4o6 Qiiarierly Journal of Concliology. 



the water, they take to the surface easily, and creep along slowly, 

 and apparently with caution, as if in search of some floating 

 substance, near which they will rest for hours. . . . The foot 

 during repose is usually retracted, and does not seem necessary 

 for mere floating purposes."* "I have this morning watched 

 one, which had reached the surface, spin its filament and descend 

 to half an inch below the surface, where it remained suspended 

 for some time. It occupied three hours in spinning this short 

 thread. I think it consists of more than a single filament. . . . 

 The surface of the water was again depressed or cupped." He 

 states that the filaments vary from one to four, were far apart in one 

 instance, but rarely could more than one thread be seen; and 

 that the animal has the power of raising itself to the surface 

 again by means of its thread. 



M. Bouchard-Chantereux has recorded that the young of S. 

 corneum possesses the same power of spinning a thread. I have 

 myself seen the latter anchor itself by a mucus filament. The uses 

 of these threads to the Pulmonobranchs appear to be: — 



I St. They enable the moUusk 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 branchial 

 valve for the entrance of more atmospheric air. 



2nd. It is a much easier method of locomotion. 



3rd. It is a much quicker mode of travelling; for if the 

 surface traversed be smooth, as the side of a glass vessel, it will 

 take the moUusk 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. 



4th. As a great part of the lifetime of the Limnseidse, especi- 

 ally the Physce, is spent in floating upon the surface of the water. 



\ 



* Without doubt they are kept afloat by the mucus cable. 



