294 THE ZOOLOGIST. 



extended from its shell, to descend through the water, making a 

 thread as it goes, and to remain suspended in the water upon a 

 thread thus made. This, however, it is believed, but rarely 

 happens. Several of the observations quoted below, it is true, 

 imply descent and suspension ; but, as the thread is generally 

 invisible, it is possible that the animals, in some cases, may have 

 been descending, or resting, upon threads already spun and fixed 

 during ascent. The animal's ability to ascend or descend is 

 attributed by Mr. Tye wholly to the condition of the lung-sac ; 

 the creatures are lighter than water, he says, when the sac is in- 

 flated, and heavier than water when the air of the sac is exhausted 

 or expelled. It must be remarked, however, that it is when the 

 air is exhausted that the creatures ordinarily require to ascend, 

 and when the sac is fully inflated that they have to descend. It 

 seems to the present writer that the changes in the creature's 

 specific gravity are largely contributed to by the contraction or 

 extension of the animal itself into or from its shell ; and it is 

 probable that the creature, when sufficiently heavy to sink, is 

 usually too much contracted and withdrawn to form a thread. It 

 is interesting to note that Mr. Tye recognizes the fact that here, 

 as in Limax, the thread represents the mucus-trail of ordinary 

 progression, such a trail, though usually invisible in the case of 

 a Limnseid, being always present in the track of the moving 

 animal. On plants in vessels in which molluscs have been kept 

 for a few days, Mr. Tye adds, a network of mucus stretches from 

 leaf to leaf, and is readily apparent when fresh water is put in, 

 the bubbles given off by the plants then adhering to the mucus- 

 lines.* 



* The locomotory mucus, besides serving for ordinary crawling on solid 

 bodies (when it is left behind as an attached trail), and for crawling through 

 the water (when it is left in the form of a thread), serves also for a similar 

 crawling progression at the surface of water, the animal, foot uppermost, now 

 leaving the mucus in its path in the form of a floating trail. Limnaeids and 

 Physids are often seen thus crawling at the surface of the water of aquaria 

 and of ponds ; and the habit, which is common to many gastropods of all 

 orders, was long a puzzle to naturalists. Alder and Hancock (1), however, 

 who studied it in Nudibranchia (Sea- Slugs), saw the movements of the foot- 

 sole to be those of ordinary crawling, and recognized the fact that the crea- 

 ture's progress was caused by these movements against the mucus which it 

 emits and leaves in its track. The animal thus crawls along the floating 



