II SPINNING OF THREADS 29 



of the European species in which the gelatinous secretion con- 

 tains more carbonate of lime, solidification seems to take place 

 at the moment when the air is expelled, and the epiphragm in 

 these is in consequence strongly convex. 



Thread-spinning. — A considerable number of fresh-water 

 Mollusca possess the power of stretching a thread, which is no 

 more than an exceedingly elongated piece of mucus, to the 

 surface of the water, and of using it as a means of locomotion. 

 This thread bears no analogy whatever to the fibrous byssus of 

 certain bivalves, being formed in an entirely different manner, 

 without the need of a special gland. 



The threads are ' spun ' by several species of Limnaea, Physa, 

 and Planorhis, by JBitliynia tentaculata, and several of the 

 Cycladidae. They are anchored to the sui'face by a minute 

 concavity at the upper end, which appears to act like a small 

 boat in keeping the thread steady. The longest threads are 

 those of the Fhysae, which have been noticed to attain a length, 

 in confinement, of 14 inches. They are always spun in the 

 ascent, and as a rule, when the animal descends, it rolls the 

 thread up and carries it down as it goes. A single thread is 

 never spun on the descent, but occasionally, when a thread has 

 Ijecome more or less of a permanence, it becomes stronger by 

 the addition of more mucus each time it is used, whether for 

 ascending or descending purposes. Cydas cornea appears to be 

 an exception to the rule that threads are only spun on the 

 ascent. This species, which is particularly fond of crawling 

 along the under surface of the water, has been noticed to spin 

 a thread half an inch in length while on the surface, and to hang- 

 suspended from it for a considerable time. 



What the exact use of the thread may be, must to a certain 

 extent be matter of conjecture. The Limnaeidae are, in the 

 great majority of cases, compelled to make periodic visits to the 

 sm-face in order to inspire oxygen. It is also a favourite habit 

 with them to float just under the surface, or crawl about on its 

 under side, perhaps in pursuit of tiny vegetable organisms. 

 Whatever may be the object of an excursion to the surface, a 

 taut thread will obviously be a nearer way up than any other 

 which is likely to present itself; indeed, without this thread- 

 spinning power, which ensures a tolerably rapid arrival at the 

 svn-face, the animal might find itself aspliyxiated, or at least 



