CH. Vl] THE FRESH- WATER MUSSEL 191 



are sent forth in tangled cords through the exhalant siphon ; 

 occasionally singly and sometimes, according to some ob- 

 servers, through a small gap between the mantle lobes 

 situated between the exhalant siphon and the elastic 

 ligament. 



So long as the slimy tangled cords of the glochidia 

 remain attached to the parent, the latter is able to draw 

 them back within the shell again by a kind of sucking 

 action. The glochidia thus ejected are incapable of sup- 

 porting an independent existence, neither are they able to 

 swim. It is frequently stated that they do swim by 

 clapping their valves together : this was long ago denied 

 by Schierholz, and my own observations entirely confirm 

 his denial. On leaving the shell of the parent they slowly 

 sink to the bottom or are carried along by the stream, 

 their byssi floating loosely in the water. If there are 

 water weeds in the near neighbourhood the threads become 

 spread like a spider's web among the leaves and branches. 

 If however a fish comes near them they are thrown into a 

 state of extreme excitement and clap their valves together 

 with extraordinary vigour and rapidity: this may easily 

 be seen by putting the tail or fin of a fish into a watch- 

 glass containing freshly discharged glochidia. The effect 

 of this valve-clapping is to force the byssus straight out 

 from the shell. If now a byssus comes in contact with a 

 fish it sticks to it and the whole tangled mass of glochidia 

 is thereupon trailed after the fish. 



Chance movements of the fins or tail are now almost 

 certain to bring some of the glochidia into actual contact 

 with the skin of the fish. Directly this happens the valves 



