344 
MOLLUSCS. 
it doubtless serves some special purpose. Nearly all the species of this genus are 
of a whitish colour, polished or obliquely striated. They chiefly occur in South 
America, Tropical Africa, Southern Asia, and certain islands in the Indian Ocean. 
True Slugs,— Family Limacidle. 
This family contains many genera of naked slugs, and several either partly or 
wholly protected by well-developed shells; the hinder end of the foot in some 
forms terminating in a conspicuous mucus or slime-pore. The typical slugs are 
mostly elongate animals, capable of great contraction, and always pointed or 
attenuated behind. The mantle forms a sort of shield, placed over the fore-part of 
the back, and beneath this is situated a small calcareous plate, representing the shell. 
These plates occur fossil in Eocene beds. The respiratory orifice is seen on the 
right side of the shield, but rather far back. The head is prominent in front, 
bearing two pairs of retractile tentacles, of which the upper are the longest, and 
furnished with eyes at the bulbous tips. The mouth is provided with a horny 
upper jaw, which is smooth, with the cutting edge produced into a sort of beak in 
the middle. The radula has numerous transverse series of horizontal or slightly 
oblique teeth, of which the central tooth is three-pronged, the laterals about the 
same height as the central one, while the marginal teeth are narrow and acute. The 
body is united to the foot, and is more or less wrinkled; the wrinkles being most 
conspicuous when the slug is contracted, and to some extent characteristic of the 
different species. The Limacidcv live in damp places, out of doors, or in cellars, 
and hide away during the daytime under stones, dead leaves, or in fact in any place 
where it is dark and moist. They feed chiefly on decaying vegetation, but some 
are more or less carnivorous. Certain species are great pests in gardens. Limax 
agrestis, the “ milky slug,”—so called from the opaque white colour of its mucus,— 
is a very common species in England; it is very fond of strawberries, and is also 
said to feed upon earthworms. These slugs increase in numbers rapidly, and are 
said to produce several families in the course of a summer; a pair having been 
known to lay nearly eight hundred eggs. Some species of Limax are capable of 
lowering themselves to the ground from the branch of a tree by secreting a slimy 
thread. The largest species occuring in Britain is L. maximus, which has a very 
wide range on the Continent, and sometimes exceeds 6 inches in length. One may 
often notice numbers of a minute white parasitic mite ( Philodrovius ) running about 
the body of this slug, and it is 
said also to live in the respira¬ 
tory cavity, but does not appear 
to cause any annoyance or 
injury to its host. 
In the genus Vitrina the 
animal is provided with a 
fragile, external, horny shell, 
not sufficiently large to receive 
it entirely, when contracted. The shells are all very much alike, and precisely 
similar to those of Hclicarion, another genus of Limacidce, which, however, is 
distinguished by having the foot truncated obliquely behind, and furnished with 
glass-snail ( Vitrina ) and amber-snail ( Succinea ). (Nat. size.) 
