(940). The arrangement of the digestive apparatus in the Acaridans is one of the most interesting points in the economy of these creatures. Behind the mouth, M. Dujardin1 was able to detect, in Trombidium and Limnochares, a cylindrical pharynx, with distinct parietes, into which are implanted numerous muscular fibres calculated to assist in the operation of suction by dilating the pharyngeal cavity; but posterior to this no traces are perceptible of either oesophagus, stomach, or intestine, so that, apparently, the juices of organized bodies, which constitute the sole food of these creatures, must be lodged in lacunary spaces, destitute of any proper walls, in the middle of a brown parenchymatous mass, which probably performs the functions of the liver. The lacunae, into which nourishment is thus received, must necessarily be prolonged amongst the tissues and in the interspaces between the muscular fasciculi throughout the entire body, thus replacing altogether the circulating fluid; and even when living specimens of such genera (Dermanyssus, Gamcisus, Bdella) as are sufficiently transparent are submitted to examination under the microscope, although it is easy to see that the blood or other nutritive juices upon which the creatures live, and with which their bodies are filled, occupies a lobed or symmetrically multifid space, there is no appearance of any canal possessing distinct walls, but the whole seems diffused through lacunae that extend even into the bases of the legs.

* Ann. des Sci. Nat. 1834. 1 Id. 1845, t. iii. p. 14.

(941). The Acari, however, possess an anal orifice, through which excrementitious matter undoubtedly issues; nevertheless, on examining this excrementitious substance, it appears rather to present the characters of a secretion, as, for example, in the case of the genus Uropoda, where it becomes consolidated on exposure to the air into a little horny stem, upon which the creature is attached as upon a pedicle. It might therefore, as M. Dujardin observes, be possible to conceive this kind of digestion in a mass acting much in the same way as the glands upon the nutritive juices submitted to their action.

(942). In the most simply organized Acaridans, such as Acarus and Sarcoptes, no traces of any respiratory apparatus are discoverable, and respiration seems to be entirely effected by the general surface of the body. In Ixodes, Gamasus, and other Acaridans furnished with forci-pated mandibles, on the contrary, numerous elegantly-ramified tracheae, of which the larger trunks are distinguishable by a spiral filament, resembling that exhibited by the tracheae of Insects, are dispersed through the body. These respiratory tracheae communicate externally through the medium of minute stigmata, which in Oribates are situated between the first two pairs of legs.

(943). Between these two extremes in the development of the respiratory system of the Acaridans, numerous intermediate grades exist in different genera; and, in some, M. Dujardin* has pointed out a mixed kind of respiratory process very different from anything as yet observed among articulated animals; this consists in a system of tracheae terminating at a respiratory mouth situated at the base of the mandibles, and serving only for expiration, while inspiration is effected by the general integument and its appendages.

(944). To render intelligible this phenomenon, it will be necessary to lay before the reader the description given by M. Dujardin of the respiratory (or, rather, expiratory) apparatus as it exists in Trombidium. It is as follows: - At the base of the mandibles superiorly is seen an oblong orifice, bounded by two lips, the structure of which is altogether remarkable: it is a perforated eminence (bourrelet reticule a jour), the internal cavity of which communicates with two large tracheal vessels which run parallel to each other from behind forwards to this orifice. Each of the tracheal trunks, at a little distance from the orifice, suddenly divides itself into a tuft of tubular tracheae, which are without any internal spiral filament, and which are distributed without any ramifications throughout all parts of the body. On observing a living Trombidium, it is seen frequently to agitate its mandibles as though to produce some movement of the air contained in the respiratory apparatus, and if at the same time a little water be placed upon the respiratory orifice, it is sometimes seen to become inflated with little bubbles of air.

* Loc. cit. p. 17.

(945). On dissecting a Trombidium, there is seen beneath the integument, which is covered with plumose hairs, an elegant network, made of a diaphanous substance of homogeneous appearance, which appears to be in relation with the plumose appendages of the integument, and in concert with them serves to absorb the gaseous elements that are subsequently emitted externally through the tracheal orifices.

(946). In Hydrachna and the aquatic Acaridans, the expiratory system is similar to that which exists in Trombidium,, only that, instead of plumose hairs upon the surface of the body, there are stomata something resembling those of plants - that is to say, apertures covered over with a very delicate membrane, beneath which is a subcutaneous network, such as exists in the terrestrial species.

(947). The nervous system in Trombidium, and probably of the other Acari, presents a very remarkable arrangement, consisting entirely of a single large globular ganglion, from which nervous filaments are given off, both before and behind. The researches both of Trevi-ranus and M. Dujardin deny the existence of anything like a supra-cesophageal ganglion or nervous collar around the oesophagus.

(948). The eyes of the Acaridans are generally four in number, sessile, and approximated together in pairs upon the dorsal surface of the cephalothorax. In some cases, however, the eye is solitary, and composed of eight or ten minute facets.