196 THE FRESH- WATER MUSSEL [CH. VI 



siderable distance in front of the shell, the cilia on its 

 surface moving all the while with great rapidity; the 

 shell is then drawn forward over the foot by a rapid 

 muscular contraction. Even when crawling over a smooth 

 microscope-slide the foot does not slip back but remains 

 stationary when this action takes place. As soon as the 

 contraction commences the cilia suddenly cease moving 

 and stand out from the surface of the foot, like the bristles 

 of a brush, absolutely motionless and rigid : in this con- 

 dition they remain until the foot again begins to glide 

 forward. It is possible that the cilia which are in contact 

 with the glass, or other surface, may actually adhere to it 

 and thus fix the foot. The appearances suggest very 

 forcibly that, at the moment of drawing the shell forward, 

 the pressure within the tissues of the foot becomes so 

 great that the cilia cannot assume any other position than 

 one perpendicular to the surface. 



Nothing is known of the growth and adolescence of 

 the young mussels; their minute size and transparency 

 render them exceedingly difficult to capture even in the 

 limited area of an aquarium. 



Powers of Endurance. It is interesting to find that 

 mussels are able .to withstand severe cold, even to the 

 extent of freezing and this even after removal from the 

 shell. I have seen an Anodonta frozen solid on two 

 successive nights as it lay in the dissecting dish and yet 

 return to life, as manifested by pulsations of the heart and 

 activity of the cilia, when slowly thawed. This is true not 

 only of the adult animal but also of the glochidia within 

 the gills. This power cannot fail to be of great value in 



