SNAILS AND LIMPETS. 173 



members of tlie genus Planorbis are m.ore or less suitable 

 for the aquarium, for they seem, as a rule, to feed upon 

 the confervoid growth in preference to any other. They all 

 live in slow-running or stagnant water. There are eleven 

 species of the genus in this country, and most of them 

 are fairly common. The chief characteristic of the Planorbes 

 is a long body contained, with plenty of room to spare, 

 in a flat-coiled shell. The disc or foot of the animal is 

 short, and is attached to the body by a kind of stalk. 

 The tentacles are long and slender. Both sexes are imited 

 in the same individual. They have no opercidum. Some of 

 them, when out of reach of water, have the power of 

 closing the apertures of their shell with an epiphragm. 

 They are, as a rule, herbivorous, though they have oc- 

 casionally been seen to partake of animal food. The snails 

 of this genus, when irritated, sometimes emit a red liquid; 

 and some of them, if not all, are at times thread-spinners. 

 There are few, if any, snaUs more suitable for a tank than 

 Planorbis comeus (Fig. 111). It has long been a great 

 favourite with aquarium-keepers. It is handsome, hardy, 

 large, and not given to destroying useful vegetation. It is 

 prolific, and ready to breed whUe in confinement at almost 

 any period of the year. P. corneus is found in slow-running 

 or stagnant water, but it is rather local. However, it can 

 nearly always be bought very cheaply of most dealers in 

 necessaries for the aquarium. It deposits its eggs upon 

 stones, leaves, and stalks of plants, and, while in confinement, 

 upon the sides of the tank as well. The eggs are contained 

 in an orbicular capsule, and vary in number from twenty to 

 forty-five. The fry are hatched, according to the temperature 

 of the water, in from sixteen to thirty days. The epidermis 

 of the young shell is covered with a kind of down, which has 

 been likened to velvet pile. The body of the animal is black 

 above and slightly greyish underneath. The mouth is 

 brownish-red. The tentacles are long and elegant. The 

 shell is flat-coiled in shape, and of a dark reddish-brown 

 colour, sometimes appearing almost quite black. There are 

 five or six whorls, and the sutures are rather deep. The shell 



