370 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



junction of the prostate with the gland, this single layer 

 of cells gives place to a mass of irregularly shaped, some- 

 what elongated cells which encroach a good deal upon 

 the cavity of the gland in this region. In a mature worm 

 the appearance of these cells is very different. They 

 become much enlarged, so that they tend to obliterate the 

 cavity of the gland. Their outlines become less distinct, 

 and the nuclei, which remain near the outer end of the 

 cells, are very difficult to identify. This increase in the 

 size of the cells is accompanied by a change in their 

 contents. The cytoplasm becomes less granular and very 

 vacuolated, and its appearance suggests the presence of 

 some secretion which stains readily with "plasma" 

 stains, such as picro-indigo-carmine (PL VII, fig. 45). 

 Outside the epithelium is a layer of longitudinal muscle 

 fibres placed somewhat obliquely, and outside this again a 

 delicate peritoneal covering of flattened cells. 



4. The Penis. The terminal portion of the spermi- 

 ducal gland is modified to form a protrusible penis which 

 opens to the exterior on the ventral surface of segment 11. 

 The structure of the penis is somewhat complicated. Its 

 cavity is continuous with that of the gland, and it is 

 surrounded by a penis sheath, which, when the penis is 

 retracted, is double, the outer layer being continuous with 

 the epidermis of the body wall (PI. VII, fig. 45, pe.s.). 

 We may, therefore, speak of the outer and inner penial 

 sheaths, both consisting of a single layer of epithelial 

 cells, somewhat cubical in shape and provided with 

 conspicuous, rounded, nucleolated nuclei (PI. V, fig. 22, 

 pe.s. and pe.s. 1 ). The cuticular secretion, which forms a 

 delicate layer outside the cells of the epidermis, is 

 continued over the cells of the penial sheath, and in the 

 retracted condition of the penis may completely fill - the 

 space between the outer and inner penial sheaths. The 



