210 MANUAL OF ZOOLOGY sect. 



very similar to that of the eyes. The function of the lateral 

 sense organs is unknown. 



The leech is monoecious. There are nine pairs of testes 

 (Fig. 119, ts), in the form of small spherical sacs situated in 

 segments 12-20. Each gives off from its outer surface a 

 narrow efferent duct, which opens into a common vas deferens 

 (v. d). In the tenth segment the vas deferens increases in 

 width and forms a complex coil, the vesicula seminalis 



Fig. 122. — Diagram of principal blood channels of leech : d. s, dorsal sinus ; /. 

 lateral vessel ; v. s, ventral sinus containing nerve-cord. 



(v. se/n), from which is continued anteriorly a somewhat 

 dilated muscular tube, the ductus ejaculatorius (d. ej). 

 From each ejaculatory duct a narrow tube passes to the base 

 of the penis (/>), a curved eversible muscular organ which 

 opens on the ventral surface of the second annulus of the 

 tenth segment, in the middle line. 



The ovaries are coiled filamentous bodies, each enclosed 



