1902 Encyclopedia > Arachnida > Arachnida - Order VII: Araneidea - Internal Organisation

Arachnida
(Part 25)



(25) ARACHNIDA - ORDER VII: ARANEIDEA - INTERNAL ORGANISATION

INTERNAL ORGANISATION. - The Muscular System of spiders is very similar to that of the rest of the Articulata. The muscles are formed of numerous parallel fibres to the different portions of the tegumentary system, and move the various parts of the body by the contraction of their fibres. Those which set in motion the falces, the maxillae, and the coxae of the legs, are attached to the thorax, and radiate to all these parts from a centre, marked externally by what has been before spoken of as the thoracic indentation, or fovea. The other joints of the legs move, one after the other, by means of small and exceedingly fine muscles, of the two kinds ordinarily known as extensors, to raise the joint, and flexors, to lower it. All these muscles act with great rapidity, and enable the legs to move in every direction according to the mode in which the various joints are articulated. The differences in the relative strength and volume of the muscles are very great; those, for instance, by which the fangs of the falces are moved are exceedingly powerful; the abdomen being generally soft, its integument does not furnish such firm points of attachment as that of the thoracic region, and yet the spider can move its abdomen with the greatest ease in every direction. Beneath the integument, at the fore part of the under side, two ligaments are attached to a membranous plate, situated under the branchial operculum, and at the hinder part, round the ring which encircles the spinners and the anus. The fore half of these ligaments is cartilaginous, while the hinder half is composed of contractile muscular fibres, and by means of those muscles the spider moves its abdomen. Two other pairs of muscles may also be particularly mentioned, one pair inserted under the membrane in the junctional pedicle, the other leading round the respiratory organs: the office of these is to open or close the branchial opercula, and to draw the sexual organs backwards or forwards. The spinners are moved by special muscles similar to those of the legs. (Simon, Hist. d. Araign.)

The Organs of Digestion consist of a large sac, or stomach placed in the cavity of the cephalo-thorax, and fastened to its arch by a strong muscle; this muscle passes round and through the stomachal sac; into the stomach a tube (lesophagus) leads from the mouth, and from it ten lateral branches or caeca (five on either side) issue, passing down the sides to the origin of the legs, and, curving round, end in a reservoir placed below the stomach. This reservoir is connected with the stomach by the muscle mentioned above, and divided from it by a solid plate into which this muscle is inserted; it, in fact, forms a kind or second or lower stomach into which the juices forming the spider's food are forced from the upper one by muscular action.

Out of the stomach the alimentary juices go into a tube or intestine, which is much contracted in size in passing through the junctional pedicle into the abdomen, but enlarges a little within the latter, this tube curves over for some distance in a line parallel with the dorsal integument; it then drops down suddenly, forming a large loop, and so passes into a large rounded pouch (rectum), in which the undigested portions of food collect, and thence pass out through the anus, situated, as before observed, at the extremity of the abdomen just above the spinners. The abdominal intestine is imbedded in a mass of fatty globules, from which bile is secrted; the bile passing by means of biliary ducts (into which urinary organs also pour their contents) into the rectum. Digestion is effected by means of a gastric juice secreted by appropriate glands between the lateral caeca, and thence poured into the stomach (Simon, l.c. after Duges).

Walckenaer, Ins. Apt., following Treviranus, gives a slightly different account of the digestive apparatus of Tegenaria domestica (Walck.) Probably a certain amount of variation would be found in the details of form and structure of the stomach and intestinal canal, and other organs of digestion, in different species; but the general plan of the digestive arrangement is the same. It is an undecided question whether the oesophagus of spiders can pass any substance except the juices of their prey. See O.P. Cambridge in Entomologist, May, 1870, pp. 65-67; and Ann. and Mag. N.H. Dec. 1872; and F. Pollock, ld., Oct. 1872.

