vni] MOVEMENT AND MIGRATION 125 



to the present those forms belonging to those families 

 which are known to be positively marine in their habit 

 show no great difference from allies inhabiting fresh 

 water, and are in one case indeed (Paranais) common 

 to fresh brackish and saline waters. As to earth- 

 worms, the number is also extremely limited, and 

 Pontodrilus is up to the present the only genus which 

 is known to inhabit a marine situation almost ex- 

 clusively. It has, moreover, been shown that both 

 earthworms and their cocoons are susceptible to salt 

 water and are killed thereby. Thus the facilities 

 which these animals possess of crossing tracts of ocean 

 are limited by this fact alone, besides other impedi- 

 ments offered by tracts of water as such. We may in 

 fact entirely discount the possibility of earthworms 

 floating across arms of the sea of any extent at any 

 rate. For they do not swim or float, but sink in 

 water. Possibly when the alimentary tract was 

 entirely empty of earth the worms might float ; but 

 it is always full and even if evacuated during their 

 passage to the bottom waters the body thus freed 

 would hardly rise. However the noxious qualities of 

 sea water to earthworms is a sufficient barrier to their 

 traversing even narrow straits. On the other hand 

 it might be suggested that torn up trees especially 

 with the roots and clinging earth still attached might 

 harbour worms and thus transmit them to foreign 

 shores. It has been suggested that in this or 



