No. 2.] ajvatoa/v of bdellodrilus illuminatus. 523 



on each side of the space in which the end of the penis Hes 

 (Figs. 20, 24, and 26, g). The gland cells are arranged in an 

 almost spherical group around a deep hilum, or sinus, which 

 serves as a duct, and opens directly into the cavity of the 

 bursa. The cells are pyramidal, with broad, deeply staining 

 bases containing the nuclei ; and elongated necks which are 

 highly granular, but stain less deeply, and converge toward 

 the sinus, in which the cuticle is interrupted (Fig. 26). The 

 presence of these glands complicates the shape of the cavity 

 of the bursa. Ventrally (below the glands) it is transversely 

 extended, and, as the glands stand out from the posterior wall, 

 antero-posteriorly constricted ; higher up (between the glands) 

 it is narrowed in its transverse dimension, and extended antero- 

 posteriorly ; dorsally it expands, but is much encroached upon 

 by the projecting end of the penis. Figs. 19, 20, and 24 will 

 make this clear. 



Over the free projecting end of the penis the epithelium is, 

 of course, reflected, and then continues with its cuticular cover- 

 ing as the lining and eversible portion of that organ. Here 

 the cells become rather smaller, but regain their size in the 

 muscular sperm sac, into which the cuticle also continues. 

 This penis epithelium is, as it were, much too long for its 

 muscular sheath, and is consequently thrown into folds or 

 slightly spiral coils within the sub-epithelial space (Figs. 19 

 and 20, pc), the narrow lumen appearing as a conspicuous wavy 

 double contour. In protrusion these waves are straightened ; 

 partly pulled out by the contraction of the minute muscular 

 fibrils, partly squeezed out by the pressure of the circular 

 muscle coat, and partly by the shortening of the whole organ, 

 owing to the contraction of the longitudinal muscles. By the 

 combined action of these forces the epithelium is everted, and 

 projects from the external opening of the bursa as a double- 

 walled tube, slightly bulbous at its free end, and covered with 

 a smooth cuticle. Whether or not the bursa itself is everted 

 naturally in this species is a question. By careful application 

 of pressure while under the microscope I have been able to 

 force it through the external opening ; but it always carries 

 the sixth nerve ganglion with it under such circumstances; 



