SNAILS AND SLUGS 49 



the coloured outside layer ; later a backing of calcareous matter 

 is added, and the strength thereby increased. From the apex 

 of the shell a spiral " suture " runs round and round the shell 

 to the upper edge of the " mouth " ; this suture is the line along 

 which the growing shell-margin has become united with the 

 already existing portions. Starting from the apex and travelling 

 down the suture, the direction of growth is seen to be always 

 towards the right hand. If the reader will imagine himself to 

 be walking down the suture he will at once perceive that he 

 is continually turning to his right. Such shells are known as 

 " dextral." A few of our land snails, e.g. Clausilia, possess 

 " sinistral," left-handed shells. 



On the under surface of the shell a portion of the " lip " is 

 reflected so as to cover a small opening, the " umbilicus." This 

 leads into a hollow pillar, the " columella," formed by the union 

 of the successive turns (" whorls ") of the shell. In some species 

 of Helix, e.g. pomatia (the Roman snail), caper ata, which is common 

 on dry short-turfed downs, and others the umbilicus is quite 

 conspicuous. If a shell be cut in half with a fret-saw, or care- 

 fully chipped away with scissors, the columella is exposed as 

 the axis of the spiral. 



The outer, coloured, horny layer of the shell, the periostracum, 

 is formed only by the skin just behind the thick fleshy edge, or 

 " collar," of the mantle which is visible in the living animal all 

 round the lip of the shell. The calcareous middle layer is also 

 formed only in the same region ; but the innermost mother-of- 

 pearl or nacreous layer is added by the entire surface of the 

 mantle. Hence if the shell be injured the animal is able in a 

 short time to patch up the hole with a white layer of mother-of- 

 pearl, but cannot renew the coloured periostracum. Such repairs 

 are affected in a very short time a few hours being sufficient for 

 the formation of a new serviceable piece. Normally, however, 

 the shell is thickest near the lip, except when growth is in progress. 

 The chalky matter is derived from that contained in the plants 

 eaten ; in limestone districts the shells are very much thicker than 

 those found on soils deficient in calcareous salts. 



As is well known, the snail can at will either extend a large 

 portion of the body beyond the limits of the shell, or on the 



VOL. II. 4 



