SPINNING MOLLUSCS. 299 



distance; and, by then slowly drawing the rod upwards, he 

 succeeded in raising the Snail out of the water, a space of about 

 seven inches, suspended by its thread, which, though difficult to 

 see in the water, now became distinctly visible. Mr. Tye chiefly 

 observed Physa hypnorum ; he states, however, that P.fontinalis 

 uses its thread in a similar way, though less frequently. According 

 to this observer, the young, as soon as they issue from the egg, 

 are capable of spinning a thread and rising to the surface of the 

 water : — 



" If my readers wish to see for themselves this habit of travelling, as 

 used by the mollusca, let them take a few adult Physa hypnorum . . . 

 place them in a glass vessel with some small pebbles at the bottom and a 

 little weed . . . and keep them until they deposit spawn. As soon as the 

 young are free from the spawn mass they will commence spinning, and 

 practise it so often that the process may be seen at any time." 



All the threads observed were spun upwards during ascent to 

 the surface ; the longest were the work of P. hypnorum, and 

 were spun in a vessel in which the water was fourteen inches 

 deep ; they extended from the bottom to the surface. When a 

 P. hypnorum, spinning its upward thread, was much disturbed, 

 it was seen to abandon the idea of reaching the surface, and to 

 turn and descend the unfinished thread, altering its position, 

 Mr. Tye says, with much dexterity and ease by bringing its 

 extremities together, and changing the point of attachment of 

 the thread from the tail to the head. This is certainly a curious 

 performance ; but, allowing for the reversed condition of specific 

 gravity, it will be noted that it exactly corresponds with the 

 turning and ascent of a thread by Limax ; the buoyancy of Physa, 

 of course, keeps the thread taut, just as does the weight of Limax. 

 When the animal completes its thread, it attaches it at the 

 surface of the water, a minute concavity at the upper end acting, 

 according to Mr. Tye's description, like a small boat, of air, and 

 sustaining the thread ; but it is more correct to say, perhaps, 

 that the thread is continued at the surface in the form of a 

 floating slime-trail, and that, thus anchored, it slightly cups the 

 water's surface-film. When the Physa returns from the surface 

 by descending its thread, Mr. Tye further observes : the thread- 

 not invariably gathered up and carried back — sometimes remains 

 attached to the surface ; and in that case, it may be used, both 



