INTERNAL ANATOMY .OF. BUELLA. 491 



with that only; there clearly was not any separate hiad-g'ut. I was not able to trace 

 any commLinication between the Malpighian vessel aad the ventricuUis, although I took 

 great pains in searching for it in numerous specimens, still I do not deny the possibility 

 of its existing at some time of year or at some period of life ; all I say is, that I could 

 not find it, and that the walls both of the ventriculas and of the excretory organ appeared 

 to me to be perfectly clear, without any vague places, in those specimens whicli I 

 examined. Bdella would not be at all a favoural)le genus in which to investigate this 

 question ; the walls of the ventriculus and excretory ox'gan are not so firm and sub- 

 stantial as in Tlnjas, and in mature specimens the latter is so enveloped by the former 

 as to make the exact relations of the two difl&cult to trace. I should not therefore base 

 any opinion on the subject upon Bdella ; but, so far as it goes, I was not able to trace 

 any communication between the ventriculus and the excretory organ in this creature any 

 more than I could in Thjas ; still it is possible that such a communication might exist 

 and be extremely difficult to see. It is quite certain that neither any second opening, 

 such as that of Hydrodroma, nor any separate hind-gut, sucli as Schaub describes in that 

 Hydraclinid, exists in any species of Bdella which I have investigated : it is equally 

 certain that I have never found any trace of food-material or faecal matter in the 

 excretory organ of any Bdella ; the contents have invariably been solely the white 

 crystalline matter (appearing black by transmitted light on account of its opacity) which 

 one usually finds in the so-called Malpighian vessels of Acarina. 



In considering the question of whether the mid-gut ends l^lindly or communicates witli 

 a hind-gut and anus, it must not be forgotten that the creatures of this group are almost 

 all strictly predatory and suctorial ; they never consu.me any solid food, but live entirely 

 upon the juices of their victims ; this applies to Bdella. 



Were it not for Schaub's paper on Ihjdrodroma I should consider that the so-called 

 Malpighian vessel in Bdella and other allied creatures was the homologue of the hind- 

 gut in other Acari, and from disuse had become a Malpighian organ only ; but of course, 

 if Hijdrodroma really has a hind-gut in addition to a similar Malpighian vessel, it would 

 seem to prove that in all other investigated creatures of the Trotnbklmm-gron'p the hind- 

 gut must have become obsolete, and vanished without leaving a trace of its former 

 existence. 



In Bdella the excretory organ, whether it be the liomologue of the hind-gut or not, is 

 exceedingly simple : it is an elongated sac (figs. 31<, E & 7), rather widest at the anus, or 

 what is usually called the " anus," narrowest about the middle, and swelling out again 

 somewhat at the anterior end. The organ is distensible, but is neither branched, 

 folded, nor plicated; it is capable of considerable expansion and contraction; it runs 

 along the median line of the body close to the dorsal surface, through the cuticle of 

 which it generally shows as a strong white band ; its anterior end is usually about in the 

 perpendicular plane of the coxa3 of the second pair of legs, i. e., pei-pendicularly above 

 the hinder part of the sub-oesophageal portion of the brain ; it varies somewhat in length, 

 and it immediately underlies the dorsal skin as far back as the genital aperture, then it 

 turns somewhat downward to reach the anus, which, however, is subterminal. In young 

 males its coiu-se may be plainly seen passing between the two great hind caeca of the 



