament, which in its turn raises or lowers the tracheal sac, and by these means the air is either drawn into or expressed from it (E. Simon, I.e., after E. Blanchard). i.-~ According to Treviranus, I.e. (vide Jones, Anim. Kingdom, p. 416), who, however, makes them four pairs on each side, there are, in the Araneidea, four pairs of spiracles (four on each side of the cephalo-thorax) situated immediately above the insertions of the legs. He also notes four stigmata (in two pairs) on the upper Bide of the abdomen, and a pair on the under side ; the former un doubtedly exist, in some species at least, but the latter appear to be imperforate, and are probably only points of muscular attachment. It does not appear that the tracheae leading from the thoracic spiracles have ever been traced; they are, perhaps, exceedingly delicate in their walls, and probably of no great length, and hence after death, when the air has left them, they would be exceedingly difficult to detect. There remains much, doubtless, to be yet dis covered in regard to these and other minute details of the internal organisation of spiders. The Nervous System. In the order A caridea, as it has been above observed, the nervous matter is concentrated into one large mass or ganglion, whence it branches out to the rest of the body. In the Pycnogonidca the nervous system consists of a longitudinal series of large ganglia, of which the four abdominal ones are sessile. In the Scorpionides, also, it consists of a longitudinal series of eight ganglia distributed throughout the whole body, and united by a triple nervous chord ; and now in the Araneidea (as in the Solpu- gidea, Thclyphonidca (Phrynides), and indeed in the Phalangidea also), we seem to have a compound of these two modifications. Thus, in the Araneidea the nervous matter is, in general, concentrated into two masses, one placed within the fore part of the cephalo-thorax, just above the oesophagus ; this (the cephalic or so-called cerebral) ganglion sends out fine nervous threads to the eyes, falces, and labium ; the other mass (thoracic ganglion) is situated just behind the former, and occupies a large portion of the thoracic cavity; it is of a flattened disc shape, and is united to the cephalic ganglion by short but broad connectors, forming a narrow collar round the oesophagus. This thoracic ganglion is formed by an approximation and soldering together of the different ganglia which exist sepa rately in the scorpions. A proof of this is furnished by the fact, 1 In a recent paper, "Ueber die Respirationsorgane der Araneen," by Philipp Bertkau," Arch. f. Nat., xxxviii, pp. 208-233, pi. 7, recorded, Zool. Record., ix. p. 206, which the present writer has not had an opportunity of studying, the author appears to have come to the same general conclusions on the respiratory system of spiders as those above mentioned, and also to have discovered, in some spiders, other orifices leading to tube-tracheae in front of the spinners. that in the young of some of the Araneidea, furrows of greater or less depth show the junctional points of the different ganglia. From the thoracic ganglion lateral nerves issue to the legs and palpi; besides which a fascia of nerves runs backwards through the pedicle into the abdomen, and these subdividing, branch out to all its different organs (Simon, I.e.) A modification of the above exists in the Theraphosides, where there is a special enlargement, or ganglion, just within the fore part of the abdomen, before the nervous fascia subdivides and branches out into that part of the body; pro bably this modification would be met with also in many other spiders (Cuvier s An. Kingdom, pi. 2, Arachnides ; and Treviranus, I.e., pi. 5, fig. 45). The Organs of Reproduction in the female consist of two long ovaria placed longitudinally within the ventral surface of the abdo men ; these unite and form a short broad oviduct, having an external opening of various form beneath .the upper end of the abdomen between the spiracular orifices, as before noted under the head of external structure. In the male the organs for the secretion of the seminal fluid consist of two long narrow convoluted tubes, occupying the same relative situation as the ovaria in the female, and also opening out wardly through a common but exceedingly minute orifice in a similar position. No intromittent organ has been discovered in any instance ; and it is not long since that the probable way in which the impreg nation of the female takes place (at least in some cases) has been at all certainly noted. It has been before remarked that the peculiar corneous lobes and spiny processes connected with the digital joint of the palpi palpal organs 2 in the male, are used in the course of copulation ; but araneologists were not (and probably are not now) agreed as to their real office in generation. A distinguished author, Mr John Blackwall, went so far as to prove, as he believed, by a series of carefully conducted experiments, that the seminal organs of the male spider, above described, are in some cases wholly unnecessary for the impregnation of the female at least so far as any external use or application is concerned and that the palpal organs are the efficient agents in the impregnation of the female spider (Report of Brit. Assoc. 1844, pp. 68, 69). This, however, seemed to be contrary to all reason and analogy, and led to the hypothesis that there was some communi cation by a duct or ducts between the spermatic vessels in the abdo men and the palpal organs (Hermann, 1868 ; Verhandlung d. ZooL Bot. Gesellsch. in IVien, xviii. pp. 923, et seq.) This idea, however, appeared to be negatived by the failure of Duges and other eminent anatomists to discover any duct in the palpus, where, if present, it would be comparatively easy to find. Another hypothesis grounded on the fact that the palpal organs have in some cases been observed to be applied repeatedly to the mouth between their numerous applications to the vulva was, that there might be some duct through which the spermatic fluid could pass into the alimen tary canal, and so be conveyed to the oesophagus, and thus be taken from the mouth by means of the above-mentioned application to it of the palpi. The discharge of the spermatic fluid in birds into the lower intestine appeared to support the possibility of this hypothesis being true (0. P. Cambridge, Trans. JV. Z. Instil., vol. vi. 1873, p. 190). Another opinion, long since, and even now, held by some arachnologists, is that the male spider collects the seminal fluid with its palpal organs into the concavity of the digital joint from the minute orifice above noted ; but no instance has ever been noticed in which a spider has been detected in such an employment of its palpi. An instance has been recorded in which there were during copulation (?) repeated embraces, and at each embrace a perfect apparent coition was effected between the sexual apertures of the male and female spider, the palpi not being used at all (0. P. Cam bridge, I.e., p. 191). Other arachnologists, again, have recently observed that the male spider before the act of copulation "emits from the sexual aperture a drop of sperma on a kind of small web made for the puipose, which drop he then takes up in the genital bulb of the palpi " (A. Menge, Ueber die Lebensiv. d. Arachn. p. 36; and A. Ausserer, Beob. ueber die Lebcnsw. der Spinnen, p. 194). This would appear to settle the question ; but while, at the same time, it is remarkable that an observer so accurate in observation and so painstaking as Mr Blackwall should never have observed male spiders taking the spermatic fluid from the abdominal aperture, as stated by Menge some thirty years ago, it is also remarkable that Mr Blackwall should unconsciously afford the strongest evidence in favour of the facts mentioned by Menge and Ausserer. Duges, in suggesting, long since, that the male spider took the seminal fluid from the aperture in the abdomen with its palpi, asked whether the palpal organs may not alternately act as an absorbing siphon and an ejaculatory organ. Mr Blackwall (Jour. Linn. Soc. , vii.) mentions this question of Duges, and then states a fact which at once answers the first part of it in the allirmative, and tends to confirm 2 These organs are not developed until the last moulting of the spider s skin, and their development (unmistakable when complete), is an unerring sign of the spider s sex aud maturity. Until this period the digital joint is more or less rounded and tumid, but it can even then

seldom be mistaken for that of a female spider.