XC1V INTRODUCTION. 



times met with, however, at a very short distance from 

 the entrance to the marsupium. In a specimen of Fluxtrn 

 papyracea, in which ova were present (in some cases near 

 the bottom of the zooecium, in others higher up in it, in 

 others, again, close to the opening into the ooecium), I 

 observed one jerking itself spasmodically, as if it might 

 be freeing itself from the investing envelope; and after 

 repeatedly witnessing its vigorous movement, I could 

 have no doubt that it might pass by means of the con- 

 traction and extension of its substance from the cell to 

 the ovicell. 



In species which are not furnished with the marsu- 

 pium, other means of egress must exist. In some cases 

 the embryo is probably liberated after the disappearance 

 of the polypide, of whose destruction it may perhaps be 

 the cause. In Vesicularia spinosa I have found the 

 mature rose-coloured larva in the tenantless zooecium 

 equipped with its cilia and ready for escape. 



In one or two forms a special opening has been de- 

 tected, near the base of the tentacles, through which the 

 ova pass. Van Beneden was the first to notice this in 

 Farrella * ; and recently Ehlers has detected a similar 

 orifice, placed in much the same situation, in Hypopho- 

 rella f. In the case of Farrella the ova seem to escape 

 at a very early stage of development, and before the ap- 

 pearance of cilia. 



But the most remarkable provision for the liberation of 

 the embryo remains to be noticed. For our knowledge 

 of it we are indebted in the first instance to the observa- 



* M6m. Acad. Roy. Brux. xviii. " Rech. sur 1'organisation d. Laguncvla,'' 

 p. 18 (sep.), pi. i. 

 t Op. cit. p. 66. 



