INTRODUCTORY CHAPTER. 



1 1 



growth, the animal has formed at the edge of the shell 

 then existing. These spiral shells are almost always formed 

 with the whorls turning towards the right, which arises 

 from the left side of the mantle being the most active, 

 in consequence of the position of the heart and blood- 

 vessels of the animal on that side * : in a few genera, 

 however, the heart is situated on the right side, and 

 then the whorls of the shell are turned to the left, and 

 the shells in which this conformation takes place are 

 called reversed, or sinistral^, in opposition to the common, 

 or dextral.% 



The growth of bivalve shells 

 is carried on in the same way, each 

 valve being simultaneously en- 

 larged, as will be seen in the 

 annexed figure, where the addi- 

 tions may be easily traced. 



The power of forming shell does not appear to be con- 

 fined to the edge of the mantle ; but exists throughout its 

 whole extent ; this has been proved by breaking and 

 removing part of the shell of a living snail, at a distance 

 from the edge or lip : the animal immediately sets about 

 filling up the gap, and effects it in the course of a few days 



* Dr. Grant makes the following observations on this subject, in 

 his Lectures on Comparative Anatomy, at the London University: — 

 " I am inclined to ascribe this spiral twist of shells to the action of 

 the heart and the respiratory organs, which are situated on the left 

 side, and to the descent which the foot of the animal makes over the 

 columella, in order to reach a solid surface to creep upon. The dila- 

 tation of the heart bends the anterior free part of the body to the 

 right side, and would cause the shell to revolve on the same plane, 

 were that cause to operate alone ; but the constant descent of the 

 foot over the surface of the pillar causes the plane of revolution to 

 change at every instant, and by these two causes the spiral form of 

 shells appears to be produced." 



f From sinister, Lat., left. \ From dexter, Lat., right 



