THE FREE-LIVING FLAT WORMS (TURBELLARIA) 327 



Both sets of organs have a common genital pore or are provided 

 with separate external openings. In many cases the male organs 

 mature earlier than the female and degenerate as the latter develop 

 so that a study of various stages of growth is necessary to give 

 complete knowledge of the organs. The rhabdocoels show great 

 diversity in structure ranging from those with simple ovaries and 

 testes to those with an elaborate system of accessory glands and 

 ducts that much resemble those of the triclads. The male copu- 

 latory apparatus or cirrus is often remarkably complex and may, 

 as in Dally ellia, present the chief characters for identification of 

 species. 



Some rhabdocoels produce two kinds of eggs, the thin- walled 

 transparent summer eggs which may undergo development within 

 the body of the parent, and the thick-walled winter eggs which have 

 a hard, brown shell and develop in the outer world. In other 

 species only the hard-shelled eggs are produced. In the Catenuli- 

 dae asexual reproduction by the formation of buds or zooids at 

 the posterior end of the body is met with commonly. More than 

 one bud may be produced before separation takes place. 



Planarians (Fig. 590) show less variation in the structure of 

 the sexual organs. The testes, usually numerous, lie both above 

 and below the digestive tract and extend from anterior to posterior 

 end. The seminal vesicle opens into the muscular bulb-like 

 cirrus, the apex of which projects into the male genital atrium, 

 which in turn leads into the common atrium. Two ovaries are 

 placed far forward. The numerous yolk glands open into the 

 oviducts as they pass back and either unite to form a common 

 duct which enters the genital atrium or open separately into the 

 posterior part of the uterine duct. Fertilization apparently occurs 

 in the uterus which lies just back of the pharynx. 



Some triclads manifest only sexual reproduction; others have 

 regular alternating periods of sexual and asexual reproduction; 

 while a number do not have a definite life cycle since sexual ma- 

 turity occurs at irregular intervals and often only among a limited 

 number of individuals. In these forms reproduction is ordinarily 

 asexual. Dendrocoelum lacteum attains sexual maturity and de- 

 posits its cocoons during the winter months. In Planar ia maculata 



