86 POND LIFE 



and has eight short tentacles. Hydra vulgaris is orange-brown 

 in colour, with six tentacles as long as its body. A hydra 

 often found with six remarkably long tentacles is H. Oligactis. 

 All these live and multiply at an enormous rate in an aquarium. 

 They make excellent living slides, and if a drop of a weak 

 solution of ordinary salt is added beneath the cover glass, the 

 hydra will oblige by firing a regular broadside of its thread- 

 like weapons. 



CHAPTER IX 



SNAILS 



WATER- SNAILS do not usually meet with much interest, partly 

 because their life histories are little understood. However, 

 the study of the freshwater molluscs is not so dull as one 

 might think. 



Water-snails are perhaps of most concern to the agricultur- 

 ists, who have suffered heavy losses by the death of sheep. Not 

 that the snail has any evil intentions, but a parasite known as 

 Distomum hepaticum uses certain snails as hosts during part of 

 its life history. The host, unaware of the damage it is thereby 

 doing, leaves the water and wanders over the grass-land. The 

 liver fluke then escapes and encysts itself on the grass, to 

 eventually be swallowed by some unfortunate sheep. 



But even if an agriculturist, care must be used not to give 

 all water-molluscs a bad name, because it happens that only 

 two of the many species of snails so common in ponds and 

 streams are guilty Limnaea peregra and Truncatula. 



The pond-hunter will often find specimens of a spiral but 

 flat snail, members of the Planorbis. These snails are par- 

 ticularly charming whilst young in fact the appearance of 

 their diminutive flat spiral shells is most original. Sometimes 

 the Planorbis will walk along carrying its shell at practically 

 right angles to its body, and seemingly requires to make some 

 effort to draw its encumbrance with it. 



Members of the Ancylus are very similar in appearance to 

 limpets. Ancylus fluviatilis is common in some fast-running 

 streams, Ancylus lacustris prefers still or slowly flowing water. 

 The former with its peculiarly beaked shell is a very fascinat- 

 ing little object. The freshwater mussels the Sphaeridae, 

 Unionidae, and Dreissenidae are very frequently found. The 

 last-named has only one British species, and is thought to have 

 been imported many years ago in timber. 



