164 JOURNAL OF CONCHOI.Or.y. Vol.. lO, NO. 6, ArRII,, I902. 



return upon the thread has been observed in similar circumstances by 

 the writer. Contrary to the opinion of Harte, the animals certainly 

 glide from objects intentionally, and the whole process is as much 

 a voluntary action as their ordinary locomotion. But what advantages 

 do the animals derive from the use of the thread? It must be ad- 

 mitted that none is very apparent. One observer (45) who saw slugs 

 descending from vines supposed that they did so in preference to 

 crawling down the rough stems ; and it has been stated by several 

 authors that the use of the thread is that of coming down from heights 

 more quickly and easily than by ordinary creeping. Their course in 

 such cases has the advantage of being direct, but this mode of travel 

 ling does not seem to be particularly easy, and from its imperfection 

 the creatures often fall. It is suggested by Tye (31) that spinning is 

 chiefly called for when the creatures are seeking food, and it is true 

 that they have thus been known to reach favourite food otherwise in- 

 accessible, e.g., protected blooms of orchids, etc., to which slugs 

 sometimes descend in greenhouses ; but it is doubtful whether the 

 advantage thus derived is worthy of consideration. Many slugs live 

 almost exclusively on the ground, and are well nigh omnivorous ; 

 those which live on trees subsist on fungi and microscopic vegetation 

 found on bark and decaj'ing wood, and in common with others which 

 live on precipitous rocks, they graze like limpets as they creep along ; 

 they have usually an unlimited pasturage, and thus are not in the 

 position of those animals which eat up all before them, and have from 

 time to time to seek new feeding-grounds. Binney (20) suggests that 

 the faculty may be a means of sudden escape from enemies, particu- 

 larly of birds ; but the descent is comparatively slow, and a slug 

 hanging by a short thread could easily be secured by a bird ; from 

 smaller enemies, the creatures may possibly escape by hanging sus- 

 pended, as spiders do during raids by ants : but the writer would not 

 attach much importance to this consideration. It is chiefly when they 

 find themselves in exposed situations that slugs have urgent need to 

 alter their positions, and it is at such times, the writer thinks, that they 

 chiefly allow themselves to glide from their supports. During noc- 

 turnal excursions the creatures must often arrive at positions which 

 become unbearable with the changed conditions of day, the breaking- 

 out of the sun, disappearance of moisture from the atmosphere, etc. ; 

 for it will be remembered that these molluscs, containing much water, 

 requiring a constant flow of slime for progression, and having no ex- 

 ternal shell for protection from evaporation, find prolonged exposure 

 not only harmful but often fatal. Naturally, in these circumstances, 

 the creatures have an instinctive horror of exposure, and they con- 

 stantly seek out not only shady but covered retreats. Some of the 

 slugs seen descending in greenhouses were probably escaping from 



