Gasteropod Molluscs. 85 



Bivalves are wonderfully prolific ; the freshwater 

 mussel has been estimated to lay between two and 

 three millions of eggs in a season, and the oyster, 

 it is computed, will produce over half a million of 

 eggs in a year. 



Classification. Bivalves are subdivided accord- 

 ing to the number of the adductors, according to 

 the equality or inequality of the two adductors when 

 both are present and (when there are two equal ad- 

 ductors) according to the presence or absence of a 

 pallial sinus (page 82). The oyster is an example of 

 the group which has one adductor. The mussel, of 

 the group with two unequal adductors. The fresh- 

 water mussel and the cockle are examples of the group 

 with two equal adductors and no pallial sinus, and 

 the gapers, stone borers, and razor-shells, belong to 

 the section with equal adductors and a pallial sinus. 



CHAPTER XVI. 



HEAD-BEARING MOLLUSCS. 



CLASS III. Cephalophora. The snail, whelk and 

 limpet are examples of a class of molluscs, each of 

 which has a distinct head furnished with sensory 

 organs, such as eyes, ear-sacs and feelers. These 

 possess a mouth armed with teeth, arranged on a rib- 

 bon placed at the bottom of the mouth cavity ; this 

 band can be drawn backwards and forwards by a set 



