BIOLOGY OF PHYSA 65 



having lung respiration have spinning power. He says in 

 speaking of the thread: " I have never seen a member of this 

 order descend by a thread unless it had first ascended by one, 

 in which case it might return upon the same thread. It would 

 no doubt be possible for it to descend by a thread if its air cham- 

 ber was sufficiently empty to allow of its sinking ; but atmospheric 

 air being essential to the creature's existence, it ver}^ rarely 

 voluntarily descends without a supply, and never in such a 

 case by a thread." "The method of anchoring these threads 

 to the surface of the water is singular. A minute concavity 

 at the upper end acts like a small boat, and thus sustains the 

 thread. When one of these mollusks descends by the thread 

 it spins in ascending, it generally carries back the thread with 

 it, gathering it together by a muscular action of the foot ; although 

 these threads are sometimes fixed and made to last a consider- 

 able time. The longest threads I have seen are those of the 

 Physa and I have had in a vessel containing 14 inches depth of- 

 water. a number of them fixed by P. hypnorum, up and down which 

 they were creeping for eighteen or twenty days together. Perma- 

 nent threads are kept in position and strong enough for use by the 

 addition of a film of mucus each time a mollusk crawls over 

 them; and I may here explain what I wish to convey by saying 

 that the process of spinning is to a certain extent an involuntary 

 act. When a snail crawls (either a terrestrial or an aquatic 

 species) it leaves behind it a trail of mucus, which is discharged 

 for the purpose of lubricating the foot in its passage over any 

 surface, and if the continuity of this mucus be not ruptured, we 

 have a thread in all respects analogous to those I am speaking 

 of." Tye says that the use of these threads seems to be: (i) 

 " They enable the mollusks to reach the surface of the water 

 gently when no other means present themselves, and to return 

 to its original station, which it often does, after having ascended 

 to the surface of the water and opened its bronchial valve for 

 the entrance of more atmospheric air. (2) It is a much easier 

 method of locomotion. (3) It is a much quicker mode of trav- 

 eling; for if the surface traversed be smooth, as the sides of a 

 glass vessel, it will take the mollusk twice the time to creep 

 as to float by a thread, while if the surface be uneven, as the 

 sides of a pond, or the leaves of a plant, it would be longer still 

 in creeping. (4) As a great part of the life of the Lymnaeidae, 



