462 NATURAL HISTORY. 



oyster. Those of the univalve molluscs are capable of protect- 

 ing themselves when withdrawn inside the shell by a horny 

 plate called the " operculum,"* which completely closes up the 

 aperture, and which may be seen in the periwinkle. The clos- 

 ing membrane found in the common snail, if taken in the win- 

 ter, is called the epiphragma,f and is supposed to be hardened 

 mucus. 



The Gasteropoda move by means of a fleshy disc or foot on 

 the under surface of the body, and by the alternate expansive 

 and contractive movements of this foot, the creature is enabled 

 to crawl. The Gasteropoda inhabit both land and water, un- 

 like the bivalves, which are exclusively inhabitants of the latter 

 clement. 



If the shell of a Gasteropodous mollusc be broken, it has the 

 power of repairing the injury by secreting fresh layers of shell 

 from the mantle within. 



The SLUGS are well-known invaders of our gardens, and, 

 together with the snail, the caterpillar, and the mysterious 

 " blight," are objects of the gardener's most intense hatred. 

 The Black Slug is usually found by hedge-banks, and in grassy 

 meadows. It seldom ventures out by day, especially if the day 

 be bright, but at night, when the dew is on the ground, it 

 may be seen trailing its dark length through the herbage, or 

 eagerly devouring the leaves. J The small Grey Slug (Limax 

 cinerea) is more common in gardens than the Black Slug. 



THE COMMON SNAIL. 



Several species of SNAILS inhabit this country, among 

 which the Edible Snail (Helix pomatia), the Belted Snail 

 (Helix nemoralis), and the common Garden Snail (Helix 

 aspersa), are the most conspicuous. The Edible Snail was 

 imported into England by the Romans, who prized them 

 highly, and fattened them in a building erected for that 

 express purpose, as indeed they are now in some parts of the 



* Lat. a cover or lid. t Gr. 'Ejri0/>ay//a, a cover. 



t While employed at night in decoying moths, by means of a fragrant compound 

 of sugar, beer, and rum, spread on the trunks of trees, I used constantly to find my 

 bait attacked by huge slugs of all kinds, descending and ascending towards the sweet 

 but dangerous banquet. 



