INTEENAL ANATOMY OF BDELLA. 505 



spermatozoa in various stages of development, according to the age of the cell. These are, 

 of course, the sperm-mother cells (figs. 35, 42) ; the inner wall of each cell eventually 

 breaks down, and the sjiermatozoa are discharged into a hollow chamber forming 

 the interior of the portion of the organ which has the nucleated cells as its outer tunic. 

 When the organ is sectioned, it often appears solid ; this is because the chamber is so 

 full of secretion and spermatozoa that its lunaen is obliterated ; often, however, it is only 

 half-full, and then its real nature is seen. It is more than probable that this chamber is 

 composed of a single cell, which is greatly distended, and has become a species of vesicula 

 seminalis to receive the contents of the adjoining cells. My reason for making this 

 suggestion is that tbe chamber frequently, indeed I may almost say usually, contains a 

 a single, large, most distinct nucleus lying free in the midst of its contents. In some 

 species, e. g. B. vulgaris, when the spermatozoa are thoroughly ripe, this chamber seems 

 to occupy almost the wliole interior of the testis, particularly of the front one of each 

 pair, and it is crowded with large spermatozoa in a ripe condition. The secretion with 

 which, in addition to spermatozoa, the cell is generally filled must apparently either have 

 arisen in the cell itself or entered from the embedding-sac (in £. Basteri) ; but if the 

 latter be the case, it is not clear where the passage is, nor would this account for the 

 presence of the secretion in the chamber of B. vnlr/atHs, which has not any embedding- 

 sac. 



The testes are largish and fairly well developed in the nymph when approaching full 

 growth, although of course not nearly so mu.ch so as in the adult, and they do not in 

 the nymphal stage contain ripe spermatozoa ; but they are very large compared with the 

 other male sexual organs, such as the great mucous and accessory glands and the 

 embedding-sac, which at the same period are either very small or altogether absent. 



Tlie Bmhedd'mg-sac (figs. 17, 19, 35, es) is a large hollow viscus : on each side of the 

 body, in an indentation of this viscus, the testes rest ; it is on the exterior side of the 

 testes, and when in situ is on edge in the body. Although the testes simply lie on the 

 inner side of the embedding-sac, and are easily removed by dissection, yet they are so 

 pressed into the wall of tbe sac that two hollows (fig. 19) are formed in the wall of 

 the embedding-sac, into which the respective testes fit, and in which they are buried to 

 the depth occasionally of half their thickness. The wall of the emliedding-sac is forced 

 up between the two testes, forming a considerable ridge. The lower edge of the 

 embedding-sac rests upon the inner side of the ventral surface of the hind part of the 

 body. In fig. 17 the two embedding-sacs {es) are shown with the testes in situ, but 

 they are opened out and laid back, in order to show the testes, as the two valves of a 

 cockle-shell might be in order to show the fish ; in reality, when in the body of the 

 living creature the two embedding-sacs are closed together, like a living cockle-shell, and 

 quite hide the testes, which are then on their inner surfaces. In fig. 19 one of the 

 embedding-sacs is shown after the testes have been removed. The embedding-sac 

 is somewhat folded and pressed so as to form lobes and folds, which vary in diff'erent 

 individuals ; it will be seen by fig. 35 that its two surfaces are usually much j)ressed 

 together, leaving a rather small and irregular lumen, which, however, is doubtless 



