BIOLOGY OF PHYSA 53 



IV. Nature and process of spinning. 



The process of spinning as found among mollusks is not 

 spinning at all, in the sense in which that term is used for spiders 

 or silk worms. The snail leaves the substratum, floats upward 

 through the water by reason of its buoyancy and leaves behind 

 it a fine invisible mucous thread to which the posterior end 

 of its foot is attached. The spinning consists in the formation 

 of this mucus into a filament. There are no special glands 

 or organs for the purpose and the mucus of the thread is nothing 

 more than the mucus of the foot which ordinarily, when the 

 snail is crawling, is left behind on the substratum in the form 

 of a thin ribbon. The thread differs from the ribbon in that 

 it is cylindrical and passes up through the water and is fastened 

 at one or both ends. The snail always begins the spinning 

 of a thread while it is on a solid. 



Before entering into a somewhat detailed account of spinning 

 some of the reactions of Physa to mechanical stimuli will aid 

 in a better understanding of its behavior. The following observa- 

 tions were first made on the reactions of Physa to the stimuli re- 

 ceived in its undisturbed environment, and they were subsequently 

 repeated by the use of artificial stimuli applied with a slender 

 glass rod. The reaction following a stimulus varied with the 

 strength of the stimulus. In general, allowing for the difference 

 in physiological state, a weak stimulus produced a positive re- 

 action, while a strong stimulus produced a negative reaction. 



A moderately strong stimulus upon the top of the head or 

 tentacles such as a snail receives, when it strikes the sides of a 

 containing vessel as it is moving along at a moderate pace of 

 about forty seconds per inch, causes it to pause momentarily and 

 raise the head. It then moves on. A like stimulus upon the 

 side of the head and one tentacle causes the animal to turn 

 toward the source of the stimulation. A strong stimulus on 

 one tentacle or on the side of the head or on both at once when 

 a snail is moving about one inch in twenty seconds, causes the 

 animal to turn away from the source of stimulation. This may 

 also be seen when the animal strikes these parts against the side 

 of a dish or stone when it is traveling at a rapid rate. A very 

 strong stimulus on the top of head such as a snail receives when 

 struck by a rapidly moving animal causes it to retreat momen- 

 tarilv into its shell. 



