492 ME. A, D. MICHAEL ON THE 



ventri cuius, but separate from them. In mature specimens, especially females, in 

 consequence of the great growth of the ventriculus and caeca and the distension of the 

 cells composing their walls by the absorption of food-material, and partly also in 

 consequence of the pressure of the immense genital organs, the ventirculus and caeca are 

 forced into every available space, and tlie caeca are pressed against each other in the 

 median line of the dorsum ; thus the excretory organ comes to be entirely surrounded 

 by the ventriculus and its ctcca, and seems as if it passed through their substance (see 

 figs. 34, 43). 



The walls of the Malpighian vessel consist of a strong, clearly marked external layer of 

 more or less hexagonal pavement-epithelium cells, with conspicuous oval nuclei of about 

 5 ft to 10^, and nucleoli ; they are often doubly nucleated ; within this epithelium is a layer 

 of almost cubical secreting cells. 



The contents of the Malpighian vessel in Bdella are small, crystalline, opaque, white, 

 more or less globiilar concretions of urinary material, similar to that found in the 

 Malpighian organs of other Acari ; in Bdella, however, they seem, in oldish specimens, to 

 accumulate to such an extent that they adhere together and often form a solid rod almost 

 filling the organ. This white matter usually shows through the dorsal cuticle, and often 

 looks, at first sight, like a wliite, or pale yellow, stripe running longitudinally in the 

 median line of the notogaster ; the rod when formed may sometimes be dissected out and 

 remain quite unbroken and hard. In other specimens the granules do not adhere, but, 

 although scattered, nearly fill the vessels ; in some cases, particularly where the sj)ecimen 

 is young, the quantity of white matter is small. 



The anus (fig. 34, A), if that be its proper name, is a longitudinal slit near the hinder 

 end of the creature, and is closed by two labia (cf. fig. 7), to which divaricator muscles 

 (fig. 34, mda) are attached. 



The Salivary Glands (so-called). 

 (Figs. 8, 9, 10, 11, 12, 34.) 



I retain the expression " salivary glands," because it has been used by so many 

 authors, and is perfectly well understood as referring to tlie group of glands which I am 

 about to treat of in this section : I have, however, in my paper on Thijas petrophilus (14) 

 beibre referred to, expressed considerable doubts whether this important and varied series 

 of glands really have solely the function which is implied by the name ; there does not 

 seem to be any very convincing mode of settling this question, and I hardly see how 

 to carry it further than can be gathered from the probabilities involved in the position 

 of the points where their ducts discharge. In the same paper I have entered fully into 

 what previous authors who have dealt Avith these glands in Acarina belonging to the 

 Trombidium-gvou-p of families have said regarding them ; as in the case of other organs 

 for Avhich I have done the same, I do not propose to repeat the whole bibliography 

 here, bu^t rather to confine myself to the description of the parts in Bdella, referring 

 only to former authors where it seems likely to give greater clearness or to assist in 

 settling doubtful questions. 



