LYMN^IDvE OF NORTH AMERICA. 33 



GLIDING: By this method of locomotion the animal appears to 

 slip over the surface of an object in the same manner that a boat glides 

 through the water or a skater glides over the surface of the ice. The 

 foot remains uniform in shape during this process, the shell being 

 carried in one position; this mode of locomotion is more characteristic 

 of Physa than of Lymnaa, but may be observed in many species, as 

 stagnalis, catascopium, haldemani, etc. A characteristic method of 

 locomotion in Lymnaa is to glide over the under side of the surface 

 film, the shell hanging downward. It has been observed that the ani- 

 mal leaves a path or track of mucus behind it as is the case when 

 the snail is crawling over a solid surface. The production of this 

 mucus path seems to be a necessary condition for successful locomo- 

 tion ; this statement has been verified by experiments in which the ani- 

 mal was allowed to crawl over an absorbent surface until its mucus 

 supply was temporarily exhausted and then returned to the water. 1 

 It was observed that the animal was obliged to rest quietly for a con- 

 siderable time before the flow of mucus was again started. The same 

 experiments demonstrated that each snail must prepare its own mucus 

 track, the mucus path of neighboring snails not being used. 



HUNCHING. This method of locomotion is common in such spe- 

 cies as obrussa, parva, etc., and may also be observed in some of the 

 large species. By this method the animal moves forward by muscular 

 contractions of the foot and then pulls the shell after it with a jerk. 

 This variety of locomotion is most notable when the animal is out of 

 the water, which is the normal condition of the smaller species men- 

 tioned. A phenomenon frequently noted is that of twisting the shell 

 from side to side by a sudden jerking motion. This motion is gen- 

 erally used when the animal is partly entangled in vegetation or is 

 imprisoned in any manner. 



THREAD SPINNING. Many, if not all species of Lymnaa, are able 

 to spin a thread of mucus and to hang by it suspended from some ob- 

 ject or even from the surface film of the water. Frequently a snail 

 may be seen to attach a thread of mucus to the bottom and rise to 

 the surface by this improvised ladder, the foot being rolled into a 

 tube. The same snail may fill its lung with air, and with the aid of a 

 few air bubbles to lighten the body, descend the ladder, reeling up 

 the mucus thread as it descends. During locomotion by this means, 

 the foot is extended from the shell to a considerable degree and the 

 animal spends a large amount of time in twisting about. 



iWalter, Cold Spring Harb. Mon., VI, p. 7. 



