48 The Medium 



ville, La., and is described by an eye witness (Bajkov, 1949). On the 

 morning of October 23, fish ranging between 5 and 23 cm in length 

 fell on the streets and in the yards, mystifying the citizens of the town. 

 There were areas along the main street in which the abundance of fish 

 averaged one to every square meter. The fish belonged to the fresh- 

 water species native to the local ponds. Although no large wind 

 storm occurred, numerous small "devil duster" tornadoes had been 

 noticed in the area. At Marksville the majority of fish were dead 

 when picked up from the ground, but it is perfectly possible that 

 many of them would have survived had they fallen in water. Stock- 

 ing of remote ponds by dumping fish from airplanes has often been 

 successfully accomplished. Transport by local violent air currents 

 may therefore explain the introduction of fish into land-locked ponds 

 or lake systems not connected with other fresh-water areas in which 

 the species occurred. 



Transport by Water. Since water has a much greater buoyant 

 effect than air, transport by the water medium is of vital concern to 

 many more kinds of plants and animals than transport by air. Trans- 

 port by currents plays an important role in distiibution in both inland 

 waters and the sea. However, we find the fresh- water organisms do 

 not utilize this transportation system to nearly as great an extent as 

 marine organisms. This difference may be partially explained by the 

 fact that fresh water is less dense and hence less buoyant. Also, in- 

 land waters are less permanent. Since fresh-water bodies often dry 

 up, or freeze, their currents cannot always be relied upon for trans- 

 portation. 



Correlated with the foregoing facts, we find that many types of 

 animals whose marine species have free-living larvae are represented 

 in fresh water by species with a much shorter larval life. In fresh 

 water there are many more species in which the eggs or young remain 

 attached to the adult, as in the copepod, Cyclops, and the crayfish, or 

 are retained within a brood pouch as in the Cladocera (Fig. 2.11). 



If stream animals make use of the water transport system, they must 

 have some method for getting back upstream again. Some of the 

 devices serving this purpose are most intriguing. The larvae of 

 mussels are provided with special hooks with which they attach 

 themselves to the gills of fish. Some of the fish that are thus parasi- 

 tized eventually wander upstream where the maturing larvae drop off 

 and metamorphose into adult mussels. Other stream forms resort to 

 the formation of resistant spores that may be carried by birds or blown 

 by the wind. 



Oysters and other benthonic animals remain established in tidal 



