1902 Encyclopedia > Arachnida > Arachnida - Order V: Scorpionidea - Sub-Order II: Scorpiones - Internal Structure

Arachnida
(Part 18)



(18) ARACHNIDA - ORDER V: SCORPIONIDEA - SUB-ORDER II: SCORPIONES - INTERNAL STRUCTURE

INTERNAL STRUCTURE. - The Muscular System of scorpions is similar to that of other Arachnids. It is well detailed by L. Dufour (Sacants etrangers, xiv. pp. 609-611), who divides the muscles into thoracic, abdominal, and caudal; those of the abdomen being also subdivided into tegumentary, perforant, and cardiac.

The Organs of Digestion consist of a straight narrow intestine, with little or no dilatation, running from the mouth to the anus, which has its external opening at the lower extremity of the fifth or last segment of the tail. At the junction of the stomachal and intestinal portions the biliary vessels are inserted, and from each side of the former (stomachal part) there issue five narrow caeca running into a mass of fatty matter on either side.

The Respiratory Organs, or pulmo-branchiae, have their external orifices or stigmata (as before mentioned) in four pairs on the ventral surface of the abdomen; the pulmo-branchiae, of which one is connected with each stigmatic opening, are hollow sacs, the walls of which are folded into delicate laminae, each being duplex, and all lying one upon another like the leaves of a book; the air is admitted to these through the external openings, which are closed by movable horny lips as in insects. (Jones, Animal Kingdom, 2d ed., 416). According, however, to L. Dufour (l.c. p. 617), the lips of the stigmata are immovable, and have a varied direction in different species. The air is admitted to the pulmo-branchiae by the action of a linear opening in supple membrane or diaphragm within the space between the lips and the laminae (L. Dufour, l.c.)

Organs of Circulation. - These, according to Newport (Philos. Trans. 1843, pp 286-298), consists of an elongated, dorsal muscular vessel (or heart) divided into eight chambers, separated from each other by valves, and with auricular openings or valvular orifices, at the division of each chamber, on its dorsal surface. From both sides, and at each end of the dorsal vessel, arteries convey the vital fluid for distribution to different parts of the body.

This dorsal vessel apparently acts in a manner analogous to that of the heart in vertebrate animals. The vital fluid is conveyed back to the "heart" from the pulmo-branchiae, where it has been oxygenated by means of a great number of slender canals, formed by the collection together of vessels that have their origin at the hinder part of the inner side of the pulmo-branchiae. These canals pass round the sides of the body in the hinder part of each segment, and pour their contents into the dorsal vessel through the valvular orifices above mentioned. The conveyance of the fluid from the general circulation to the pulmo-branchiae is effected by means of an intricate system of anastomosing pulmonic and capillary vessels, situated chiefly below the nervous chord on the ventral surface of the body.

From the above L. Dufour (l.c.) dissents. According to his researches, which appear to have been pursued under the great advantage of having fresh as well as living examples to observe and dissect, the heart (or dorsal vessel) is not divided into chambers at all; its chambered appearance, which misled Newport and others, arising from inevitable changes in the form of the organ after death, and long immersion in spirit of wine. Dufour gives very exact figures and descriptions of these changes. According to him the heart is a simple, undivided vessel running throughout the whole body, only narrowing at the fore part of the caput, and in its passages through the tail; and from this tube various lateral veins and arteries issue, those from the hinder or tail portion being profusely branched. The dissents entirely from Newport, Blanchard, and others, with regard to the mode above mentioned in which the vital fluid is conveyed to and from the pulmo-branchiae, but he confesses that he has not been able to find out how it is actually effected, though he believe that the pulmo-branchial laminae are permeated by vascular ramifications spread out over every one of their leaves, and that in these ramifications the fluid receives the benefit of the air admitted to the laminae. Siebold, however, says that no traces of blood vessels have been found in the pulmonary lamellae of another order, Araneidea; and it is hard to believe but that, if such details as those given by Newport in regard to the scorpion actually existed, Dufour, with the advantage of fresh examples to dissect, should have been unable to discover it. On the whole, it seems too soon, even yet, to dogmatise on the position of the scorpions from their respiratory and circulatory systems. That they have these two systems, the first by means of a dorsal vessel, with lateral branches the latter by a kind of pouch containing a modification of tracheae, is certain; and that their system is different from that of some other groups is also certain, but in the absence of a more perfect knowledge of the real nature of the difference, its true value cannot well be yet ascertained.





