192 THE FRESH-WATER MUSSEL [CH. VI 



are snapped together, the apical teeth seize a portion of the 

 skin and, folding within the opposed valves, drag it well 

 down into the glochidial shell cavity and into contact 

 with the mantle. The process is not unlike what takes 

 place when a piece of soft material is caught between the 

 teeth of two cog-wheels revolving in opposite directions. 

 When once the glochidium has laid hold it never relaxes 

 its grip. If a hard part, such as a spine of a stickleback, 

 has been seized the glochidium soon dies and drops off; 

 if however it is a soft portion, such as the gill filaments, 

 or tail or lateral fins, to which it has become attached, 

 then the tissues of the fish, irritated by the slight wound 

 and the presence of the glochidium, become inflamed and 

 rise up all round the seat of injury. In a short time a 

 complete cyst is formed entirely covering the glochidium 

 and rendering it impossible for the parasite, as it now is, 

 to detach itself. 



For the next three months the glochidium lives as a 

 parasite upon its host, the fish. At first it obtains all its 

 nourishment by intracellular digestion of the cells and 

 juices of its host by means of its mantle. Subsequently 

 the mantle undergoes a metamorphosis which results in 

 its regeneration, the original cells being replaced by new 

 ordinary ectodermic cells. Henceforth nourishment is 

 obtained from the lymph streams of the host by means of 

 amoeboid cells situated in the gut of the young mussel. 

 It is probable that the perforations of the glochidium 

 shell are of importance in enabling diffusion to take place 

 through it, especially during the earlier stages of parasitic 

 life. Meanwhile the byssus and the adductor muscle of 



