74 SYSTEMATIC DISTRIBUTION. 



as was observed by Swammerdam, and verified by 

 the more extended observations of PfeifFer, Turpin, 

 and others. They are easily seen in the egg of the 

 Limncei) Phys& 9 Ancyli) and Bithinice, which have a 

 transparent coat. (See Phil. Trans^ 1833.) 



The shells of the newly-hatched animals have been 

 frequently considered as distinct species, and some 

 very thin shells of land Mollusca, such as Vitrina, 

 have been taken for the young of other well-known 

 species, as H. hortensis* These young shells are easily 

 known by their always being of a pale horn colour ; 

 the whorles are generally rather irregular, and en- 

 large very rapidly ; and the apex of the whorl which 

 was first formed is generally large and blunt, com- 

 pared with the size of the shell. They are always 

 destitute of colour, for the animal does not deposit 

 the colouring matter until after it has been hatched ; 

 and it is therefore generally easy to distinguish in the 

 young shell (and sometimes also in the adult) that part 

 of the top of the spire which formed the shell of the 

 animal when in the egg. 



The shell is formed by the hardening of the animal 

 matter, which is secreted by certain glands on the 

 surface of the body, by means of chalky matter, which 

 is also secreted by similar glands. It has been stated 

 that the unhatched animal, very shortly after it is 

 formed, begins to make its shell; and when it is 

 hatched, deposits on the edge of the mouth of the 

 little shell which covered its body in the egg a small 

 quantity of the mucous secretion. This dries, and is 

 then lined with some mucous matter, intermixed with 

 calcareous particles ; and when this hardens, it again 

 places on its edge another very thin layer of the mu- 



