INTRODUCTION. Ixxxvii 



but I have not been able to determine the mode in which 

 the fertilization of the ova is effected. 



Joliet has observed the separation of the sexes in a 

 Lepralia ; and, according to Vogt, Loxosoma phascoloso- 

 matum is unisexual ; whilst in Tendra, according to 

 Repiachoff"^, some of the cells in a colony are herma- 

 phrodite, but the greater number of one sex only. 



The origin of the reproductive elements and their posi- 

 tion in the perigastric cavity have already been discussed 

 in the account of the endosarc (pp. xlvi-1). 



The spermatozoa are formed out of the nucleus, which 

 occui's in each of the vesicles included in the sperm-cell ; 

 and consist of a thread-like body, which is occasionally 

 vibrioid, but more commonly somewhat enlarged at one 

 extremity f. They have an undulatory movement; and in 

 some cases I have noticed a violent shaking of the clavate 

 head. 



It is a question whether the ova are fertilized by sper- 

 matozoa developed in the same cell with themselves, or 

 whether the office is discharged by those which are libe- 

 rated from other cells. Joliet has adopted the latter view, 

 and supported it by a number of observations, which 

 appear at least to show that in some cases the sperma- 

 tozoa are entirely cleared out of the zooecium before the 

 ova have reached the stage for fertilization. The very 

 fact, too, that they are sometimes ejected in such immense 

 numbers into the surrounding water (where they seem to 

 live and thrive), may be taken to prove that they have a 



* Zeitsch. f. wisscnscli. Zool. xxv. p. 129. 



t Both Farre and Van Beueclcu describe them as having iu some cases a 

 rounded or discoid head ; and the former states that by this they occasionally 

 fix themselves. 



