258 GROWTH OF THE SHELL chap. 



to Marseilles, an Avicula 78 mm. and an Ostrea 95 mm. long 

 (both being species peculiar to W. Africa) were taken from its 

 keel. These specimens had therefore attained this growth in at 

 most 154 days, for at the period of their first attachment they 

 are known to be exceedingly minute. P. Fischer relates ^ that in 

 1862 a buoy, newly cleaned and painted, was placed in the basin 

 at Arcachon. In less than a year after, it was found to be 

 covered with thousands of very large Mytilus edulis, 100 mm. 

 X 48 mm., the ordinary size on the adjoining banks being only 

 about 50 to 60 X 30 mm. 



Some observations have already been recorded (p. 40) on 

 the growth of Helix aspersa. In the summer of 1858, which was 

 very dry, especially in the south of Trance, the young Helices 

 born that year were still very small in August. About the end 

 of that month abundant rain came on, and in four or five days 

 young If. variabilis, H.piisana, and H. aspersa, eating without cessa- 

 tion, as if to make up for lost time, grew more than a centimetre 

 of shell. The lip of a young R. arlmstorum has been observed to 

 have grown, at the end of the first week in the season's growth, 

 3 mm., at the end of the second week, 6"25 mm., the third, 

 11 "5 mm., and the fourth 12'5 mm., with a finished lip." 



Careful observation has shown that in the growth of the shell 

 of Helix aspersa the periostracum is first produced ; it is covered 

 with hyaline globules, 10-12 mm. in diameter, which persist even 

 in the oldest shells. Calcareous matter is deposited on the 

 internal face of the new periostracum, at some distance from the 

 margin. It is secreted by a white zone or band of cells l^ound- 

 ing the entire breadth of the mantle as applied to the peristome. 

 Immediately behind the white zone are a series of pigment cells 

 which not only give the shell its colour but complete the 

 calcification of the shelly matter laid down by the white zone. 

 When the animal has attained its full growth and the lip is 

 finished off, the white band and the periostracum cells completely 

 disappear, and only such cells persist as contribute to the internal 

 thickening of the shell. Shell growth, in this species, is very 

 rapid. If a portion of the pulmonary sac is laid bare, by remov- 

 ing a fragment of shell, at the end of 1^^ or 2 hours there may 

 be detected a delicate organic membrane covering the hole, 

 and strewn with crystals of carbonate of lime. This thickens 

 ^ Journ. de ConelnjL xii. p. 3. ^ T. vScott, Journ. of Conch., 1887, p. 230. 



