Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/313

SPIDERS.] A RACK N IDA 293 spiders is very similar to that of the rest of the Arti- culata. The muscles are formed of numerous parallel fibres traversed by very marked striae ; these muscles are attached 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 maxilla 1 , and the coxse 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, orfovea. 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 yfcrors, 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 spinner s and the anus. The fore half of these ligaments is cartilaginous, while the hinder half is composed of con tractile 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 back wards 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 (oesophagus) leads from the mouth, and from it ten lateral branches or cteca (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 of second or lower stomach into which the juices form ing 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 intes tine, which is much contracted in size in passing through the junc tional pedicle into the abdomen, but enlarges a little within the L .t ter ; 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 secreted; the bile passing by means of biliary ducts (into which urinary organs A!SO pour their contents) into the rectum. Digestion is effected by means of a gastric juice secreted by appropriate glands between the literal cceea, and thence poured into the stomach (Simon, I.e., after I uges). Walckenaer, Ins. Apt., following Treviramis, gives a slightly different account of the digestive apparatus of Tegenaria domestica Valck.) Probably a certain amount of variation would be found in the details of form and structure of the stomach and intestinal canal, nd 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 0. P. Cambridge in Entomolo gist, May, 1870, pp. 65-67; and^?i. and Man. N. H., Dec. 1872 ; and F. Pollock, Id., Oct. 1872. Organs of Circulation. As in some other groups of Arachnids a strong muscular tube or vessel (divided into four chambers) 1 runs J Claparede (Etudes sur la circulation du sang chez Ics Aranees du Lycosa), iu describing the dorsal vessel of a young Lycosa, says along beneath the dorsal integument of the abdomen, following the integumental curve ; this dorsal vessel (or heart) varies in form ia different species, and is enveloped in a fine fibrous tissue, which forms a pericardium. 9 The several chambers of the heart communicate with the pericardium by means of small elongated orifices (auriculo- veutricles) 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 nourish ment to those parts, pass over the stomach ; and the third are imbedded beneath that organ, and emit arteries to the legs, extend ing to the very extremity of each. These three pairs of arteries reunite forwards and form a collar round the cerebral ganglion ; and from thence a tube a abdominal or posterior aorta passes back wards under the ganglia through 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 4 penetrates the branchial organs, and being there reoxygenated, is carried back to the pericardium by four brancho- cardiac vessels ; from thence it passes into the dorsal vessel or heart through the auriculo- ventricles 5 above named. Muscular contrac tions and dilatations appear to be the means by which the fluid ia propelled through the sinuses that bring it to the branchial organs for oxygenation (Simon, I.e., 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-branchiiB from the trachere, falls to the ground ; the more especially since, whatever the difference may be between the ordinary tracheaj and the pulmo-branchire, both are found in the same individual, in the order we are now considering Araneidea. Orga.ns of Resjnration. 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 trachea?. Such difference as there is, and it is indeed an important one, seems to be this, that whereas the trachea? 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-branchiaj are modified tracheae, localising, 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 organisa tion than is possessed by those Articulata furnished with merely simple trachea; ; it is a progress from a diffused to a localised system, but still it seems essentially distinct from the lungs of vertebrates, in which the Mood 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-branchirc 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- branchiae, since there is not only no real analogy to the lungs of a that at that age no such chambers exist, and questions whether or not the chambered form such as Newport and Blanchard have described in the scorpion, does or does not exist in the adult Lycosa. Vide supra, p. 285, on the heart of the scorpion. 2 The existence of this is also questioned by Claparede, who speaks of the heart as placed " not in a distinct organ pericardium but in a certain indefinite space lacune pericardique," (I.e.) 3 Claparede could not find this in Lycosa, (/. c.) 4 Claparede (1. c. ) says, on the contrary, that the blood " never pene trates between the leaves (feuillats) of the respiratory organs," but is reoxygenated in its close passage by them. 5 Claparede says that all the fluid which comes from the respiratory

organs passes into the heart through the -foremost pair of these openings.