IxXXviii INTRODUCTION. 



function to discharge beyond tho precincts of their birth- 

 place. In some instances, too, Joliet has been able to 

 obtain direct evidence of the fertilization of the ovum by 

 extracellular spermatozoa. But though in certain species 

 this method may prevail, we have no reason for supposing 

 that it is general. In Eugula avicularia I have observed 

 the spermatozoa swarming round the ovary, which was 

 situated on the funiculus immediately below the extre- 

 mity of the caecum ; and it can hardly be doubted that in 

 this case fertilization was effected either before or imme- 

 diately after the escape of the ova. In most of the cells 

 eggs in various stages of development were present. In 

 Farrella Van Beneden describes the ova as passing from 

 the ruptured ovisac into the cavity of the cell, and being 

 there surrounded on all sides by the spermatozoa. In 

 Bicellaria ciliata Nitsche has found the ovum fertilized 

 whilst still within the body-cavity, and before its trans- 

 ference to the ooecium ; and in this case there can be no 

 doubt that impregnation was due to the spermatozoa of 

 the cell in which it occurred. We should draw the same 

 inference from S mitt's account of the reproductive ele- 

 ments in Scrupocellaria scruposa*. In Paludicella, ac- 

 cording to Allman, the spermatozoa liberated from the 

 testicle at the bottom of the cell may be seen clustering 

 round the ovary at the top of it. We must, I think, 

 admit that both modes of fertilization exist. Of course, 

 in the case of unisexual species, the spermatic bodies must 

 find their way from the male to the female zooecia ; and 

 we can only suppose that they take the road through 



* He says " sSdana svarma de [the spermatozoa] upp till den plats, der 

 annu agget ligger qvar uti djurhusets 6'fre del" ('Om Hafsbryozoernas 

 Utveckling,' p. 36). 