Organs of Circulation. - As in some other groups of Arachnids a strong muscular tube or vessel (divided into four chambers) runs along beneath the dorsal integument of the abdomen, following the integumental curve; this dorsal vessel (or heart) varies in form in different species, and is enveloped in a fine fibrous tissue, which forms a pericardium. The several chambers of the heart communicate with the pericardium by means of small elongated orifices (auriculo-ventricles) placed at the constrictions where the chambers are separated from each other; through these orifices the vital fluid passes into, but cannot pass out of, the heart. This latter operation is effected by the contraction of the heart, which forces the fluid forwards into a tube or artery issuing from the first chamber; this tube-aorta-passes through the junctional pedicle and entering the thorax, divides into three pairs of arteries. The upper pair follow the dorsal line and give off blood vessels to the eyes, the falces, and other parts of the mouth; the second pair, intended to give nourishment to those parts, pass over the stomach; and the third are imbeeded beneath that organ, and emit arteries to the legs, extending to the very extremity of each. These three pairs of arteries reunite forwards and form a collar the cerebral ganglion; and from thence a tube-abdominal or posterior aorta - passes backwards under the ganglia though to the abdomen, continuing quite to the spinners; this tube furnishes numerous branches to the organs which it passes in its course. From the general circulation the vital fluid is brought into two large longitudinal reservoirs within the lower face of the abdomen, by means of sinuses or canals formed by the interstices of the muscles; these anastomose and pour their contents into the reservoirs just above mentioned; from them the fluid penetrates the branchial organs, and being there reoxy genated, is carried back to the pericardium by four branchocardiac vessels; from thence it passes into the dorsal vessel or heart through the auriculo-ventricles above named. Muscular contractions and dilatations appear to be the means by which the fluid is propelled through the sinuses that bring it to the branchial organs for oxygenation (Simon, l.c., after Emile Blanchard, and Duges).

As has been already observed in several notes, there is some difference between the above account of the circulatory system and that given by Claparede; probably the organs of circulation vary somewhat in different species. One point of difference, however, between the above and Claparede's description is of importance, that is, the difference which is mentioned in note (4) below. The idea that the blood "penetrates" the branchial organs may have been led up to by the apparently unfounded, but confidently pronounced opinion that these organs were true analogues of the lungs of the Vertebrata; whereas, if the blood does not penetrate them, but is merely oxygenated in its general passage by and round them, their real tracheary nature (as seems to be the better opinion now) is strongly confirmed; and hence the hitherto widely -received division of Arachnids into pulmonary and tracheary, based upon the idea of the distinct nature of the pulmo-branchiae from the tracheae, falls to the ground; the more especially since, whatever the difference may be between the ordinary tracheae and the pulmo-branchieae, both are found in the same individual, in the order we are now considering-Araneidea.

Organs of Respiration. - From what has just been said, it seems hardly proper to base any definition of the respiratory organs of spiders on the supposed essential distinction between the pulmo-branchiae and tracheae. Such difference as there is, and it is indeed an important one, seems to be this, that whereas the tracheae are lengthened tubes, which convey the air in very small volumes to different parts of the body, and so oxygenate the vital fluid in its passage, the pulmo-branchieae are modified tracheae, localizing, so to speak, within a peculiarly furnished sac, a considerable volume of air in the immediate course of the large sinuses, in which the used-up vital fluid is re-collected previous to its return to the dorsal vessel. This arrangement, no doubt, marks a higher state of organization than is possessed by those Articulate furnished with merely simple trachea; it is a progress from a diffused to a localized system, but still it seems essentially distinct from the lungs of vertebrates, in which it seems essentially distinct from the lungs of vertebrates, in which the blood is carried to the air and permeates every portion of the air-vessels, and not the air to the blood, as is really the case in regard to both the pulmo-branchiae and tracheae of spiders. It is not a matter, perhaps, of prime importance, but if the above views be correct, it would seem better to relinquish the name of pulmo-brancheae, since there is not only no real analogy to the lungs of a Mammal, but just as little to the gills of a fish. We would propose, therefore, to distinguish these modified and localized tracheal sacs by the name of sac-teacheoe, the ordinary ones being designated tube-tracheoe .

