MEANS OF DISPERSAL 181 



feed, when they become young fish, on the eggs of the 

 mussel. 



None of the other bivalves of our rivers or lakes offer 

 an illustration so " sensational " as that of Anodon and 

 Unio ; but in two of them, Cyclas and Pisidium, the free 

 larval stage is suppressed, and development is intra- 

 parental. They have been seen holding on to the foot of 

 various highly locomotive animals, such as the strong 

 flying water beetle, Dytiscus (Fig. 53), as well as newts 

 and frogs, thus recalling one stage in our fanciful 



FIG. 53. A Water Beetle, Dytiscus, 

 a strong nocturnal flyer, with a 

 bivalve, Cyclas cornea, holding 

 on to one of its legs. After Kew, 

 " Dispersal of Shells." 



account of the evolution of the mussel larva. It is 

 obvious that by this means Cyclas and Pisidium might 

 easily be carried from one river system to another. 



Dreissena, which in outward appearance closely re- 

 sembles the common sea mussel, offers the first serious 

 exception to our rule, since it passes through a free larval 

 stage of development, and the larvae contribute freely to 

 the floating population of lakes and rivers in various parts 

 of Europe. The genus is, however, by no means so 

 richly represented at the present day as it was during a 



