II SPINNING OF THREADS 29 



the epiphragm. In some of the European species in which 

 the gelatinous secretion contains more carbonate of lime, solidi- 

 fication 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 exceedingl}^ elongated piece of mucus, to the sur- 

 face 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^ PJiysa^ 

 and Planorhis^ by Bithynia tentaculata, and several of the 

 Cycladidae. They are anchored to the surface 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 Pliysae^ 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 

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

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

 cending or descending purposes. Cyclas 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 sus- 

 pended 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 

 surface in order to inspire oxygen. It is also a favourite habit 

 with them to float just under the sui'face, 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 insures a tolerably rapid arrival at the 



