No. 2.] AjVAT03fV OF BDELLODRILUS ILLUMINATUS. 519 



Reproductive system. — The reproductive organs of Bdello- 

 drilus differ in certain important respects from those of 

 Branchiobdella, as described by Dorner, Keferstein, Voigt, 

 Vejdovsdy, etc.; but nevertheless there is a close general 

 resemblance between the two. Resembling its allies, Bdello- 

 drilus is hermaphroditic, and possesses accessory sexual struc- 

 tures admirably fitted to the process of reciprocal fertilization, 

 which, no doubt, occurs during copulation. The general 

 arrangement of the reproductive organs is shown in Fig. i. 



The male organs occupy the greater part of the peri-enteric 

 cavities of somites five and six. They consist of two pairs of 

 testes, and two pairs of vasa deferentia, having a common 

 opening to the exterior by means of a complex unpaired atrium, 

 which is differentiated into a glandular sperm sac, a muscular 

 sperm sac, an eversible penis, and a muscular copulatory bursa, 

 with its associated glands. The testes proper can be distin- 

 guished satisfactorily only in the young, in which, if examined 

 shortly after hatching (or after removal from the cocoon just 

 previous to that event), they are easily detected in entire 

 worms or in sections as small groups of rounded nucleated 

 cells, attached near the floor of the body on each side of the 

 posterior faces of septa 4-5 and 5-6. Their origin from the 

 peritoneal lining of the coelom is evident upon the examination 

 of longitudinal sections of several successive stages of develop- 

 ment. The first steps in the development of spermatozoa 

 begin before the worm has nearly reached full size, and pro- 

 ceed continuously; the various stages floating freely in the 

 coelom, in which they complete their development. In the 

 mature worm the cavities of the fifth and sixth post-cephalic 

 somites are filled with spermatozoa in various stages of de- 

 velopment, while the testes proper have become much reduced 

 and inconspicuous. The details of this process have been 

 admirably worked out and described by Voigt; and it need 

 only be added that what observations the writer has made are 

 in accordance with his account. 



The male efferent ducts (Figs i, 19, and 20) correspond in 

 number to the testes, and their mouths open into the same 

 somites. The mouths of the anterior pair lie far forward in 



