II.] INTRODUCTION. XXV 



first male and afterwards becoming female. In the case 

 of the Concliifera or bivalves, they are probably all 

 strictly ^^ monoecious ^' and fertilize themselves. It was 

 at one time supposed that the Oyster and freshwater 

 Mussel were exceptions to this rule,, and that some in- 

 di^4duals were male and others female; but the re- 

 searches of Davaine, Moquin-Tandon, and other able 

 physiologists have disproved this idea*. The external 

 organization of the Conchifera, as well as the complete 

 isolation of the Teredines and many other kinds from all 

 intercourse with each other during the whole of their 

 lives, would make it very unlikely that they are of dif- 

 ferent sexes. Some of the Mollusca are ovoviviparous, 

 the eggs being hatched within the body and the fry ex- 

 cluded in a perfect form. This is the case not only with 

 " monoecious ^^ kinds (such as Anodonta and Kellia) , but 

 also with some snails [Helix rupestris and Pupa umbili- 

 cata) and even with " dioecious ^^ kinds, such as Paludina 

 and certain species of Littorina. Some particulars will be 

 found in the following pages with regard to the amours 

 of the land and freshwater snails ; and much more might 

 be said on this curious subject. The tender passion 

 seems to take up much of their time and attention. M. 

 Turpin has observed a pair of the common garden-snail 

 [Helix aspersa) engaged in love-making for the space of 

 more than ten hours ! 



Fecundity. — The extreme fertility of some moUusks is 

 not exceeded by that of fish. It has been estimated that 

 the oyster spawns annually at least three millions. Ac- 

 cording to C. PfeifFer the gills of a freshwater mussel of 

 moderate size contain at least 400,000 eggs ; and Jacob- 

 son has given a much higher figure (two millions) for the , 



* See also Hancock " On the Organization of the Brachio'poda" Phil. 

 Trans, vol. cslviii. p. 816. 



b 



