70 JEAN DAWSON 



held in the slime serves to startle Physa and causes it to react 

 by drawing violently away from the entangled animal. This 

 allows the latter to escape. Indeed, Physa does not make 

 an attack upon other snails even though it is in a starving con- 

 dition. The ones that succumb to hunger first, however, are 

 greedily devoured by the remaining snails. 



The habitat in which the snail is living necessarily determines 

 the kind of diet that it must feed upon. For example, aside 

 from the microscopic animal and plant life, a snail living in 

 running water such as is found in the creek-bed habitat must 

 feed almost entirely upon Cladophora. On the other hand, 

 in the pond habitats a mixed diet can readily be obtained, 

 since all debris of dead animals and plants must needs be left 

 to decay where it falls. 



II. Mucus as a food and as a means of collecting food. 



One cannot watch an active snail like Physa moving con- 

 tinuously about leaving a track of mucus behind it wherever 

 it goes, without wondering at the secretion of such great quan- 

 tities of mucus. In fact, it does not take an active snail long 

 to secrete a mass of mucus the size of its own body. This 

 mucus is insoluble in water and hence if a number of snails 

 are kept in a small aquarium, we should expect to find a per- 

 fect interlacing net work of mucous threads such as Pearl (1903) 

 found with his planarians. No such accumulation of mucous 

 threads occur, however, since they are eaten by the snail. 



It is a common thing to see Lymnaea stagnalis (and other 

 fresh water pulmonates) turn about and eat from the foot the 

 mass of somewhat hardened mucus together with what foreign 

 bodies this has incorporated. Linden (1892) has observed the 

 dexterity of Lymnaea in obtaining food from the foot, while 

 the snail is moving upon the surface film. Brockmeier (1898) 

 describes this process as " plankton fishing." The mucus serves 

 not only as a food itself, but is a most efficient means of collecting 

 food for the snail, both on solids and on the film. The trails 

 of mucus left on solids remain sticky and serve as a trap to catch 

 minute plants and animals and other foreign particles. The 

 snails eat continually from the solids, devouring the mucus 

 with all that is caught in it. It thus happens as Walter (1906) 

 records for Lymnaea, that the snail may even " engulf and 



