58 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



In a small pool near Wlirzburg, dry every summer, 

 he finds Daphnids and Asplanchna. In another close by, 

 Cypris, CulicidEB, and fly-larvae, and these are never assoc- 

 iated with Daphnids. When the conditions are favourable, 

 development proceeds at a rapid rate. Thus in 1883 

 the summer was very dry in south Germany ; no rain had 

 fallen for weeks. One night a heavy thunderstorm broke 

 and two days afterwards, v. Kennel found in rain-pools not 

 only Infusoria, Ostracods, Mesostomid Turbellaria with 

 winter-eggs, but also Branchipus with ripe eggs. The next 

 day all was dry as before. How far passive migration 

 has occurred in the case of P. alpina it is dithcult to 

 ascertain. At Wlirzburg, neither fish nor birds visit the 

 spring where it is found. But this does not hold for 

 other localities where I have seen it. If we keep in mind 

 that it is at the point where springs reach the surface that 

 this form chiefly occurs, it is not difficult to accept v. 

 Kennel's supposition (before the eggs were known) that 

 the real habitat of this animal is in the interior of moun- 

 tains and in subterranean water and we are driven to the 

 conclusion that the animal must have ranged over western 

 Europe before the glacial epoch, acquiring its present 

 distribution owing to the separation of England, Ireland 

 and Isle of Man from the Continent and one another. 

 Now however that the egg-capsules have been discovered, 

 it appears more probable that a passive agency has been 

 the main factor in a post-glacial process. 



2. Polycelis corniUa, 0. Schmidt (PI. XII, fig. 5.) 

 This species occurs commonly in fresh-water in the 

 neighbourhood of Port Erin, but in warmer water than 

 Planar ia alpina. 



