LYMN^EID^E OF NORTH AMERICA. 35 



in those regions between the parts of the foot that are fixed to the 

 substratum serves to advance the animal. 



"The presence of cilia on the surface of the foot has been re- 

 peatedly demonstrated. These cilia may be easily seen on young snails 

 by means of the microscope. In this connection it is interesting to 

 note that a young Physa primeana, ten days old, showed cilia in motion 

 not only around the edge of the foot, but over the tentacles as well." 



Rate of locomotion. The Lymnseas are usually sluggish crea- 

 tures, content to browse slowly over the surface or to remain hunched 

 at or above the surface of the water. Mr. Walter (1. c, p. 13) has 

 experimented with Lymnsea with the following result : 



Number of 



seconds required Number of 



to go 1 inch. trials. 



70 1 60 



78 80 



141 25 



220 30 

 The average time was found to be 106 seconds. 



d. RESPIRATION. 



Respiration in Lymnsea is carried on normally by means of a so- 

 called lung, which is a fold of the mantle occupying the last part of 

 the body whorl. This forms a sac or cavity into which the ai/ is 

 taken through a modified portion of the mantle which forms a tube 

 or siphon. This cavity is well supplied with blood vessels, the air 

 taken into the lung seriating the blood in these veins before it enters 

 the heart, after it has circulated through the body. 



To obtain the air for respiratory purposes, Lymnsea visits the 

 surface of the water periodically. When the surface is reached, the 

 body is swung around to the right and the siphon opens above the 

 surface with a distinct clicking sound. Apparently the snail is not 

 aware that the surface has been reached until the tip of one, or both 

 tentacles are in contact with the surface film, at which instant the 

 siphon is brought into position. This peculiarity has been observed 

 in a number of species of Lymnsea and is mentioned by Walter and 

 others. The individual, after filling the respiratory chamber with air, 



2 The following- experiment by Walter is of interest: 



"To demonstrate that the rate of progress of a single snail changes when 

 the latter is forced to make repeated attempts to reach the surface, a single 

 instance, ■which is typical of the behavior of all the snails that were subjected 

 to experiment, is given. In the 60 trials of Lymnaeus the escape from the 

 two-inch circle for the first 20 trials took, on the average, 45 seconds. The 

 second 20 trials averaged 63 seconds, while the average for the last 20 trials 

 was 101 seconds. This slowing down may have been simply the effect of 

 fatigue, but more likely it was due to the gradual exhaustion of the mucus 

 supply." 



