506 - ME. A. D. MICHAEL ON THE 



capable of expansion. The walls of the organ are formed of columnar cells varying 

 greatly in size in different parts ; they only stain lightly, but have large and clear, much 

 flattened, oval nuclei of about 14 ^u and darkly staining nucleoli of about 5 /u. 



The embedding-sac has been most frequently empty in the specimens which I have 

 sectioned ; but sometimes it has contained a fine granular secretion, which hardly stains 

 at all, but which has apparently been of a slightly viscid character. I have never 

 detected spermatozoa in it ; but it is just possible that if the secretion enveloped the 

 spermatozoa and were impervious to stain, the spermatozoa might be present and not be 

 visible, although this does not seem very probable. It is a very strange thing that I 

 have not been able to detect any entrance to or exit from the embedding-sac either 

 forming a connection with the testes or any other organ ; it appears to be a closed sac,, 

 but it is difficult to believe that it always is so. 



The embedding of the true testes in the external wall of a hollow mass is very like 

 the arrangement in the Oribatidge*. In these Acari the embedding-organ serves as a 

 vesicula seminalis. Perhaps, however, of all published delineations of the genital organs 

 of Acari, that by Prof. Berlese of a sagittal section of Actinecla corniger (i. prostigmata,. 

 pi. vii.) comes nearest to the testis and embedding-sac of Bdella Basteri. Berlese does 

 not say a word about the genital organs of his creature in the letterpress ; but, curiously 

 enough, he has mistaken the male for the female, and in the explanation of his plate he 

 marks the embedding-sac as being the ovary, and apparently considers the testis, which 

 he draws very well, to be an egg ; he shows a communication ti-om the embedding-sac 

 into the penial canal, which he calls the " plicae vulvares, or labia interna." 



The nearly full-grown nymph of Bdella Basteri shows, as before stated, the testes 

 well developed, but the creature does not possess any embedding-sac; that seems to be 

 developed in the adult only ; and moreover, strange as it may appear, it is a fact that I 

 have been unable to trace any sign ot an embedding-sac even in the adults of any ot the 

 other species of Bdella, three or four in number, which I have been able to examine. 

 The true testes in these species come right down to the ventral surface. 



Tlie TestlciUar Bridge (figs. 17, 20, 34, tb) is so much a portion of the testes that it 

 might perhaps have been more correctly described immediately after them ; but its exact 

 position woiild not be so well understood before the description of the manner in which 

 the testes of B. Basteri are partially sunk in the embedding-sac. 



The bridge is a tubular connection between the two testes on one side of the body and 

 the corresponding joair on the other side ; it would not be useful to attempt to give its 

 exact size, because it varies so much according to the amount of spermatic matter in its 

 lumen for the moment and from other causes. It has already been mentioned that the 

 two short ducts from the two testes on one side unite in a common duct which enters 

 the end of the bridge; this common duct is so intimately connected with the bridge that 

 it must be regarded as the commencement of that structure. The bridge curves round 

 immediately behind the penial canal and, in B. Basteri, is between that organ and the 

 lower part of the anterior edges of the embedding-sac ; but it is quite free from both ; is 



* ' iJritish Oribatidtc,' Michael (Eay Soc. 1884), p. 150, pi. F. figs. 1-5. 



