186 MB. A. D. MICHAEL OK A3T [Mar. 5, 



Henkin says that where this organ overlies and touches the 

 ventriculus, the investing membrane (or tunica propria) becomes 

 vague, aud 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 anteiior to the other, which is the 

 true auus ; 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. Sehaub remarks 

 correctly that previous writers had not observed this smaller 

 opening, except Haider, who calls it a preanal opening \ 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 Hydrachnid 

 which possesses an anal opening — a reasonable observation enough. 



A year later Sehaub published a very interesting paper on 

 marine Hydrachnidse, giving, inter alia, numerous anatomical 

 details relative to two species of Pontarachna ; but, oddly enough, in 

 these he only draws a single opening \^■hich he calls anus, just as 

 previous writers had in other species, \\ithout saying whether it 

 was the point of discharge of the alimentary canal or of the 

 excretory organs or of both. 



Coming now to Thy as I find precisely the state of things 

 described by Croneberg. I find a single longitudinal slit-hke 

 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 half\^ay between the epimera of the fourth legs and the 

 posterior margias of the body. I have most carefully examined 

 this plate and the surroundiag parts in many specimens, and I 

 cannot find any trace of a second opening such as Sehaub de- 

 scribes ; 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 Sehaub ; the 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 hollow-square form 

 of the ventriculus in this species, and goes to the opening before 

 described. This organ certainly ends blindly in front, and although 



^ " Die Arten imd Gattuiigen der Schweizer Hydrachnidenfauna," Mittbeil, 

 d. Schweizer entom. Gesellsch. 1882, p. 18, 



