626 OYSTERS, AND ALL ABOUT THEM. 



in the water, and a larva is seen capable of moving about. 

 The motive power of this larva is evidenced by the acceler- 

 ated rotatory motions, and by the precipitate starts which 

 enable it to traverse the field of observation. Sometimes 

 it gyrates as if on a pivot, sometimes it remains quiet and 

 motionless ; but an attentive examination will show that 

 the vibrating cilia with which it is furnished never cease to 

 execute the movements which are peculiar to them. 

 Arrived at this period of its existence, and considering the 

 minuteness of its girth, J further observation becomes diffi- 

 cult. We have, however, been able to discern the rudi- 

 ment of a shell about the seventh day. 



Fecundation succeeds without the necessity for con- 

 forming rigorously to the indications we have detailed. The 

 determination of sexes, for example, is not even necessary, 

 for in operating on a certain number of subjects there is a 

 certainty that among them will be found males and females. 

 It is the same in decanting ; but we must admit that it is 

 a useful precaution which offers real advantages, and facili- 

 tates the examination of the phenomenon of development. 



We may add that the generative elements keep during 

 several hours, and without being brought into contact, all 



% The larva has a volume nearly equal to that of the ovum. Now 

 the ovum of the angulata, if supposed to be perfectly spherical, and 

 taking the minor axis of this sphere as the diameter, measures : 

 Diameter, 52 thousandths of a millimetre; volume, 73,584 thousandths 

 of cubical millimetres. 



With the view of comparison, subjoined are the measures obtained 

 from the following species : Oyster of Dackar, unisexual ; diameter of 

 ovum, 43 thousandths of a millimetre. Oyster of Toulon (Ost. plica- 

 tula), hermaphrodite ; diameter of ovum, 95 thousandths of a milli- 

 metre. Ordinary oyster (Ost. eduhs), hermaphrodite; diameter of 

 ovum, 122 thousandths of a millimetre. 



