INTRODUCTORY CHAPTER. 15 



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 oppo- 

 sition to the common, or dextral. f 



The growth of bi- 

 valve shells is carried on 

 in the same way, each 

 valve being simultane- 

 ously enlarged, as will 

 be seen in the annexed 

 figure, where the additions may be easily 

 traced. 



London University : — " I am inclined to ascribe this spiral 

 twist of shells to the action of the heart and the respira- 

 tory 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 dilatation 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 sur- 

 face 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." 



* From sinister, Lat., left. 



f From dexter, Lat., right. 



