164 HOUSE, GARDEN, AND FIELD 



surface, for greater size means diminished power of using 

 capillary forces. A Cyclas, especially when young, will 

 also attach slime- threads to the bottom of a vessel, or 

 to floating plants, and use these as a means of ascending 

 and descending. Such threads, being perfectly transparent, 

 are not easily seen. Occasionally they become visible by 

 their reflecting a beam of sunlight, or by air-bubbles cling- 

 ing to them, especially when the water has recently been 

 changed, or by particles of fine mud becoming attached 

 and giving them a brownish tint. It is only close observers 

 who will see the slime-threads at all, and even they may 

 have to wait long. Sometimes a young Cyclas has been 

 seen to hang from a floating plant, and to rotate for hours 

 by means of the stream of water issuing from one of its 

 siphons. 



The travelling pond-snail is the commonest of water- 

 snails. Like a garden-snail or a slug, it breathes air by a 

 kind of lung, which has however no communication with 

 the mouth. This is a strange feature in an animal which 

 is usually immersed in water, and points to descent from 

 land-snails a derivation which is on all accounts highly 

 probable. Travelling pond-snails are able to leave the 

 water at pleasure, and are not uncommonly found on a 

 bank, or in wet grass, or on the roots of a tree, but they 

 love damp places and soon perish in dry air. They often 

 come up to the surface of the water to breathe, and at 

 such times a pore may be seen to open on the right side of 

 the body, close to the thin lip of the shell. This pore, which 

 is placed as in a slug or a garden-snail, leads into the lung. 

 Fresh-hatched pond-snails have the lung filled with water, 

 and the same thing is said to be true of the pond-snails 

 which dwell in the depths of great lakes, though they re- 

 place the water by air whenever they have an opportunity 

 of doing so. The freshwater limpets (Ancylus), which 

 may be considered as a kind of pond-snail, regularly fill 

 their lungs with water, and keep them filled. 



It is easy to study the form of a pond-snail which is kept 



