LYMNZIDE 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 Lymnea, but may be observed in many species, as 
stagnalis, catascopium, haldemani, etc. A characteristic method of 
locomotion in Lymnea 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.? 
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. 
HuncuInc. 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 Lymnea, 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. 

Walter, Cold Spring Harb. Mon., Vi, p:. 7. 
