Sources : Slug

Isidore of Seville [7th century CE] (Etymologies, Book 12, 5:7): Slugs [limax] are mud vermin, so named because they are generated either in mud [limus] or from mud; hence they are always regarded as filthy and unclean. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.25): Lymax [slug], a genus of turtle [testudinis], as the Experimentator says, is named from mud, because it is born in mud. It eats earth. It has four horns, but two are longer. In crawling, it extends its horns, but as soon as it is touched, it retracts its horns and curls in on itself. It is hidden in winter, it is born in spring. Its blood closes the pores and effectively prevents hair from growing. It heals broken and bruised wounds in some cases. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.69): [Bartholomaeus appears to have confused the slug (limax) with the snail (cochlea or testudine), describing some of the attributes of both.] Limax is a worme of slyme, and hath that name Limax, for he bréedeth slime, or of slime, and is therefore alway foule and uncleane, as Isid[ore] saith lib. 11. And it is a verye slowe worme in mooving, and beareth an harde shell on his backe, and closeth him therein, and is an horned worme, & hath two small horns before the mouth, and when he féeleth any grievous thing, he draweth the horns anone into his shell, and closeth himselfe in the shell, as it were within an house. And such wormes are gendered principally in corrupt aire and raine: and hée créepeth, though it be with slow pace, & commeth up to the tops of trées, and biteth and gnaweth the buds and fruite thereof, and where ever he créepeth or slydeth, he leveth after him a glemy froth and strake of uncleannesse. - [Batman]