ETJPAGTTRTJS . 527 



portion {m. v. I.), which is connected by anterior and 

 posterior branches to the two lateral masses. The 

 remainder of the system comes under Marchal's head : — 



3. The Posterior Vesicular Mass, which includes the 

 unpaired bladder in the abdomen and its connecting 

 tubes with the two paragastric masses. 



The connecting tubes are a pair of branching ducts 

 which run on the top of the alimentary canal side by side 

 till they reach the abdomen, when they unite in a large 

 thin-walled vesicle (bl.) of considerable extent — rather 

 the shape of a centrifuge tube — which is about three- 

 quarters the length of the abdomen. It is called the 

 nephrosac. The walls are composed of squamous 

 epithelium with striated cytoplasm at its base and a 

 large spherical nucleus in the centre of each cell (fig. 31). 

 There is a distinct dark border to the cells of the 

 labyrinth and vesicular masses, which is probably due to 

 a striated margin or " Harchensaum." The bladder is 

 tucked between the lobes of the digestive gland — or 

 between the ovaries in the female. 



As the excretory system lies wholly in the venous 

 blood sinuses of the body, excretion is probably carried 

 on by direct diffusion through the walls. In sections of 

 the vesicular masses cells may be seen with their inner 

 portions projecting as a clear vesicle into the cavity of 

 the organ. These vacuoles may be shed as small 

 bladders into the lumen, or the entire cells in the end sac 

 are nipped off and pass to the exterior. The excretory 

 fluid is full of such bladders and single cells. 



