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Show 188 ME. A. D. MICHAEL ON AN [Mar. 5, always occur; when they do they are not usually bilaterally symmetrical, or probably in any way permanent; they often are found to a lesser degree in other parts of the sac, and evidently provide for considerable extension and contraction of the lumen of the organ. The folding sometimes is found in directions where it would probably not result from dorso-ventral compression. In regard to the homologies of the alimentary canal and excretory organs, it may not be immaterial to remember that Wagnerx has lately found that in the embryo Ixodes the so-called Malpighian vessels are formed from the endoderm quite separately from the proctodeum, and only become connected with that organ in the latest stage of development. The histology of the alimentary canal does not present any features varying sufficiently from what has been described by other authors to make it necessary to notice them. The tunica propria is particularly clear and well marked. The lumen of the ventriculus is large, its walls composed of more closely-placed cells, forming a more even layer than is usually found in the ventriculus of Acarina; the cells are large but not so loose nor rounded as in most species, and the large groups of rounded cells projecting into the lumen and gradually becoming detached and dropping off into it, correctly figured by Henkin in Trombidium, are far less abundant here. These remarks apply specially to the male, in which, as far as I have seen, the amount of food-material absorbed by, and contained within, the cells of the ventriculus is less than in the case of the female, where the cells are often greatly distended by it. Salivary Glands (Plate VIII. fig. 16 ; Plate IX. fig. 23). I use tbe expression " salivary glands " for the glands which I am about to treat of because that expression is in general use for them; I a m not, however, satisfied that it quite correctly expresses their function. These glands are often largely developed in the Acarina, probably most so in the predatory kinds. It is already well known that some species of Hydrachnidae are amply provided with them; in the present species they assume considerable importance. Croneberg found three pairs of glands, each pair having bilateral symmetry, two pairs being more or less kidney-shaped, while the third pair are more sausage-shaped. Croneberg only draws and describes the portion of these later glands near to, and including, the efferent end, apparently not having traced them further. In his fig. 36 he draws the kidney-shaped glands as composed of numerous, largish, closely-pressed secreting-cells with clear nuclei, and the sausage-shaped glands as composed of a single layer of squarish cells surrounding a small lumen; he shows the three glands on each side of the body as communicating by small ducts with a larger joint efferent-duct. Schaub also found three pairs 1 "Beitrage zur Phylogenie der Arachniden," Jena. Zeitschr. f. Med. u, Naturw. 29 Bd. (1894) Heft i. pp. 125-152, |