HARDWICKE'S SCIEN CE- GO S SIP. 



49 



MOLLUSCAN THEEADS. 



By G. SHERRIFF TIE. 



'ONTAGU, at the beginning of this 

 century, noticed the hcabit in Phym 

 fontinalis of thread-spinning. He 

 says : " Physa fontinalis spins a 

 filament by which it lets itself 

 down from the surface after float- 

 ing." Later, Mr. Kobert Waring- 

 ton ' gave an exceedingly interest- 

 ing account of this thread-spinning 

 by Limncea glufinosa, L. stagnalis, 

 various species of Planorbis (not 

 named by him), and Physa fontinalis. 

 The latter upon one occasion formed 

 a thread so tough that he was enabled 

 to lift the snail seven inches above 

 the surface of the water by it. The 

 author includes in his list of thread- 

 spinners Neritinafluviatilis — of this I shall 

 speak further on — and concludes by 

 his belief that "all the fresh-water snails I 

 are possessed of this power." 



Now, after this well-proven fact of spinning, 

 stated upon the authority of so good an observer, 

 you would scarcely expect to find such an observa- 

 tion as this :— " The Physse, especially P. hypnorum, 

 are active in habit, whether swimming foot upper- 

 most, on the surface of the water, holding them- 

 selves stationary at different depths in the water, 

 or gliding through it in sudden jerks by an hydraulic 

 action of the foot. By bringing the lateral margins 

 of this organ into contact, the animal constructs a 

 tube for inhaling and suddenly expelling the water 

 either upwards or downwards.' Montagu stated, and 

 the statement has been repeated by Jeffreys, that 

 the anunal spins a mucous thread for letting itself 

 down in the water and rising again for respiration ; 

 but I have not succeeded in confirming this observa- 

 tion, and have great doubts of its accuracy."! 



* Zoologist, 1852, pp. 3634-5; 1855, p. 4533. 



t LovcU Reeve, "British Landand Fresh- water Mollusks," 

 pp. 150-1. 1863. 



No. 111. 



Mr. Reeve does not tell us how he proved his 

 assertion about the " hydraulic action of the foot," 

 and does not seem to have tried to ascertain how 

 they " hold themselves stationary at different depths 

 in the water,"— coolly " doubts " Montagu's state- 

 ment about the "mucous thread," and does not 

 notice Mr. Warington's observations at all. I may 

 state that a moUusk is only capable of " holding 

 itself stationary at different depths in the water " 

 when attached to a thread, and that no " hydraulic 

 action " of the foot takes place. When a mollusk 

 is forming a thread, the "lateral margins " of the 

 foot are brought together, forming a channel for the 

 natural flow of mucus down the sides of the foot 

 to the tail; thus adding to the thread, which is 

 gradually extended. The existence of a thread may 

 be proved, as stated by Mr. Warington, by passing 

 a rod under the creature, by which means it can be 

 swayed to and fro. 



I have taken great interest in this thread-spin- 

 ning, and long before I had read Mr. Warington's 

 excellent notes I had been observing this seeming 

 phenomenon, and had tabulated the species abso- 

 lutely seen by myself in the act, and noted the 

 conditions under which moUusks are capable of 

 producing and using a thread. 



Let me here explain that the words thread and. 

 spinning are used descriptively, and it must not be 

 supposed that these threads, or the production of 

 them, bear any analogy to the spinning of spiders- 

 In the case of the mollusk the thread is gelatinous— 

 in fact, is formed of the slime of the creature, the 

 process of forming it being, to a certain extent, an 

 involuntary act, although it is used for a set 

 purpose ; whereas the spider's thread is silken, and 

 its formation is entirely under the control of the 

 creature. Neither are they to be confounded with 

 the byssal filaments of the Mytilidm, Pectinid<e, 

 Dreissena polymorpha, &c., these latter being of a 

 fibrous nature, and the product of a special organ. 

 As members of the order Pulmonohranchiata — 

 breathers of atmospheric air— spin and use threads 



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