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Show 180 ME. A. D. MICHAEL ON AN [Mar. 5, is also occasionally an irregular central protoplasmic mass, and the whole is joined by delicate threads. It seems not improbable that the absence of chitinization from the exterior tunic of these glands may be correlated with the much greater chitinization of the external cuticle in Thyas than in Hydrodroma. Schaub and Haller appear to have found that the mouth of each of these glands was surrounded by a thick ring of chitin, and was in connection with a more or less triangular chitinous sclerite bearing a small spine, which may be regarded as protecting the opening ; neither of these conditions, however, is to be found exactly in Thyas petrophilus. The dermal glands of this species discharge to the exterior either through a largish central hole in one of the numerous smaller chitinous plates in the cuticle (fig. 25, cs.), each of which plates bears a small hollow spine (ps.), or else at the edge of a plate, usually in the former manner ; the sclerite is, however, distinctly a plate with numerous pores, which bears both the hair and the mouth of the gland; there is not one solid ring-like ridge surrounding the mouth and another triangular ridge supporting the hair. I have not been able to make certain of any really definite connection between the dermal glands and these smaller dermal plates, as it seems to m e that the number of plates does not agree with the number of glands, and that some of the plates have not the central opening ; it is, however, extremely difficult to be absolutely sure on this point. In a few instances I have found near where the duct emerges a minute and extremely delicate membranous sac within the gland, which sac contains an almost globular structure formed of open irregular network, which stains deeply (fig. 26). The Alimentary Caned and Excretory Organ (Plate VIII. figs. 14 15; Plate IX. figs. 23, 27). I join these two systems, because, in effect, it is impossible properly to separate them in the Hydrachnidae, and indeed in some other families of the Acarina, e. g. the Gamasidae. The alimentary canal in Thyas petrophilus differs considerably from everything which, to m y knowledge, has been described in the family, or indeed in the Acarina at all; although undoubtedly it is a modification of the same general plan. At the entrance from the mouth to the pharynx I find organs which I suppose are those described by Schaub as " palpenartige Gebilde "; I a m not, however, able to regard them as of the nature of palpi; they seem to me, in m y species at all events, to be small masses destined either to fit together very closely, and indeed to interlock, and thus form a valve closing the entrance to the pharynx, or else to be separated at the will of the creature, thus completing the pharynx in its office as a sucking apparatus. The pharynx itself with its muscles (figs. 24, 27, ph.) has the same lanceolate form shown by Schaub in Hydrodroma, and has an average length of about -15 mm., by a breadth, in its |