490 ME. A. D. MICHAEL ON THE 



to describe botli together. lu my late paper on Tlnjas petroph'dus (14), which is 

 frequently referred to in this treatise. I have entered so fully into the history of wliat 

 former authors have said upon this point that I do not think it would he proper tO' 

 repeat it all here, and will only give such a short statement of it as is necessary in order 

 that what I have to say about Bdella may be easily understood. 



Croneberg (4) describes the ventriculus of Ei/lais extendens (Hydrachnidije) and of 

 Trombid'mm (5) as a viscus closed posteriorly and not connected with any anus ; he says 

 that there is not any passage for ftecal matter or solid remains of food out of the 

 ventriculus, and that the anus-like opening, which certainly exists, only gives exit to 

 the excreta of the Malpighian vessels and does not communicate with the alimentary 

 canal. This opinion of the extremely careful Kussian anatomist was in direct opposition 

 to the previously expressed oj)inion of Pagensteclier (17) relative to Tromhidimn; he 

 considered the opening to be the anus, and what Croneberg calls the " Malpighian 

 vessel " to be the rectum and to be in communication with the mid-gut in the ordinary 

 manner. Henkin, in his researches into the anatomy of Trombidimn fuUginostiin (6), 

 failed to find any communication between the ventriculus and hind-gut or Malpighian 

 vessel, whichever it should be called, but he thought that there must be one, although he 

 could not see it; he says that where the Malpighian vessel of Croneberg overlies the 

 A-entriculus the walls of the latter organ become vague, and that he should think 

 there must be a communication tliere, although he could not find it. Next came 

 Schaub : speaking about Hydrodroma (Hydrachnidis) (18), he agrees with Croneberg that 

 the anus-like opening is not an anus, and that it is only the point of discharge of the 

 Malpighian vessel, which does not communicate with the ventriculus or any part of the 

 alimentary canal ; but he says that just anterior to the so-called anus there is a much 

 smaller opening, which is the real anus; and he says that to this smaller opening a 

 distinct hind-gut passes from the ventriculus, with which it is in communication. There 

 is no doubt that this smaller opening exists in Sydi'odroma, and Haller* had seen 

 and figured it before Schaub did. Haller simply calls it a preanal opening, without 

 suggesting its function. No one has seen this smaller opening in any creature of the 

 Tro7nliid'mm-gYo\n[) except Hydrodroma, and no one except Schaub has ever recorded 

 seeing this hind-gut distinct from the Malpighian vessel. 



A year later Schaub published a paper (19) on Pontarachna (Hydrachnidse) : there he 

 only figures a single opening, which he calls the " anus," precisely as previous writers 

 had done in other members of the group ; he does not say whether it is the exit of the 

 Malpighian vessel, or of the hind-gut, or of both; nor whether there is a hind-gut 

 distinct from the excretory organs or not. Karpelles (8) ignores all previous autliors 

 and treats the excretory organ of Bdella arenaria confidently as the rectum, but says 

 that he never could find any food-ball in it. 



In Thy as petroph'dus (Hydrachnidse) (14) I certainly observed but one opening, and 

 that decidedly communicated with the excretory organ (Malpighian vessel) and 



* "Die Aiteii unci Gattungeii der schweizcr HjdracLnidenfaune," Mittheil. Schweizer. entom. Gosellseli. ISbl', 

 p. 18. 



