CH. Vl] THE FRESH-WATER MUSSEL 201 



surface waters and is carried hither and thither by the 

 waves and tidal currents. This mode of dispersal is how- 

 ever impossible for an animal inhabiting running fresh- 

 water, for the veligers would inevitably be carried along 

 by the stream and in time swept out into the sea. Here 

 then we see a cause which has led to the retention of the 

 young within the brood-chamber of the parent. Other 

 fresh- water animals exhibit the same peculiarity; many 

 species not only of molluscs, but also of crustaceans, worms 

 and coelenterates, either by maintaining the young attached 

 to the parent or by stocking the egg with abundant food 

 material, defer the acquisition of a free life by the embryo 

 to a far later period than is the case with their marine 

 relatives. Exceptions exist, but it would be going beyond 

 the scope of this book to discuss them. There is little doubt 

 that the prevalence of parental care for the young among 

 the fresh-water fauna is the result of running streams 

 making their way to the salt waters of the sea. An in- 

 teresting exception is afforded by Dreissensia (Dreissena) 

 polymorpha. This species inhabits both brackish estuarine 

 waters and fresh. It has a free-swimming veliger larva 

 which certainly could not make headway against a stream. 

 Nevertheless the species has spread up the Thames and 

 into the Oxford and Birmingham Canal, while on the 

 Continent it has travelled up the Rhine, along the Main 

 and Danube Canal and into the Danube. These exten- 

 sions of its distribution are of recent date and are in fact 

 due to human intercourse. The adult I), polymorpha 

 fixes itself by its byssus to various, relatively firm, objects, 

 such as logs and the bottoms of boats, and in this way has 



