210 THE INVERTEBRATA 



one-quarter or one-fifth the length of the body. This pore leads into a 

 space known as the genital atrium. Into the genital atrium open the 

 separate ducts leading from the male and female portions of the 

 generative system, together with other accessory organs. The homo- 

 logies of the various accessory portions of the generative organs in 

 the three different groups are difficult to ascertain. Names are often 

 used which were applied to organs before their homologies were 

 ascertained, and this increases the confusion. 



In studying the generative systems in actual specimens elaborate 

 reconstruction from sections is often necessary, as the heavy pig- 

 mentation obscures them when the animal is viewed by transmitted 

 light. In transparent specimens careful staining will bring to light 

 most of the parts but it often requires considerable skill and practice 

 to identify these parts. 



The organization of the platyhelminth generative system may be 

 reduced to a general plan as follows. The testes are round bodies, often 

 very numerous, having a lining of cells which give rise to the sperma- 

 tozoa. From the testes lead out ducts, the vasa ejferentia, which, 

 uniting, form the vas deferens. There are usually two vasa deferentia 

 collecting the sperm from the testes on either side of the body. The 

 ends of the vasa deferentia are often distended and act as vesiculae 

 seminales. The vasa deferentia unite and lead into a pear-shaped bag 

 with very muscular walls. This is the penis. At rest it opens into the 

 genital atrium, but during copulation it is extruded through the genital 

 pore to the exterior and pushed into the genital pore of another in- 

 dividual. The penis is usually seen very easily, being one of the most 

 conspicuous parts of the genital apparatus. 



The female portion of the generative system consists of the ovary, 

 which produces the ova, and the vitellarium, which supplies the ova 

 with yolk and a shell. The shell substance is liquid and hardens later. 

 This division into ovarium and vitellarium (or "yolk gland" as it is 

 sometimes called) occurs throughout the Platyhelminthes, but it is 

 probably an elaboration of the more usual arrangement of forming 

 the yolk in the ovary, an arrangement which occurs in the primitive 

 Acoela and in the Polycladida. The ovaries discharge their ova into an 

 oviduct which is enlarged near the point of this discharge and thus 

 forms a receptaculum seminis. Here fertilization occurs. The oviduct 

 next receives the opening of the vitelline ducts. After the opening of 

 the vitelline ducts the duct continues as the ductus communis, and 

 leads into the genital atrium. At the junction of the oviducts and 

 vitelline ducts there is a thickening of the walls of the duct and 

 certain glands, the "shell" glands, pour a secretion on to the egg 

 which probably assists in hardening the shell. This thickening is 

 indistinct in the Turbellaria but is very marked in the Trematoda, 



