BIOLOGY OF PHYSA 45 



II. MUCUS AND MUCOUS THREADS OF PHYSA. 



I. Secretion of mucus; its properties. 



II. Formation of an epiphragm. 



III. Food and mucus correlated. 



IV. Nature and process of spinning. 

 V. Conditions affecting spinning. 



1. Food. 



2. General activity or tonus affecting spinning. 



3. Habitats affect spinning. 



VI. Some principles underlying spinning. 

 VII. Biological significance of spinning. "r-* 



VIII. Brief discussion of the literature on spinning. 



1. Secretion of mucus; its properties. 



Mucus is secreted over all the exposed parts of the snail's 

 body but does not flow perceptibly from the glands except upon 

 the ventral side of the foot. The presence of slime on any ex- 

 posed part of the animal's body may be easily demonstrated 

 by touching the integument with a fine rod or bristle. The 

 mucus may be felt sticking to the rod or, when the bristle is 

 withdrawn, may be seen to be stretched out into a fine short 

 thread. The experiment may be tried when the snail is crawl- 

 ing on the substratum either in or out of the water but the 

 demonstration of slime on the integument is rendered much 

 easier if snails are taken that have become accustomed to being 

 handled, otherwise they may retreat into their shells at the 

 slightest disturbance of the water. The slime, secreted by the 

 snail's integument has presumably the same nature whether 

 found on the ventral side of the foot or on the other exposed 

 parts of the body. The description which follows is for the 

 former alone. 



The mucus or slime varies in physical properties according to 

 the length of time since its secretion. Freshly secreted mucus 

 is a clear, viscous, jelly-like substance, resembling in appear- 

 ance the white of an egg. (When it is fresh and under water 

 or in a thin layer on the substratum it is practically invisible 

 and difficult to study.) Fresh mucus is at all times more or less 

 stringy and may be pulled out into threads of greater or less 

 length according to the physiological condition of the snail 

 at the time. The ordinary thin trail of the mucus is not visible 

 at any time under water unless the snail has crawled over a 



