INTEENAL ANATOMY OF BDELLA. 517 



creature gets a little older the crescent becomes somewhat deeper, more approaching a 

 horseshoe form (fig. 24, ov) ; but after impregnation, and when the ova are mature, the 

 Avhole becomes one shapeless mass of large eggs A\hich enclose and hide the oviduct. The 

 ova are formed and more or less matured in short pedunculated oocysts, each ovum 

 apparently forming its own oocyst by pushing out the exterior tunic of the ovary, thus 

 forming a sac in which the ovum lies. Exactly how the ovum gets from the oocyst into 

 the oviduct is not by any means clear to me in Bdella, or, indeed, in many of the other 

 Acarina, although it is evident enough in the Oribatidns and most Gamasidaj. 



The Oijidnct (figs. 21, 25 & 43, od, & 14). — In all described Trombidiidoe and Ilydrachnidse 

 there are two oviducts whicli unite to form a common tube (the vagina) ; in Bdella, how- 

 ever, there does not apj^ear to be any such arrangement ; in all s^^ecies which I have 

 examined there ajipears to be one single fleshy tube leading from the centre of the ovary to 

 the external labia. Some authors would probably divide this tube into oviduct, uterus, 

 and vagina ; in reality, however, I do not see any distinction of regions which is at all 

 permanent : it is one unbroken tube, in some parts a little larger, in others a little smaller, 

 but still the one tube, and the enlarged portions are apt to vary with age and contents. 

 It is only in the young specimens that this tube can be clearly seen or dissected out; 

 when distended by maturing or mature eggs it becomes a great irregular sac, the walls 

 of which have lost their extremelv thick and irlandular character to a ijreat extent. The 

 oviduct when in situ is not straight or perpendicular ; it is in effect rolled round the 

 ovary ; so that when dissected out the whole forms a rounded mass, which can be uncurled 

 in fresh specimens of the young female, but which it is extremely difficult to stretch out 

 when the eggs are mature. The ovary and oviduct of a young specimen of B. vulgaris 

 partly uncurled, so as to show them, are delineated at fig. 25 ; it will be seen that the 

 oviduct here is longish and is corrugated, showing evident signs of capability of 

 expansion ; at its distal end it enlarges considerably, and were this permanent it might, 

 perhaps, be called a uterus, but I do not see that it is so. Eeyond this enlarged portion, 

 i. e. between it and the external labia, is a short flexible portion of the tube, which might 

 be called the vagina, if that name be used at all. In B. Basterl (fig. 24) I have not 

 seen any such enlargement in the young female. The figure is drawn as In sltn, only a 

 little uncurled, but the oviduct is, I think, much shorter and thicker in pro^wrtion than 

 in B. vulgai-is. The external labia and membranous portion of the duct are not repeated 

 in this figure ; they would not vary from the other species. 



The oviduct is an organ with extremely thick fleshy walls (fig. 44) (in some places as 

 thick as "1 mm.) and consists of an almost structureless tunica propria, within which are 

 usually several row s of deeply staining cells, containing a granular protoplasm and small 

 round nuclei (about 2>n) which do not stain much more deeply than the general cell- 

 contents. The AA'alls vary greatly in thickness in difi'erent parts, and according to the 

 amount to which the organ is distended in the particular place at the time. The cells 

 are extremely loose and irregular, crowded together, l)ut not so as to press strongly 

 against one another ; thus they have rounded and otherwise curved outlines, and are not 

 forced into hexagonal or other angular forms. The layers are often folded so as to 

 make great projections into the lumen of the duct. The outer layer of cells, adjoining 



SECOND SKKIES. — ZOOLOGY, VOL. VI. 70 



