208 SNAILS AND SLUGS [CH. VII 



made by the glandular organs just described. If however 

 a portion of shell is cut out from the midst of one of the 

 whorls, within two hours a thin organic membrane con- 

 taining crystals of calcium carbonate is secreted and 

 rapidly closes the hole. By subsequent internal additions 

 the new piece is thickened and made as strong as the 

 surrounding uninjured parts but it is devoid of perios- 

 tracum. On the other hand if the injury is inflicted upon 

 the shell-lip of a growing snail both the outer non-cal- 

 careous "periostracum" and the internal calcareous portions 

 are alike repaired. The calcareous matter is in the form 

 of that modification of calcium carbonate known as 

 aragonite. 



The normal rate of shell-growth probably varies 

 considerably according to climatic conditions, the nature 

 and amount of food-supply, and so on. According to 

 Lowe 1 the shell increases but little for a considerable 

 time, and never becomes mature before at least one period 

 of dormancy during which no growth occurs. Many 

 species are said by him to burrow in the ground and 

 conceal themselves during periods of shell growth. In 

 this opinion he is at variance with Collinge 2 . Be that 

 as it may, there is no doubt that when growth does occur it 

 is very rapid. H . variabilis, H. piscina, and H. aspersa have 

 been known to add more than a centimetre of shell in four 

 or five days. Nearly all snails' shells exhibit in addition 

 to the lines of growth a few well-marked lines some 

 distance apart and often separating areas of different 



1 Phil. Trans. 1854. 



2 Naturalist, 1891 ; Conchologist, n. 1892. 



