30 USE OF THREADS 



surface, the animal might find itself asphyxiated, or at least 

 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 resting 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 by cer- 

 tain of the Cydostomatidae^ by some Cerithidea^ several Missoa 

 and other marine genera, prominent among which is Litiopa 

 homhyx^ w^hose 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 by threads 

 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 Ameri- 

 can species of Limax^ besides those of Tehennophorus^ possess 

 this singular property. Limax arhorum 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, but 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 excursions in search of food, 

 its condition enables it to do so, but that when no such neces- 

 sity is pressing, the thread-forming mucus is not secreted, or is 

 perhaps held in suspense w^hile the glands assist in lubricating 

 the food before digestion. ^ 



Food of Land and Fresh- water Mollusca. — 1 ^Wori ater, 

 the great black slug, although normally frugivorous, is unques- 

 tionably carnivorous as well, feeding on all sorts of animal mat- 

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

 It is frequently noticed feeding on earthworms ; kept in cap- 

 tivity, it will eat raw beef ; it does not disdain the carcases of 

 its own dead brethren. An old man near Berwick-on-Tweed, 

 going out one morning to mow grass, found a black slug devour- 

 ing, as he supposed, a dead mouse. Being of an inquisitive 



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



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



