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Show 186 MB. A. D. MICHAEL ON AN [Mar. 5, Henkin says that where, this organ overlies and touches the ventriculus, the investing membrane (or tunica propria) becomes vague, and that he thinks that there must be communication at this point, although he could not find it. After this came Schaub's paper on Hydrodroma : he described a very different state of affairs ; he saw all that Croneberg and Henkin saw, and a great amount more ; he agrees with Croneberg that the excretory organ which opens at the so-called anus does not communicate with the ventriculus; but he says that this opening is not the anus at all, but is simply the opening of the excretory organ, and that, in Hydrodroma, the same chitinous plate on the ventral surface which contains the so-called anus also contains a much smaller opening, immediately anterior to the other, which is the true anus ; he draws and describes a well-developed hind gut leading from the ventriculus to this true anus in addition to, and quite separate from, the excretory organ. Schaub remarks correctly that previous writers had not observed this smaller opening, except Haider, who calls it a preanal openingl, and only noticed it in Hydrodroma; he is inclined to object to former authors having called the larger opening the anus, and he suggests that it is scarcely probable that Hydrodroma is the only Hydracbnid which possesses an anal opening-a reasonable observation enough. A year later Schaub published a very interesting paper on marine Hydrachnidae, giving, inter alia, numerous anatomical details relative to two species of Pontarachna ; but, oddly enough, in these he only draws a single opening which he calls anus, just as previous writers had in other species, without saying whether it was the point of discharge of the alimentary canal or of the excretory organs or of both. Coming now to Thyas I find precisely the state of things described by Croneberg. I find a single longitudinal slit-like opening (figs. 10, 23, A.) closed by two labiae and situated in a small chitinous plate (fig. 10, ap.) in the median line of the body about halfway between the epimera of the fourth legs and the posterior margins of the body. I have most carefully examined this plate and the surrounding parts in many specimens, and I cannot find any trace of a second opening such as Schaub describes ; to this single opening one organ and one only goes ; this organ is the excretory vessel described by all the authors, and its whole inner surface is usually thickly clothed with the white excretory matter before mentioned. There is not any second viscus such as that figured by Schaub; tbe ventriculus ends blindly (fig. 23, v.). The excretory organ overlies it in the central line, and even hides almost all of its azygous anterior diverticulum when looked at from the dorsal side; it then turns downward, passing through the large opening left by the hollowr-square form of the ventriculus in this species, and goes to the opening before described. This organ certainly ends blindly in front, and although 1 " Die Arten und Gattungen der Schweizer Hydrachnidenfauna," Mittheil, d. Schweizer entom. Gesellscb. 1882, p. 18, |
