104 ANIMAL BIOLOGY 



used for respiration in the water, in the land snails and 

 many fresh-water species the gills have been lost and the 

 surface of the cavity adapted for breathing air. The 

 garden snail feeds mostly on the tissues of plants, but it 

 will also devour meat and various other kinds of food. 

 It travels most at night leaving evidences of 

 its journeys in the form of slime tracks which 

 result from the mucus secreted by the foot. 

 During the winter, and sometimes in periods 

 of drought, the snail draws into its shell and 

 secretes a porous limy substance over the 



A pond snan~ mouth of tne she11 called the epiphragm. Thus 

 Physa, hav- sealed up, the snail lives in a dormant state 

 spfrai^coli. 86 until the advent of more favorable conditions 



of life. 



Many of the lung-breathing relatives of Helix live in 

 fresh water. The common pond snails, Limnaea, Physa 

 and Planorbis may easily be kept in aquaria where one 

 may watch their many interesting peculiarities of behav- 

 ior. These forms usually come to the surface for air, 

 and, after filling the lung, descend. 

 One curious habit of many pond 

 snails is the spinning of mucus 

 threads from the bottom to the 

 surface film. The snails crawl up 

 and down upon these threads in 



their periodic excursions to the FlG . 88 ._ Conus eburneus . 

 surface for air. 



The sea abounds in gastropods of the most varied forms, 

 sizes and habits. Some of these are carnivorous and prey 

 upon other molluscs. One often finds a bivalve with a 

 round, smooth hole bored through its shell. This tells 

 the story of some carnivorous gastropod which had bored 

 into the helpless bivalve with its rasp and devoured its 



