PHYSIOLOGY OF THE INVERTEBRATA. 423 



opens externally by separate ducts — the vasa deferentia — on 

 each side of the ventral surface of the body. 



The Gcpliyrca are also unsegraented worms, but they are 

 dioecious (i.<'., the sexes are distinct), and the spermatozoa and 

 ova are developed from the epithelial cells, which line the walls 

 of the perivisceral cavity, or they are developed in simple 

 caical glands. The earlier stages in the development of these 

 animals are similar to those of the Oligochaia and PolyclicBta. 

 The adult state is attained from the embryonic condition by 

 a metamorphosis. In the genus Boncllia, the males, which 

 are minute, rudimentary, and Planarian-like, live in the 

 uterus of the female.* 



The Hirudinca are hermaphrodites, f The male organs of 

 Hirudo consist of nine pairs of testes, situated in successive 

 segments. The first pair are in the segment behind that 

 which contains the eversible penis ; and the others are in the 

 following eight segments, a little in front of the nepliridia or 

 segmental organs. From each testis a duct passes into the 

 vas deferens (situated laterally), which, in front of the anterior 

 pair of testes, becomes much coiled, forming the vesicular 

 seminalis. The latter opens into a duct, which passes for- 

 ward to the ventral median line, and, along with the same 

 duct of the vas deferens of the other side of the body, opens 

 into the prostate gland. A duct proceeds from the prostate 

 gland, which forms a sheath of the eversible penis, and opens 

 in the twenty-fourth segment. The spermatozoa are enclosed 

 in a case, called a spermatophore. The female organs are 

 situated in the segment behind that which contains the penis. 

 They consist of two small ovaries provided with oviducts ; the 

 latter open into the uterus, which is surrounded by an albumin 

 gland. The uterus proceeds into a vagina, which opens by a 

 sniall orifice situated between the twenty-ninth and thirtieth 

 segments. It will be noticed that the external orifices of the 

 genital organs are unpaired. 



• This is different from Bilharrda (one of the Treniatoda), for the male 

 retains the female within its own body. 



t Except Hidriobdella and ilalovbdella, w hich arc dioeciout-. 



