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Show 270 ON A CASE OF HOMffiOSIS IN THE GENUS ASELLUS. [Mar. 20, The palp is misshapen and rudimentary. It consists merely of an irregular process with slight traces of segmentation. The molar process is rather less than the normal in length. Its triturating end is ill-formed, and does not bear the elaborate structures found in the same place in the normal mandible, the apex being simply covered with a thick plate of chitin. Apex of extra mandible of Asellus aquaticus, to show details. p', palp of abnormal mandible. This mandible was in life carried flat to the body, as shown in the drawing, but 1 have not succeeded in determining winch morphological surface was upwards. It is not evident which this should be, but presumably it is the face, which in the normal mandible is next the mouth. I regret that I could find no feature sufficiently differentiating the two surfaces; but from the fact that, as shown in the figure, the diminishing series of teeth are on the exposed surface, it seems likely that the presumption referred to is correct. In a normal mandible this series is much less clearly defined. The animal was approaching a moult, and the newly-formed skin, which could be seeu through, showed that at the next ecdysis the appendage would not be noticeably changed. Ln the details of the toothing and in the number of the plumose setae (11 instead of 13), the animal departs slightly from Sars's type; but on examining other specimens 1 find that these characters vary a good deal, as might be expected. Elsewhere11 have discussed the significance of cases of this sort, 1 Especially ' Materials for the Study of Variation,' Chap, v., where an account of previously recorded cases is given. To these should be added a case of an Astacus having a somewhat aniennuliform structure with two jointed filaments replacing one of the eyes : Hofer, B., Verh. d. deutsch. zool. Ges. iv. 1894, p 8-) fig. -' "' |
