70 



upon Captain Guise expressing a wish for specimens, to send him two, 

 almost by return of post. 



Physa hypnorum. This very active creature — from the facility 

 with which it ascends to the surface, returns to the bottom, or 

 holds itself suspended in the tank-waters in which it may be confined, 

 for the purpose of observing its habits — ^has given rise to the idea that 

 it does so by means of threads. A very small exercise of the reflective 

 po\vers on our parts, entirely setting aside observation of what really 

 takes place, will suffice to demonstrate the fallacy of any such notion, 

 for the following reasons : — 



Istly. Because any threads, in a densely populated vivarium, would 

 either constantly interfere with the free motions of any other creatures, 

 or, firom their fragility, with their subservience to the purpose for which 

 they were intended. 



2ndly. That their mucous consistency would be constantly diminish- 

 ing, in the constant action upon them, of so solvent a fluid as water. 



3rdly. That ordinary attention to the movements of the creature, 

 will show that it can ascend at will from any part of the vase to the 

 surface in the most direct line, and that it does so in the manner 

 about to be described. 



By bringing the edges of its foot closely together that member 

 is converted into a tube, from which the column of water therein 

 enclosed, is expelled with considerable force, either upwards or 

 downwards, as the animal may be disposed to ascend or descend, 

 being in fact a modification of the mode of locomotion adopted by 

 the cuttle-fish, which effects its rapid movements by ejecting water 

 through a funnel. 



Physa fontinalis, and some of the other Lymnaeidae, exhibit the 

 same powers, though in a less remarkable manner. 



NOTES. 



Dreissena polymorpha, when seen moored by its byssus, would 

 lead us to .suppose that any forcible detachment would be fatal, or 

 attended by circumstances of extreme inconvenience ; but experience 

 teaches us that it can detach itself at will from any position which 

 has become disadvantageous to it, and take up any other more con- 



