TURBELLARIA. 217 



the vasa def erentia : * these open into glandular sacs the vesiculae 

 seminales (Vd\ which themselves open into the penis. The penis 

 is an eversible organ, which can be protruded through the genital 

 pore. 



In the Polydada the ovaries as well as the testes are follicular 

 and scattered, and there are no yolk-glands. Moreover, the, external 

 openings are separate, the male being in front of the female. In 

 some Rhabdocoela also the generative openings are separate. In the 

 Acoela the ovaries are compact, but the testes are follicular, and there 

 are no generative ducts, so that the generative cells have to make 

 their way through the parenchyma to the external generative 

 opening or openings. 



The penis is protrusible and very commonly armed with hooks, 

 which are said to assist the animal in retaining hold in copulation, 

 and also to serve as weapons of offence. In Prorhynchus and 

 Stylostomum the penis opens into the mouth. 



A curious instance of vegetative repetition is presented by some 

 Polydada, in which there may be (Anonymus) several pairs of penes 

 and of male openings placed in two rows on the ventral surface. 



In some cases it is said that true copulation takes place ; but in 

 many forms (e.g., Polydada) the sperm masses are deposited on the 

 body of another worm, or even in a wound in the body-wall made by 

 the impact of the penis. In such cases of sperm injection the 

 spermatozoa must make their way to the receptaculum or uterus 

 by travelling through the tissues of their host. These creatures 

 do not always confine their attentions to individuals of their own 

 species, for Lang observed a Pseudoceros crawl over a Thysanozoon 

 and make several wounds, in each of which a sperm mass was 

 deposited. 



Some Rhabdocoela form two kinds of eggs summer eggs, which 

 have thin shells, and are retained in the uterus till hatching; and 

 winter eggs, which have thick brown shells, are laid, and last through 

 the winter to be hatched out in the spring. The eggs are either 

 enclosed singly in egg-shells, or several of them are included with 

 a number of yolk -cells in one shell or cocoon. The Polydada 

 agglutinate a number of eggs together in an albuminous mass. 



The fresh-water Turlellaria as well as many marine forms undergo 

 a simple direct development, and in the young state are often difficult 

 to distinguish from Infusoria. Some of the Polydada undergo a 



* It is possible that there are no vasa deferentia, but that the spermatozoa 

 make their way through the parenchyma to the vesiculae seminales. 



