218 MOLLUSCA. 



shell, they can entirely cover and protect it. Thus, 

 it appears, the true office of these sails is exactly 

 that of keeping themselves applied to the shell at 

 all times, in reserve for the moment when the ani- 

 mal coming to the surface removes them, and spread- 

 ing them, raises them as sails. In fact, the series of 

 suckers on the sail-arms, when the membrane of the 

 sails is wrapped about the shell, is placed exactly 

 over the keel of it in such a manner that each 

 sucker corresponds to each point in which the ribs of 

 the shell terminate, until they reach the two margins 

 of the spiral." Madame Power concludes, that the 

 shell is formed by a secretion from the membrane of 

 the sails, the wrinkles of which cause the ribs of the 

 shell ; and thus is explained the singular fact, so 

 puzzling to former naturalists, who supposed the 

 shell to be formed from the skin which it covered, 

 as in other shelled Mollusca, that the inward surface 

 does not at all correspond with the folds of the ani- 

 mal's body within. The spread sail exhibits a silvery 

 surface, with rings surrounding each other, a black 

 spot in the middle, bounded by beautiful gold colour ; 

 the neighbourhood of the suckers along the keel and 

 spiral is of a vivid purple. The shell, during life, is 

 by no means brittle, but so flexible as to admit of the 

 two sides being squeezed together without injury, and 

 this pliancy can be recovered even after drying, by 

 simply soaking it in water. We have already said 

 that there is no muscular attachment of the body 

 to the walls of the shell ; the animal seems to retain 

 its position by the pressure of the sac against the 



