180 FRESHWATER FAUNA 



outgrowth from the wound they inflict closes over and 

 encysts them, and they are carried about by their host 

 and nourished at his expense till they attain the state of 

 fully-formed young mussels. Then escaping from the 

 cyst they drop to the bottom of the stream, and relin- 

 quish themselves to a sedentary life. In this way they 

 become dispersed through the river they inhabit, and may 

 even be transferred from one river system to another. 

 As to the history of this cunning device we are entirely 

 in the dark, but something might possibly be learned by 

 observation of the fry of marine mussels. Pectens are 

 known to make swift progression in the adult state by 

 a series of leaps a yard or two in length, produced by a 

 flapping of their valves, while Lima flies with the light- 

 ness of a butterfly. If young spawn are given to the 

 same kind of locomotion, we might imagine that this was 

 the first step towards acquiring the trick of the larval 

 pond mussel. The next advance would be to hold on to 

 objects of any kind, both animated and not. Grown-up 

 bivalves have not infrequently been found tightly nipped 

 on to the feet of newts and other animals (Fig. 55). From 

 this to the selection of a fish might follow as a conse- 

 quence of the advantage of obtaining nourishment, the 

 advantage of dispersal following as an unforeseen result. 

 All this, however, is a mere exercise of fancy, the poorest 

 sort of substitute for knowledge ; and besides, it would 

 appear that larval freshwater mussels by the flapping of 

 their valves do not fly, but simply produce an extension 

 of the byssus. 



It may be of interest to observe in passing that the 

 mussel does not exact service from the fish in ail cases 

 without paying for it in kind. Thus in Central Europe 

 a fish called the bitterling deposits its eggs by means 

 of a long ovipositor in the mantle cavity of Unio or 

 Anodon, where they undergo their development, and 



