Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/295

ACARIPS.] ARACHNIDA 275 Anchorarii, found in aquatic species, and used for retaining their position by means of terminating points. 3. Fusi- formes, without any claw on the last joint. 4. Filiformes, as their name implies, filiform, or not at all tumid. 5. Antenniformes, chiefly differing from the last in their greater length. G. Valvoiformes, flattened, or excavated, or sheathed in form. 7. Adnati, or united throughout the greater part of their length to the labium, and always slightly developed. The eyes in very many Acaridians are entirely wanting; When present they vary in number, being generally two, four, or six, and are placed on the cephalo-thorax ; some times, as in the family Hydrachnides, they apparently con sist of mere spots of pigment beneath the cuticle; 1 in some other cases there is but one eye, which is composed of a varying number of small facets. INTERNAL STRUCTURE. The Organs of Digestion con sist, in many species, of a short intestinal canal, which branches out into lateral caeca, " a sapartie 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. 1845, torn. iii. p. 14) show in two families (Hydrachnides 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 (I.e., p. 17) speaks of trachea?, in the family Trombidides, having an external orifice at the base of the falces on tlieir upper side, the use of these tracheiB 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 (occasioned, as we have seen, by the almost complete fusion of the head and thorax in the abdomen) extremely simple. In the families Trombidides and Acarides, 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 improbable 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 of circulatory organs have been found in such of the lower acarids as have been subjected to minute dissection, and hence the supposi tion 2 that the intestinal canal, by means of muscular move ments 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-viviparcus ; in the 1 Rymer Jones, An. King., 2d ed. p. 411. 2 See Packard s Guide to the Study of Insects, p. 627. 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 herma phrodite, but this, though true of the Pentastomides, is uncertain in respect to others of this order. Partheno genesis, 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, how ever, 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 Claparede s Studies on Mites (Studien ai> Acariden), and Siebold in Kulliker s Journ. Sc. Zool., 1868, part iv. GENERAL OBSERVATIONS. Acaridiaus are universally distributed ; they are to be found under stones, dead leaves, or bark of trees, in the ground, in water, in unrefined sugar, upon dried meat, fruits, cheese, and putrid animal matters, upon all of which they feed. Some are parasitic, both externally and internally, in the flesh of different animals, living upon the juices of the creatures they infest. One species, Sarcoptes Scabiei, is the cause of tha disease called the " itch ; " another is our troublesome "Harvest- bug." Some are parasitic upon insects of different kinds ; others are said to have been found in the brain and eyes of man. All are small, great numbers almost microscopic ; the smaller kinds, particularly of the Acarides, are very difficult to preserve as cabinet objects, and from this, as well as from their minuteness, the order, generally, does not receive that attention to which, from their diversified forms and modea of life, Acaridians are certainly entitled. From various considerations of structure, the Acaridea may be divided into ten families, each containing one or more genera. These families may shortly be characterised as follows : Fam. 1. Pcnlastomides. Body annulate, vermiform, lancet-shaped ; the segments of the cephalo-thorax continuous with the body, aud furnished with four strong claws or rudimentary legs; this appears (Rolleston, Forms of Animal Life, p. 118) to be the larva or imma ture state, the adult form being destitute of limbs (which are replaced by four hooks two on each side of the mouth), and reversing the ordinary progress of many acarids, from a fewer to a larger number of legs ; organs of respiration and circulation, none (Huxley). On this abnormalfamily, of which one genus and eighteen species have- been described, see Cobbold s Entozoa, pp. 393-402, and authorities there quoted. (See figs. 1, 2.) FIG. 1. Pentattoma tcenioitks, Rud. FIG. 2. The savne, in the i

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