48 PLATYHELMINTHES TURBELLARIA CHAP. 



which is often very complex, and may consist of a curved chitinoid 

 hook or a coiled loop (Promesostoma), of hooks (Proboscidae), or of 

 an intricate arrangement of plates (Proxenetes) ; or the penis may 

 take on a complex corkscrew-like form (Pscudorhynchiis). The 

 (frequently armed) female genital canal usually possesses a bursa 

 seminalis for the fertilisation of the eggs, but a receptaculum 

 seminis or spermatheca may serve for the reception, the bursa, 

 for the lodgment of the spermatozoa of another individual. The 

 fertilised ovum is provided with a supply of food-yolk and with 

 a shell, which may be formed in a special diverticulum, the 

 " uterus." The development of these organs strains the resources 

 of the animal to the utmost, and in some Proboscidae the 

 alimentary canal is squeezed out and disintegrates, in order to 

 make room for them. 



A few Mesostoma (M. ehreribergii, M. productum, M. lingua) 

 produce two kinds of eggs thin- and thick-shelled. The latter 

 are laid throughout the summer, and lie dormant through 

 winter. The young which hatch in spring out of these 

 " winter " eggs develop rapidly, and when only 7 to 8 mm. long 

 {i.e. one-third the size of the parent) already possess functional 

 genital organs ; the penis, however, is rudimentary, and incapable 

 of being used for copulation. Hence it is probable that this 

 stunted progeny self-fertilise their thin-shelled or " summer " eggs. 

 After the formation of these eggs the same parent is said 

 (Schneider J ) to produce thick-shelled or winter eggs, but how- 

 ever that may be, the first young which hatch from the thin- 

 shelled ova are produced in great numbers at a time (April to 

 May) when food is abundant. These grow rapidly to the full 

 size, and then having attained maturity, cross -fertilise one 

 another's ova, which become encased in a thick brown shell ; and 

 it is these numerous "winter" eggs that lie dormant through- 

 out the autumn and winter. Many Mesostoma, and practically 

 all other Rhabdocoela, however, produce only thick-shelled eggs, 

 and in all cases it is probable that to these many species owe 

 their wide distribution, the exact range of which is, however, 

 unknown, as is also the means of dispersal. 



1 Untcmichungen ii. Plalyhelminthen, Giessen, 1873, p. 101. 



