110 JEAN DAWSON 



It reacts alike to all stationary objects. The reaction resulting 

 from contact with such an object depends, as has been previously 

 stated, upon the momentum of the snail and the character of 

 the substance, whether hard or yielding. Physa reacts to motion 

 i/t^-cKirin the habitat in the following ways: — Gently flowing water 

 \ does not visibly affect the snail, while rapidly flowing water 

 icauses it to contract and hug the substratum. Snails which 

 meet, respond positively or negatively according to their momen- 

 tum. The motion of the water caused by the swimming of 

 tadpoles, frogs and insect larvae does not call forth visible reac- 

 tions unless the moving object comes very close. If these animals 

 touch the snails, the latter react momentarily. Tadpoles were 

 introduced into a habitat in which the snails were, at least for 

 some time, totally unused to the presence of such animals. 

 The sudden onslaught of a tadpole caused the snail to contract 

 violently, expel the air from the lung and drop to the bottom. 



This reaction suggests, perhaps, the Anlage of fear in the 

 higher animals. After the tadpoles had been in the aquarium 

 for some time, the snails react when suddenly struck; but they 

 do not expel the air from their lungs, and seem to receive less 

 of a shock, perhaps the Anlage of a startled reaction in the 

 higher animals. The snails in the aquarium have undergone a 

 change in their physiological state and now react to the blow 

 from a tadpole like the snails that are accustomed to these 

 animals in the field. 



From the lists of the environmental factors (see Table IV) that 

 are capable of giving stimuli in the different habitats, it is seen 

 that some snails are receiving many different kinds of stimuli 

 per hour over and above the number received by others. Through- 

 out the work with Physa it has been noted that snails from some 

 habitats become accustomed much more quickly than others 

 to being handled ; in other words, they are more adaptive. Those 

 that are accustomed to living alone in an undisturbed environ- 

 ment take longer to become accustomed to being handled than 

 those living in a habitat in which a variety of animals are 

 actively moving about. A snail may be approached without 

 frightening or startling it, if some motion to which it is accus- 

 tomed is imitated, while it is difficult to approach a snail that 

 lives alone in a still-water habitat without startling it. It appears 

 from the above, that under the same conditions, snails in like 



