SNAILS AND LIMPETS. 165 



1 



As -the tastes of the various species of Mollusca difEer, 

 some species preferring to eat the healthy growing plants, 

 others the decaying, others confervoid growth, and a few 

 delighting to partake of a little animal food as a relish to their 

 vegetable, it will readily be seen that while some water- 

 snails are useful in the tank, others are injurious there, 

 inasmuch as they will be likely to destroy or hurt the weeds 

 which are so necessary to the welfare of the aquarium. 

 Those snails which prefer to feed either upon decaying 

 vegetation or upon confervse are, of course, the most suitable 

 for the ordinary t».nk. The Univalve MoUuscs are valuable 

 in the aquarium, not only from their readiness to act as 

 scavengers and to clear away the confervoid growth which 

 both checks the growth of the plants and interferes with 

 the transparency of the glass, but also because their eggs 

 and fry provide the fish with about the best of foods. How- 

 ever, it must not be supposed that they will mow away the 

 algae from the sides of the aquarium with the precision 

 with which labourers cut down hay in the fields. Their work 

 must occasionally be supplemented with the gentle rubbing 

 of a bit of sponge tied to a short cane, or, what perhaps is 

 better, a piece of coarse brown paper so fastened. Snails can 

 keep the glass perfectly clean, but it is not wise, for the sake 

 of the plants, to have so many in the aquarium as to force 

 them to do this. I once placed a few half-grown snails 

 (Lirmiaea stagnalis) in a very small aquarium, the sides of 

 which were so covered with confervse that nothing could be 

 seen through them, and by the end of five or six days the 

 glass was as clear as if it had been polished with a cloth. 

 The snails had nothing else but the confervas to eat. 



It is an interesting sight to see a snail at work upon the 

 . transparent sides of an aquarium. The flat portion of the 

 mollusc, which is pressed against the glass, is called the foot 

 or disc, and the animal progresses by alternately expanding 

 and contracting this foot or disc. The mouth of the snail will 

 be continually opening and shutting, and from the lower part 

 of it a curious yellowish- grey band wUl be seen, between each 

 opening and shutting, to come forward and scrape the glass. 



