USE OF THREADS 



seriously inconvenienced, before it could succeed in taking in 

 the desired supply of oxygen. With the Cycladidae, which do 

 not breathe air, such an explanation is out of place ; in their 

 case the thread seems to be a convenient means of restin^r in 

 one position in the intervals of the periods of active exercise to 

 which several of the species are so much addicted. 



The power of suspension by a thread is also possessed \)j 

 certain of the Cyclostomatidae, by some Cerithidca, several Eissoa 

 and other marine genera, prominent among which is Litiopa 

 horiihyx, whose name expresses its power of anchoring itself to 

 the Sargasso weed by a silken thread of mucus. Several 

 species of slugs are known to be able to let themselves down liy 

 tlireads from the branches of trees. Limax arhorum is especially 

 noted for this property, and has been observed suspended in 

 pairs during the breeding time. According to Binney, all the 

 American species of Limax, besides those of I'ehennojyJiorus, 

 possess this singular property. Limax arho7'um appears to be 

 the only slug which has been noticed to ascend, as well as 

 descend, its thread. It has also been observed ^ that when this 

 species is gorged with food, its slime is thin and watery, and 

 unable to sustain its weight, ])ut that after the process 

 of digestion has been performed, the mucus again becomes 

 thick and tenacious. It appears therefore that when the 

 animal is hungry and most in need of the power of making 

 distant excm'sions in search of food, its condition enables 

 it to do so, but that when no such necessity is pressing, the 

 thread -forming mucus is not secreted, or is perhaps held in 

 suspense while the glands assist in lubricating the food before 

 digestion.^ 



Food of Land and Fresh-water MoUusca. — Arion ater, the 

 great black slug, although normally frugivorous, is unquestion- 

 ably carnivorous as well, feeding on all sorts of animal matter, 

 whether decaying, freshly killed, or even in a living state. It is 

 frequently noticed feeding on earthworms ; kept in captivity, it 

 will eat raw beef; it does not disdain the carcases of its own 

 dead brethren. An old man near Berwick-on-Tweed, o-oina; out 

 one morning to mow grass, found a black slug devom'ing, as he 

 supposed, a dead mouse. Being of an inquisitive tui'n, and 



1 W. Harte, Proc. Duld. N. H. Soc. iv, p. 182. 

 ^ See on the whole subject of threads G. S. Tye, Joivrn. of Conch, i. p. 401. 



