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 Limacibjs. 



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 Limacidce 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 (Philodromus) 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 Helicarion, 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.) 



