168 PEESH-WATEE AQUAEIA. 



It is necessary to take care that no dead snails remain in 

 the aquarium, or they wiU quickly corrupt the water. 



Fig. 114 will perhaps be useful to some readers in helping 

 them to understand the descriptions of the various snails 

 suitable for the fresh-water aquarium which will be given. 



The pectinibranchiate (possessors of comb-like gills, adapted 

 for breathing beneath the surface of the water) fresh-water 

 snails of Britain are divided into three families, viz. : (1) 

 Neritidoe, (2) Paludinidoe, (3) Valvatidw. 



Neritina fluviatilis is the only representative we have of the 

 family Neritidoe. It inhabits running streams. It will not 

 live long in confinement, and it is only mentioned here in the 

 hope that the assertion of its unsuitability for the aquarium 

 wiU save some individuals of this species much unhappiness, 

 and their captors not a little disappointment. 



The following short description of this mollusc may help 

 those who do not know it to identify it : The body of the 

 animal is of a dirty- white colour, spotted with black ; head 

 almost black; tentacles long, slightly transparent, and 

 streaked with black; eyes large and black, and placed on 

 short pedicels at the base of the tentacles ; shell nearly ^in. 

 long, of a yellowish-brown colour, marked with spots of white, 

 brown, purple, or piak ; there are three whorls, the last very 

 large in proportion to the rest; spire very short; sutures 

 rather deep ; operculum haK-moon shaped, and yellowish, with 

 an orange border; the shells are often covered with some 

 chalky substance. These snails generally deposit their eggs 

 upon the shells of one another. 



All the members of the family Paludinidne (dwellers in 

 marshy places) possess an operculum, and. each individual is 

 either male or female. 



Palvdina oontecta (whose specific name implies that it is 

 operculate) is one of the handsomest of the British fresh- 

 water snails. It is found in ponds, canals, lakes, and slow- 

 running rivers in many parts of England, but especially in 

 the South. It is an irritable animal, falling from the stones 

 and plants to which it may be adhering as soon as it is 

 touched or startled. The snail is ovoviviparous, the eggs 



