TWO AQUATIC OLIGOCH^TES. 29 



The innermost layer consists of glandular cells. The regularly- 

 arranged ciliated cells of the vas deferens become polygonal at 

 its hind end like those of the funnel, and suddenly losing their 

 cilia pass into the glandular epithelium 'of the atrium. The 

 glandular cells are large and have a feebly staining cytoplasm 

 containing numerous deeply staining secretory granules of various 

 sizes ; the nuclei always lie at the basal part of the cells during 

 the period of active secretion. The cell membranes are exceed- 

 ingly thin, and are difficult to make out. The inner cavity of 

 the atrium is always irregularly crescent- shaped in cross section 

 at its broadest part. 



The process of secretion takes place as follows. The diffusely 

 scattered secretory granules of the atrial gland cells first assemble 

 in the distal part of the cells, in more or less distinct groups. 

 The granules of these groups then unite and form large globules, 

 which again unite to form large masses ; these become entirely 

 homogeneous and are finally expelled into the atrial cavity. As 

 soon as the expulsion has been effected the cells resume their 

 activity ; and the next secretion is even proceeding in the proximal 

 part of the cells before the expulsion of previous secretion has 

 been completed. 



d. Prostate. The prostate gland opens into the middle of 

 the atrium, where the outer layers of the atrial wall are 

 drawn out to receive the duct of the former, the two organs 

 being separated by a sphincter muscle consisting of a few fibres. 

 The organ is hemispherical in young specimens, but of irregular 

 form in full grown ones, in which it surrounds the atrium more 

 or less and occupies a large part of the cavity of the segment, 

 sometimes pressing on septum XI/XII. The general aspect of the 

 organ is well described by Dieffenbach in Tubifex. He says, 

 " Sie besteht aus einzelnen länglichen Drüsenlappen, die von der 

 Mündungsstelle aus sich fächerförmig ausbreiten und in deren 

 Mitten ein feiner Kanal verläuft, dem die einzelnen, stark granu- 

 lösen, mit grossem Kern und Kernkörperchen versehenen Drüsen- 

 zellen aufsitzen." The prolongations of the lobe canals, sometimes 

 uniting with others, open into the interspace between the circular 



