and Fish kept in a limited portion of Water. 275 



These water-snails have the extraordinary power of moving 

 along the surface of the water with gi'eat rapidity with their 

 shells downward, the foot being attached as it were to the atmo- 

 spheric air. The Planorbis also can fix itself, without any appa- 

 rent means of attachment, by its side to the flat surface of the 

 glass, and will remain thus for several days. /jixruii j^^^tft 



In watching the movements of the Limnea, I was for sOm6 time 

 under the impression that they had a power of swimming or 

 sustaining themselves in the water, as they would rise from the 

 bottom of the pond, a portion of the rock-work, or a leaf of the 

 plants, and float for a considerable period, nearly out of their 

 shells, without any apparent attachment, and by the contortions 

 and gyrations of their body and shell, move some little distance, 

 in a horizontal direction, from the point which they had left. On 

 more carefully watching this phsenomenon, however, I found they 

 were attached by a thread or web, which was so transparent as to 

 be altogether invisible, and which they could elongate in a similar 

 way to the spider; they also possessed the power of returning upon 

 this thread by gathering it up as it were, and thus drawing them- 

 selves back to the point which they had quitted. These facts 

 were clearly proved in the following manner : — A Limnea stag- 

 nalis had glided its way along a young and short leaf of the VaU 

 lisneria which terminated below the surface of the water, and 

 having reached the extremity launched itself ofi* from it ; after 

 moving about with a sort of swimming or rolling motion in a 

 horizontal direction for some time it lowered itself gradually, and 

 in effecting this the long flexible leaf of the Vallisneria was bent 

 with an undulating motion, corresponding exactly with every 

 movement of the snail, clearly showing that it had a firm attach- 

 ment to the extremity of the leaf. On another occasion a L. glu- 

 tinosa gradually rose from the surface of a piece of submersed 

 rock, and when at the distance of about 3 or 4 inches from it 

 stayed its progress, floating about in a circumscribed horizontal 

 direction for some time ; at last it rose suddenly and rapidly to 

 the surface, evidently from the rupture of its thread of attach- 

 ment. The most convincing proof, however, of this fact that I 

 can perhaps adduce, and one that I have often repeated with all 

 the before-mentioned Limnea, is that when the snail has been 

 some inches distant from the supposed point of attachment, a 

 rod or stick has been carefully introduced, and slowly drawn on 

 one side between them in a horizontal direction, and by this 

 means the snail can be made to undulate to and fro, obeying 

 exactly the movement of the rod : this requires to be done very 

 gently, as, if too much force is used, the web is broken, and the 

 snail rises rapidly to the surface. 