In some spiders, Theraphosides (including the giant spiders of the family better known as Mygalifes), the respiratory organs are solely sac-trachea; these are in this case four in number, and placed in two paitrs oin 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 sactracheae, an orifice leading to tube-tracheae. In some few spiders a kind of supernumerary spiracular slit or opening is visible near to the ordinary one, but nopt always similarly placed. A pair of others are also visible closed behind the sexual aperture in Cambridgea fasciala (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 undesdcribed species of Drassides, and some also exist in the well-known European spiders, Argyroneda aquatica. 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-branchieae in the family Scorpionides. The following short description is abstracted from E. Simon, l.c., following Wmile Blanchard: - "The respiratory apparatus is contained in sacs beneath the stigmata; these sacs are globular and flattened, formed by a silvery white membrane. Each of them contains 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 communication with the outer air admitted through the spiracular oridices. The walls of these numerous saccfirom 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 movement of this organ, idastile or sustole, 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, l/c/ after E. Blanchard).

According to Treviranus, l/c/ (vide Jones, Anim, Kingdom, p. 416), whom 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 side of the abdomen, and a pair on the under side; the former undoubtedly 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 even 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 remain much, doubtless, to be yet discovered in regard to these and other minute details of the internal organization of spiders.

The Nervous System. - In the order Acarideae, 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 Pycnogonida 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 Solpugidea, Thelyhonidea (Phrynides), and indeed in the Phalangidea also), we seen to have a compound of these two modifications. Thus, in the Araneidea the nervous matter is, general, concentrated into two masses, one placed within the fore part of the cephalo-thorax, just above the osesophagus; 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 separately in the scorpions. A proof of this is furnished by the fact, 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 issues 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 , l.c.) 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 out into that part of the body probably this modification would be met with also in many other spiders (Cuvier's An. Kingdom, pl. 2, Arachnides; and Treviranus, l.c. pl. 5, fig. 45).

The Organs of Reproduction in the female consist of two long ovaria placed longitudinally within the ventral surface of the abdomen; 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 orifiuces, 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 outwardly through a common but exceedingly minute orifice in a similar position. No intromitten organ has been discovered in any instance; and it is not long since that the probable way in which the impregnation 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-in the male, are used in the course of copulations; 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 communication by a duct between the spermatic vessels in the abdomen and the palpal organs (Hermann, 1868; Verhandlung d. Zool. Bot. Gesellsch, in Wien, xviii pp. 923 et. seq.) This idea, however appeared to be negatively by the failure of Duges and other eminent anatomists to discovery 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 application to the vulva- was, that there might be some duct through which the spermatic fluid could pass into the alimentary 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 (O.P. Cambridge, Tran B.Z. Insitit., 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 to 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 (O.P. Cambridge, l.c. 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 purpose, which drop he then takes up in the genital bulb of the palpi" (A. Menge, Ueber die Lebensw. D. Arachn. P. 36; and A. Ausserer, Beob. Ueber die Lebensw, 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 made 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 affirmative, and tends to confirm Menge's and Ausserer's statement: "A male Agelene labyrinthica confined in a phial, spun a small web, and among the lines of which it was composed I perceived that a drop of white milk-like fluid was suspended; how it had been deposited there I cannot explain, but I observed that the spider, by the alternate application of its palpal organs, speedily imbibed the whole of it."

That the spermatic fluid is conveyed to the female parts of generation by the male palpi is thus pretty certain; and it seems not improbable that there are more modes than one by which the palpi are supplied with the fluid. See also critical observations on this subject by Dr. T. Thoreel, 1873, Synonyms of Europ. Spid. Pp. 591-595.






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