NATURAL AND ARTIFICIAL BOATS. 



and therefore the force of the stream is partially neutralised. 

 These molluscs absolutely swarm in the Cherwell, and in the 

 multitudinous ditches which drain the flat country about 

 Oxford into that river as well as the Isis. 



Belonging to the Gasteropods, the Water-snail can crawl 

 over the stones or aquatic vegetation, just as the common 

 garden snail or slug does on land. But it has another mode 

 of progression, which it very often employs in warm weather. 

 It ascends to the surface of the water, reverses its position so 



PUPA SKIN OF GNAT ACTING AS BOAT. 



that the shell is downward, spreads out the foot as widely as 

 possible, and then contracts it in the centre, so as to form it 

 into a shallow boat. 



The carrying capacity of this boat is necessarily small, but 

 as the shell and nearly the whole of the animal are submerged, 

 and therefore mostly sustained by the water, a very small 

 amount of flotative power is suflicient for the purpose. Some- 



