152 INTRODUCTION TO ZOOLOGY chap, xi 



when actively feeding, so that the exhaleut and inhalent apertures 

 show and also the foot by which it fixes itself; (2) of the internal 

 organs as displayed ,in a dead mussel when one 'valve of the shell 

 and one of the mantle have been removed ; (3) of a glochidimn 

 larva seen under the microscope. These can be obtained from 

 the outer gill of a female mussel any time during the autumn. 

 The movements of the little larvae are interesting to watch. 



Dreissena will also flourish in captivity, but Marine Bivalves 

 are best studied alive by the sea, as they will not thrive in a 

 small tank. 



