V] TURBELLARIA 97 



divided by a shallow depression into two lobes. It is situated in 

 front of the mouth near the anterior end of the animal, embedded 

 in the parenchyma. It gives off a pair of nerves which run forward 

 to the tip of. the body, and another pair of rather stout nerves 

 which run back, one on each side of the pharynx, to the tail. The 

 nerves give off fine branches which are distributed all over the body. 

 A pair of eyes of a simple structure lie on the upper surface of the 

 brain. 



Mesostoma like Hydra is hermaphrodite. Both eggs and 

 spermatozoa are expelled from a single genital pore situated 

 behind the mouth. This pore leads into an ectodermic sac called 

 the genital atrium. 



The male organs consist of two long sac-like testes which lie 

 above the alimentary canal and are directly continuous with short 

 ducts which connect them with the genital atrium and are termed 

 the vasa deferentia (Lat. vas, vessel; defer ens, carrying down). 

 These ducts unite to form the muscular tube called the penis, 

 which communicates with the genital atrium through which it can 

 be protruded. It is inserted into the genital atrium of another in- 

 dividual and in this way the spermatozoa are transferred from one 

 individual to another. This forcible transference of spermatozoa 

 from the male to the female is termed copulation. The proximal 

 portion of the penis is swollen up to form a bulb called the vesicula 

 seminalis, in which the spermatozoa are stored up before being 

 transferred to another individual. 



The female organs consist of a large ovary on each side, 

 divided by constrictions into numerous lobes; these are not well 

 marked in the species represented in Fig. 41. The whole of 

 one ovary and the greater part of the other produce only small round 

 eggs incapable of development termed yolk-cells, these portions 

 of the ovaries being called yolk-glands or vitellaria. The yolk-cells 

 serve as food for the developing eggs. The basal lobe however of 

 one of the ovaries (11, Fig. 41) produces ova capable of development; 

 this is the ovary (sensu stricto) orgermarium. The two oviducts, 

 or, as they are generally styled, the vitellarian ducts, are directly 

 continuous with the yolk-glands and lead directly into the genital 

 atrium. Near their common opening a thick muscular pouch opens 

 into the atrium. This is the spermatheca which receives the 

 spermatozoa from another individual, and emits them on to the ova 

 as they pass its opening. Around the spermatheca are certain glands 

 called the shell-glands, which also open into the atrium. The secre- 



S. & M. 7 



