SNAILS AND SLUGS 53 



in different species. A new dart is produced subsequently for 



use on future occasions. The spawning season is in July and 



August, but the eggs of one season are not 



all deposited simultaneously, but in patches 



of thirty or more at a time. They are placed 



in moist spots, under stone heaps, dead 



leaves, etc. Each egg is contained in a 



leathery, calcareous shell, is spherical and 



about J inch in diameter. The young snails 



emerge in about four weeks time, and by the 



time winter sets in have attained a diameter 



of nearly \ inch. When one year old their 



diameter is about i inch. Sexual maturity FlG - 33--Eggs of snail 



, , , . . , , within their shells. 



is reached when three years have elapsed. 



The shell of an old snail can always be distinguished by the 



fact that the margin of the shell " lip " is rolled back. 



In winter time the snail retreats into some sheltered crevice 

 or buries itself in the earth, and closes the mouth of the shell 

 by secreting a lid (the " epiphragm " or " hibernaculum "), 

 which is largely calcareous in composition. This is at first liquid 

 slime charged with chalky matter, and fills the shell-mouth. 

 The animal then blows a bubble of air from the pulmonary aper- 

 ture, and so separates the calcified slime from the body, at the 

 same time causing it to bulge outwards. At the same moment 

 the body is withdrawn deeper into the shell, and external atmos- 

 pheric pressure flattens the epiphragm or even drives it in a 

 little. Sometimes several epiphragms are made one behind the 

 other. The dry, hardened lid is porous, so that respiration can 

 take place through it. 



Slugs in their general organisation resemble snails. It is 

 true that they have no protecting shell to cover the part of 

 the body which contains the major part of the internal viscera, 

 as have snails; but in the yellow cellar-slug (Limax flavus), 

 and a large pale grey species spotted with black (L. maximus), 

 a small shell is present in the skin of the dorsal region, and in 

 Avion ater, the large black slug, calcareous crystals are found in 

 the same region ; while Testacella carries a small but plainly 

 visible shell as a little cap on the hinder part of the body. 



