192 LAKES AND RIVERS, 



in the ditch or brook is so well known to every 

 parent or guardian. The young have a natural love 

 of finding something which is new to them, which re- 

 quires trouble to look for, and involves labour and time 

 spent in the open air. True this natural inclination 

 seldom gets beyond picking out of the water what may 

 be for the moment attractive, only to cast it aside as use- 

 less or to break it up in ignorant thoughtlessness ; but 

 the teacher or parent may usefully direct the natural 

 taste, and - turn what was only an unthinking love of 

 play into a habit of observation and a most pleasurable 

 way of training the mind, with advantages of an almost 

 priceless kind to the future life of a young man, though 

 he be not destined to become a naturalist, or have 

 that turn of mind which might make him in the long 

 run one of the scientific men of his day. In no 

 branch of knowledge can a habit of observation and 

 analogy be more pleasantly, easily, and certainly 

 taught than by encouraging and directing the young 

 in their country rambles to look for, examine, and 

 collect the shells, with their living inhabitants, pebbles, 

 or insects of their neighbourhood, 



These fresh-water bivalves are distinguished by the 

 leaf-shape of the gills, which are in pairs on both sides 

 of the body. They have no distinct head ; the foot is 

 tongue-shaped, and, in some instances, can be con- 

 siderably elongated ; it is used by the animal for creep- 

 ing along or fastening itself to other objects "by a 

 lyssus or bundle of muscular threads." Like the class 

 Moncecia among plants, both sexes are comprised in 

 the same individual ; the respiratory organs are the 

 gills. The mantle is divided into two lobes ; this en- 



