VARIETIES OF OYSTERS. 155 



ooze. Lister appears to have trusted too much to his 

 friend, and not to have learnt for himself the fact that 

 oysters have not the slightest power of locomotion, except 

 in their embryonic state. Bishop Sprat's account of our 

 oyster fisheries, which has been so often quoted in works 

 on natural history, was chiefly compiled from this commu- 

 nication of Dr. Willis. The " spat," said to be like a drop 

 of candle-grease, is a pure fiction. 



From April to July the ova are continually excluded 

 from the ovary and discharged into the gills, where they 

 are hatched. Every batch of fry in succession is then 

 committed to the sea, and the young commence life a free 

 animal, like other bivalves, swimming, or rather flitting 

 about with considerable rapidity, by means of numerous 

 cilia which fringe their circumference. Each is enclosed 

 in an extremely thin and prismatic semi-globular bivalve 

 case. Mr. Eyton says, speaking of the appearance and 

 habits of the oyster fry : "The semitransparent animals, 

 with two reddish elongated dots placed on each side beside 

 the cilia (which were in constant and rapid motion), were 

 exceedingly tenacious of life, the cilia moving until the 

 water was dried up upon the glass. Some that I placed in 

 a little salt and water were alive the next dav.' " After a 



j 



short enjoyment of freedom, they attach themselves to a 

 stone or some other object, and the mantle soon begins its 

 object of secretion, and converts the case into a shell ; the 

 latter becomes agglutinated to some extraneous body." 

 . the cilia and eye-like spots disappear, and the 

 permanent organs are developed. This metamorphosis 

 has its parallel in the Cirripedia and other classes of inver- 

 tebrate animals. The parent oyster is slow in recovering 

 from its long continued parturition, and it is not fit to eat 

 until about the middle of August. Indeed, it is not con- 



