(6) ARACHNIDA - ORDER I: ACARIDEA - INTERNAL STRUCTURE
INTERNAL STRUCTURE . - The Organs of Digestion consist, in many species, of a short intestinal canal, which branches out into lateral caeca, "a sa partie stomachale," and has an anal orifice on the lower side of the abdomen, more or less near to its posterior extremity (Walck, Ins. Apt. iii. 135). The researches of Dujardin (Ann. Sc. Nat. 184445, tom. iii. p. 1444) show in two families (Hydranhnides and trombidides) a curious modification of these organs. In this, the juices upon which these creatures exist are sucked in, apparently, into simple cavities in the substance of the body; these cavities are without walls of any kind, and from them the juices circulate through the body, and thus form its support.
The Organs of Respiration consist generally of tracheae, communicating outwardly with the air by means of minute orifices, "stigmata," which, in the genus Oribates, are situated between the first and second pairs of legs. In one numerous family (Acarides) no special organs for breathing exist, respiration being apparently effected by the general surface of the body. Different genera exhibit these two modes of respiration in various forms of union. M. Dujardin (l.c. p. 17) speaks of tracheae, in the family Trombidides, having an external orifice at the case of the falces on their upper side, - the use of these tracheae being for expiration, while inspiration is performed through the general surface of the body and the plumose hairs attached to it, or, as in the family Hydrachnides, by "stomata," i.e. apertures covered with very delicate membrane.
The Nervous System in Acarids is ganglionic, as in the rest of the Articulata; and, as we should expect from the simple form of the body (occasional, as we have seen, by the almost complete fusion of the head and thorax in the abdomen) extremely simple. In the families Trombidies and Acaride, and probably in the rest also, the nervous apparatus consists of :one large globular ganglion, from which nervous filaments are given off, both before and behind."
With regard to the Circulatory System in the Acaridea too little, apparently, is yet known to make it possible to speak certainly. It is highly simprobable but that, in some of the higher groups, distinct organs exist, though such do not yet appear to have been discovered. The results hitherto are negative; no traces circulatory organs have been found in such of the lower acarids as have been subjected to minute dissection, and hence the supposition that the intestinal canal, by means of muscular movements and contractions, operates in the irregular propulsion of the vital fluid to various parts of the body.
The Reproductive System of the Acaridea is very simple. The external organs consist of an opening on the ventral surface, generally between the coxae of the hinder pair of legs Acarids are both oviparous and ovo-viviparous; in the latter case the young are produced through a large orifice or vulva nearly one-third of the length of the body, and closed by two valves. Some are supposed to be hermaphrodite, but this, though true of the Pentastomides, is uncertain in respect to others of this order. Parthenogenesis, however, certainly exists in some species. The ova appear commonly to be produced in the substance of the general tissue of the body without the presence of any ovarian apparatus with distinct walls; it is certain, however, that ovaries are present in some species-in the family trombidides, for instance, in which a tubular double branched ovarium was discovered by Dujardin. Some of the Acaridea, as Tetranychus, produce silk, and spin webs, but the silk-secreting organs have yet to be discovered; neither do any external organs, such as spinnerets, appear yet to have been noticed. The development of Acaridians from the egg is a subject of great interest and importance, for which, see Claparedes's Studies on Mites (Studien an Acariden), and Siebold in Külliker's Journ. Sc. Zool., 1868 part iv.
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Arachnida - Table of Contents