Nervous System. - A longitudinal row of nine - according to Newport, eight - ganglia, connected by a double nervous chord, is situated in the median line of the body, below the intestinal canal. The first ganglion consists of two lobes, the upper one of which, occupying a position somewhat analogous to that of the brain in the Vertebrata, is so called (cerveau) by Dufour. From these lobes the optic and other nerves issue, some of them being connected with the various parts of the mouth and the legs. A nervous collar, according to some authors, surrounds the oesophagus. Dufour, however, disputes the existence of this collar, and explains how the idea of it arose, l.c., pp. 556-557. Of the remaining ganglia four are, according to Dufour, situated in the abdomen, and from these four there spring on each side and underneath, branching nerves, which run to each of the pulmo-branchiae respectively and their adjacent parts, as well as to the viscera and tissues of the abdomen. The first abdominal ganglion is in close connection with the large thoracic one, and is in some instances so little developed that Newport others, overlooking it, derived the nerves running to the first pair of pulmo-brancheae from the thoracic ganglion itself, and thence allowed but three to the abdomen.

The caudal ganglia four in number, do not correspond to the number of segments, which are five, or counting the terminal bulb, six. From each caudal ganglion two lateral nerves only issue, except from the last, whence there issue three pairs; the extra ones, running back, and branching out into the muscles of the poison bulb, doubtless add greatly to its sensibility and irritability.

The Organs of Reproduction occupy, both externally and internally, a similar position in both sexes. The external aperture is duplex, situated between the first and second segments of the abdomen, in front of and, in a manner, between the comb-like appendages, and covered by an operculum. Internally those of the male consist of a duplex set of vessels for the elaboration, collection, and conveyance of the seminal fluid to the external orifices, each orifice having its own set. The vessels, answering to testes, in which the seminal fluid is secreted, consist each of a tube forming three large quadrilateral, flexuose, and free, anastomosing meshes placed longitudinally; those of one set sometimes communicating with the other at the lower mesh by a short, strong connecting tube. According to Treviranus the three meshes on either side anastomose with each other, having, in fact, a common connection throughout, but this L. Dufour considers to be an abnormal case; the vessels for the collection of the seminal fluid, versiculoe seminales, are three for each set, and all open into he channel of emission; this is produced backwards in the form of a long, fusiform, sheath-like channel, lying along the flanks of the abdominal cavity, and attenuated at both extremities; within this channel is another slender, corneous sheath-like organ, prolonged to the genital aperture, where it is exceedingly fine. This is "le fourreau et l'armure de la verge" (Dufour), and within it is the intromittent organ itself, having a whitish cartilaginous, thread-like appearance. No recorded observations have yet decided the mode in which these parts of generation are used: the Scorpionides being nocturnal in their habits makes such observations very difficult, but all analogy is in favour of copulation by the exsertion and introduction of the thread-like organs above described into the female parts of generation. These consist of two ovaria, each composed of a membranous tube, forming four large quadrilateral meshes in a longitudinal line on each side, anastomosing with each other and with those of the other side; each of these ovaria ends in a simple oviduct (sometimes with, sometimes without ovisacs) leading to the vulva or external orifice. The general similarity of these female organs with those of the male is at once apparent. A curious point noticeable in these parts in the Scorpionides is their duplex character, and hence the question as to their mode of operation becomes of greater interest. It is remarkable that the only other Arachnids in which an intromittent organ is as yet known are among the Phalangides, a group widely separated from the Scorpionides in other characteristic details, though approaching much nearer to the pseudo-scorpions. Scorpions are ovo-viviparous, and, according to Dufour (l.c.), their period of gestation is of great length, extending even to fifteen or sixteen months; but for five of these months, which occur in the winter season, Dufour concludes, from frequent observations, that the whole genital apparatus is, like the animal itself, in a dormant state, and that therefore no progress in the development of the ova takes place during this period. On he very important branch of this subject - the embryology of the scorpion- reference only can be made to the works of Heinrich Rathke, Reisebemrkungen aus Taurien (contained also in Burdach's Physiology, Bd. Ii. p. 242, et seq.) and to thay of Dr Elias Metschnikoff, "Embryologie des Scorpions," Z. Wiss. Zool., Bd. Xxi pp. 204-232, taf. Xiv-xvii.; also separatre, Leipzig, 1870.





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