INTERNAL ANATOMY OF BDELLA. 507 



not in any way sunk in tlie embedding-sac or joined to tlie penial canal, althongli in 

 ripe specimens all the organs become pressed against those adjoining them from the 

 immense development of tlie genitalia. It forms a passage for the spermatozoa from 

 the testes on each side of the body, and in specimens containing ripe spermatozoa it is 

 usually crowded with the ripest of them. From this one Avould naturally anticipate that 

 there would be some passage from the bridge into the penial canal, which it immediately 

 adjoins; but, strange to say, this is just what I have been entirely unable to find by any 

 amount of searching ; there does not seem to be any enti-ance from it into the penial 

 canal, and I think I may say positively that there was not any such entrance in aay 

 specimen which I have examined. 



The testicular bridge is probably the homologue of the short junction or bridge 

 between the testicular masses on the two sides of the body found in Thijas petropJdlus 

 (14. p. 193, fig. 17), although the situation is very different; the bridge in Thyas 

 being at the extreme posterior of the testicular masses, whereas that of Bdclla is central. 

 There is a kind of progression from the almost horseshoe-shaped testicular mass of 

 Tromhidimn fuUginosnm (6. fig. 17) to that of Thyas, above referred to, where, 

 although tlie liorseshoe form is more or less preserved, the two sides are almost 

 separated ; and fiu-ther to the organs of BdeUa, where the two sides are wholly separated 

 posteriorly and only joined by a central bridge. 



The histology of the bridge does not vary much from that of the testes ; the same 

 layer of darkly staining, clearly-nucleated cells which forms the exterior tunic of the 

 latter extends continuously over the former, and when the genital products are not 

 fully developed constitutes the principal thickness of the organ, the lumen being small. 

 At a later stage the lumen is greatly distended by the spermatic products, and the walls 

 of the organ become thinner; there is never any sign, liowever, of the cells of the 

 bridge becoming sperm-mother-cells, or of their giving rise to spermatozoa. 



The Mucous Glauds {glandulce mucosa-) (figs 17, 31 & 36, gm, & 37), as I call them, 

 for the purpose of not binding myself to homologies, are almost, if not quite, the largest 

 organs in the body of the adult male Bdella. The form of each is that of a great 

 sausage-shaped sac bent upon itself in the middle, so that the two ends almost touch, 

 and the two halves also almost touch at their inner edges and are parallel to each other; 

 the great curved central bend is at the anterior portion of the organ, the ends being 

 directed backward (fig. 36). The bend is not horizontal but perpendicular, so that the 

 one half of the organ lies above, not at the side of the other ; the end which, as herein- 

 after explained, comimmicates with the testiciUar bridge is the lower, and is usually 

 more or less swollen and irregular in form, while shortly before the swelling there is a 

 slight constriction. The walls of the organ are extremely thick ; they are composed of a 

 substantial tunica propria externally ; on the inner side of this a single row of large, more 

 or less columnar, somewhat irregular cells, varying in B. Bastcrl from about "06 mm. to 

 about -1 mm. in length and about 15 f* to 25 ^ in width, which have extremely dense 

 dark-staining walls where they adjoin each other, l. e. on their sides, and much thinner 

 walls at the ends. The exterior end of the cell is filled with a layer or mass of protoplasm 

 about -02 to -03 mm. thick on the average, which stains freely and contains the strongly 



