'Pond-snails 327 



tlic bivalves previously described, but simply a secre- 

 tion from the ordinary slime -gland which most snails 

 possess. The fresh-water snails that have to glide 

 over deep impalpable mud require to pour out this 

 slime to nmke a safe track for themselves, just as for 

 quite opposite reasons the land-snail has to use it to 

 make a smooth path over dry or rough substances. 

 Some of these slime-ropes of Physa have measured 

 14 inches. Most of the threads are spun as the snail 

 ascends, and only rarely {Spliwrium for instance) is 

 it spun in the descent. Sometimes they are left 

 for days and used over and over again ; at other 

 times the descending snail gathers up the thread as 

 it goes, and probably reassimilates it. Some of the 

 Planorbids and Limnseids spin these threads, and use 

 them in the same manner, but the land-slugs, as we 

 shall see, spin them for different purposes. 



