No. 2.] AA^ATOA/y OF BDELLODRILUS ILLUMINATUS. 52 I 



diameter before emptying into the anterior side of the enlarged 

 inner end (muscular sperm sac) of the penis. The glandular 

 sperm sac lies in contact with the anterior side of the penis 

 sheath, where it is held by the vasa deferentia (Fig. 19). At 

 about the middle of its length it presents a slight ventral 

 enlargement which receives the vasa deferentia, beyond which 

 it curves abruptly upward. The thick glandular walls are 

 made up principally of deeply-staining columnar or irregularly 

 pyramidal cells, with distinct basal nuclei, and reticulated 

 granular protoplasm (Figs. 22 and 23). At the place of com- 

 munication with the muscular sperm sac these cells perforate 

 the muscular coat of the latter, and become continuous with 

 its lining epithelium (Fig. 23). No cilia are present in the 

 sperm sac. 



The copulatory region of the atrium is formed by a single 

 invagination of the entire thickness of the body wall ; the 

 epidermis becoming the epithelial lining, and the longitudinal 

 and circular muscle coats, respectively, the longitudinal and 

 circular muscle coats of the atrium. Proximally the in- 

 vagination becomes expanded into a conspicuous sub-spher- 

 ical copulatory bursa (Figs. 19 and 20), the posterior dor- 

 sal portion of which is continued into the sub-cylindrical 

 penis sheath (Fig. 19). The thick muscular walls of the bursa 

 are principally derived from the longitudinal muscles of the 

 body. These continue their longitudinal direction over the 

 bursa and penis sheath, increasing in thickness on the anterior 

 wall of the former, while on the latter only a single layer of 

 closely appressed fibres is developed. They cease entirely at 

 a constriction which marks the upper end of the penis and the 

 beginning of the muscular sperm sac (Figs. 19 and 20). 

 At the point where the bursa passes into the penis sheath a 

 reflection of a portion of the longitudinal fibres (which pushes 

 the epithelium before it) into the cavity of the former, gives 

 rise to the basal portion of the penis, which, when retracted, 

 projects as a conical process downward and forward into a 

 space between the bursal glands (Figs. 19, 20, and 26). 



The circular muscle fibres become in the walls of the bursa 

 quite distinct from the epithelium and more closely associated 



