152 THE INFLUENCE OF INANIMATE SUREOUNDINGS. 



live in pure fresh water. When I was still a student, Oor~ 

 dylophora lacustris (fig. 40) was found only in estuaries and at 

 the mouths of rivers where the water was at any rate occa- 

 sionally salt or brackish; it was discovered almost simul- 

 taneously in England and Belgium, and somewhat later I 

 found it in the Schlei, near Schleswig. Since that time, 1854, 

 the animal has in many places migrated into rivers; it has 

 already reached the Seine at Paris, and has got into the fresh- 



FiG. iO.— Cordi/lop?iora lamstris (from P. E. Schiiltze), a bracldsh-water polyp which 

 within the last ten years has gradually migrated into pure fresh water. 



water aquarium of the Jardin des PI antes there, where it is 

 said to be very common. Its migrations in the Elbe were still 

 more remarkable. After reaching Hamburg, and even, if I am 

 not mistaken, finding its way into the Alster, it took possession 

 at the same time of the great water-pipes of the city, in which 

 it lived, associated with the well-known bivalve, Dreissena 

 polymorpha, in siich enormous quantities as to impede the flow 

 of water through the pipes. This ca.se is the more interesting 



