22 BULLETIN': MUSEUM OF COMPARATIVE ZOOLOGY. 



could have arisen from a receptacle of the usual form is by an invagina- 

 tion of that portion of the receptacle from which the duct arises, the duct 

 being thus carried into the interior of the vesicle. If after such invagina- 

 tion the terminal portion of the duct should become dilated, there would 

 be formed a structure comparable to the inner vesicle. Should the recep- 

 taculum of appendiculate trematodes arise as the result of such a process, 

 the space between the outer wall of the receptacle and the wall of the 

 inner vesicle would represent the original cavity of the receptaculum, the 

 inner vesicle would be simply an enlargement of the duct which had come 

 to lie within the true receptacle, while the pore at the fundus of the inner 

 vesicle would correspond to the original opening of the duct into the 

 receptacle. 



In case the receptacle of Hemiurus has arisen by a process of this kind, 

 the wall of the inner vesicle would represent a double layer of epithelial 

 cells. I could find no evidence that the wall consisted of two layers ; 

 but it is not much more difficult to conceive of two layers of epithelial 

 cells being modified to form an apparently structureless membrane than it 

 is to imagine the same change in a single layer of such cells — a modifica- 

 tion which is knowu to take place, not only in the case of the wall of the 

 seminal receptacle, but also in that of the walls of nearly all the organs 

 of the body. 



If the union between the oviduct and the duct from the seminal recep- 

 tacle takes place outside of the shell-gland, the resulting duct, the obtype^ 

 immediately enters the shell-gland, through which it passes in a more or 

 less horseshoe-shaped course, emerging near the anterior end of the gland. 

 The wall of the ootype is thicker than that of any other part of the sexual 

 organs and in it are imbedded numerous oval nuclei. It is joined near 

 its beginning by the short unpaired part of the vitelline duct. 



The sliell-yland is a compact spheroidal mass of cells surrounding the 

 ootype. It is situated midway between the two vitellaria, a little behind 

 the posterior margin of the ovarium, and more ventral than either of these 

 organs. 



The vitellaria are two irregularly oval bodies situated just posterior to 

 the ovarium, one on either side of, and not far from, the median plane. 

 (Fig 1 ; Plate 4, Fig. 42). Their longest diameters are parallel with 

 the longitudinal axis of the body. In cross sections of the body they are 

 seen to occupy a dorso-lateral position. They are commonly slightly 

 lobulated, though they sometimes have a regular oval outline. 



Each gland is about one-third as wide as the body and nearly twice as 

 long as it is wide, the average measurement of several specimens being 



