54 THE BOOK OF NATURE STUDY 



As a rule slugs are more active than snails ; their rate of 

 progression is about a mile in eight days, but snails would take 

 over a fortnight to traverse the same distance. Some species 

 which frequent trees and bushes, e.g. Limax marginatus, are 

 able to lower themselves head first to the ground by a cord of 

 slime, and return to the branch whence they set out by the same 

 means. Some water-snails (Lymncea) are able to ascend to the 

 surface from the depths and travel down again to the same spot 

 by the aid of a similar slime-cable anchored for the purpose to 

 some firm object on the bottom. In the last case the animal 

 ascends by virtue of the buoyancy of a bubble of air which is 

 protruded, but not detached, from the pulmonary aperture. 

 The object of the ascent is to obtain a fresh breath of air. 



The slugs which occur in this country are classified into two 

 main groups, the Arionidce and the Limacidce. Members of these 

 families may be distinguished by the following features. Arionidcz 

 possess a slime gland on the tail, and the pulmonary opening 

 lies in front of the centre of the margin of the mantle ; whereas 

 Limacidce are devoid of a slime gland, and their pulmonary open- 

 ing is situated behind the centre of the mantle margin. 



The largest and one of the most common examples of the 

 former family is the " Black Slug/' Arion ater. This species 

 occurs in a great variety of localities, being not only abundant 

 in gardens and cultivated fields, but also on moors and boggy 

 peat lands where other molluscs are seldom found. Its food 

 consists of decaying rather than fresh vegetation. At times 

 the animal becomes carnivorous, even to cannibalism, and will 

 even descend to devouring the excrement of other creatures. 

 The young are usually pale yellowish white, but gradually 

 assume a darker colour as they grow older. The deepening 

 colour first appears upon the tentacles, whence it spreads down- 

 wards to the back and edge of the foot. Mature individuals 

 vary greatly in colour, some being red, others lead colour, others 

 quite white, while many exhibit mixtures of colours. The 

 cause of these variations in colour is not known ; probably several 

 factors contribute. It is, however, noteworthy that specimens 

 from the warmer regions of our islands are as a rule more 

 brilliantly coloured than those obtained in the colder northern 



