8 NATURE'S TEACHINGS. 



times, on a fine day, whole fleets of these natural boats may 

 be seen floating down the stream, thus obtaining a change of 

 locality without any personal exertion. 



In perfectly still water, where no current can waft the 

 Limnaea on its easy voyage, it still is able to convey itself from 

 one place to another. By means of extending and contracting 

 the foot, it actually contrives to crawl along the surface of the 

 water almost as readily as if it were upon the under side of 

 some solid body, and, although its progress is slow, it is very 

 steady. Another very common British water-snail, the Pouch- 

 shell (Physa fontinalis), has almost exactly the same habits. 

 Reference will be made to the Pouch-shell on another page. 



The capacity for converting the body into a boat is not con- 

 fined to the molluscs, but is shared by many other animals. Take, 

 for example, the well-known marine animals, called popularly 

 SEA- ANEMONES. As they appear when planted on the rocks, 

 they look as incapable of motion as the flowers whose names 

 they bear. Yet, by means of the flattened base, which they 

 use just as a snail uses its feet, they can manage to glide 

 along the rocks in any direction, though very slowly. 



The base is capable of extension and contraction, and by 

 elongating one side of it, fixing the elongated portion, and 

 then raising the remainder of the base towards it, the animal 

 makes practically a series of very slow steps. This mode of 

 progression may often be seen in operation on the glass front 

 of an aquarium. 



The same property of expansion and contraction enables 

 the Sea-anemones to convert their bodies into boats, and float 

 on the surface of the water. When one of these animals 

 wishes to swim, it ascends the object to which it is clinging 

 say the glass of the aquarium until it has reached the air. 

 It then very slowly, and bit by bit, detaches the upper part of 

 the base from the glass, allowing itself to hang with its 

 tentacles downward. These, by the way, are almost wholly 

 withdrawn when the animal is engaged in this business. By 

 degrees the whole of the base is detached from the glass except 

 a very tiny portion of the edge. The base is next contracted 

 in the middle into the form of a shallow cup, and, when this is 

 done, the last hold of the glass is released, and the animal 

 floats away, supported by its hollowed base. 



