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Show 198 MB. A. D. MICHAEL ON AN [Mar. 5, packed, elongated cells of about -01 mm. diameter, and in section exhibit similar cellulation all through ; they are quite solid without lumen ; but I have not been able to trace any duct from them. _ I thought at one time that they probably discharged into the posterior part of the tubular salivary glands; but after careful investigation I am not, up to the present, able to state that this is the case, although the two organs are in tolerably close juxtaposition ; and the function of the glands therefore remains uncertain to me. The Palpal Organs (Plate VIII. fig. 22). These organs might probably be included in the last section as glands of unknown function," but I do not wish to pledge myself to the assertion that they are glands, although I incline to think so. They are largish paired organs, one on each side of the body; the posterior portion is an elongated lobe with a rounded hinder end; about a third of the length of this lobe (the posterior third) lies under the brain, but is not in any way connected with it, there is a separate nerve from brain to palpus. The lobe runs in an almost direct course from below the brain to the palpus, but it diminishes considerably in diameter before reaching that appendage, and where it enters has less than one third of the diameter of the thickest part of the lobe. Within the first joint of the palpus the palpal organ swells out again and forms a second elongated lobe, not nearly so thick as the first; at its distal end this bends slightly downward, and enters the second joint, where it again diminishes in diameter, and then runs forward until nearly the distal end of the palpus, keeping an almost uniform thickness (the anterior part is not shown in the figure). The organ is composed of large irregularly-placed cells, as far as can be judged from the nuclei, which are few but very distinct and of considerable size ; but I have not been able to detect the lines of demarcation between cell and cell in any of m y preparations. The organ is solid, i. e. there is not any lumen, and I have not been able to trace anything like a duct from it. I a m not aware of anything which has been described in the Acarina which can be identified with it or considered the homologue of it; nor have I ever seen such an organ in any other species that I have examined: the structure most resembling it, that I a m acquainted with, in the Acarina is the spinning-glaud partly in the palpus of Tetra-nychus; but the present species is aquatic and there is not any reason to suppose that it has any power of spinning; moreover the palpi are not furnished with a spinneret, such as is found in Tetranychus. The palpus is almost certainly a raptorial organ, it assuredly is not tactile; but there is not any poison-fang or spine that I can discover, and the mandibles are evidently the killing-organs. For these reasons I think it best not to suggest a function for these palpal structures and to leave the matter for future investigation. |