166 HOUSE, GARDEN, AND FIELD 



it may remain connected with the surface-film by a thread ; 

 and that it may go up and down with the help of old 

 threads extending from the top of the water to the bottom. 

 This is not all ; threads are certainly seen at times to 

 stretch from one leaf or twig to another, and under favour- 

 able circumstances tracks of slime several feet long and 

 half an inch wide, have been seen to cross the surface of 

 standing water. 1 Any one who finds a water-snail sus- 

 pended between the surface and the bottom should in- 

 vestigate the circumstances with all possible care, as more 

 facts are still desired. 



Pond-snails of several species may be kept in an 

 aquarium. Even small glass globes, holding a pint of 

 water or less, will do perfectly well, if adequate food and 

 air are supplied. Fill the globe with water a fortnight in 

 advance, and set it in a sunny window ; the sides will soon 

 be coated with a green film of minute green water-weeds, 

 and a pond-snail requires no more. Watch the operation 

 of feeding. The upper lip is raised, and the sides of the 

 mouth drawn back ; a brown mass protrudes for an instant, 

 and then the mouth closes. This action is repeated many 

 times in a minute, and each time a morsel of the green 

 weed is licked off and swallowed. Those who possess the 

 requisite skill in dissection can open the mouth, and 

 examine the parts. There is a transverse cutting blade, 

 the mandible, which projects from the upper lip, and can 

 be made to descend, like a guillotine ; there is also a 

 lingual ribbon, a long, brownish, flexible membrane, set 

 with countless pointed, backward- directed teeth, which rasp 

 the food ; it is this which protrudes from the mouth at 

 the moment of opening. The snail, as it travels over the 

 glass, clears a narrow track, and by examining the track 

 with a lens, we can see exactly how much of the green stuff 



1 Crowther, " Mucous tracks of Limnoea stagnalis,"yi?wr//. of Conchology, 

 viii. p. 230 (1896). The published observations respecting the mucous 

 threads of snails have been collected and discussed by H. Wallis Kew in 

 the Zoologist for July, 1900. 



