122 LIMIT TO GROWTH. 



fibres. This muscle, by its contractions, brings the 

 foot and the shell closer together, and compresses the 

 body ; on relaxing, it allows the shell to be raised by 

 the elasticity of the body. The snails of spiral shells 

 are bound to them by two muscles, which arise from 

 the pillar ; and, having penetrated the body below its 

 spiral part, run forward under the stomach, and spread 

 their fibres in several slips, which interlace with those 

 of the muscles proper to the foot, the substance of 

 which they enter. 



A limit has been assigned to the growth of shells and 

 of their inhabitants ; and at a certain period consi- 

 derable changes take place in the disposition of the 

 mantle, and in its powers of secretion. Frequently 

 it suddenly expands into a broad surface, and adds to 

 the shell what may be termed a large lip. No sooner 

 is this accomplished than sometimes the same part again 

 shrinks, and the mantle retires a little way within the 

 shell, still continuing to deposit calcareous layers, which 

 give greater thickness to the adjacent part of the shell, 

 and at the same time narrow its aperture, and materially 

 alter its general aspect and shape. Thus it happens 

 that the shells of the young and of the old individuals 

 of the same species are very different, and would not 

 even be recognised as belonging to the same tribe of 



