GASTEROPODA. 537 



presented by the nacreous portions of shells, commonly 

 called mother-of-pearl, are produced solely by the disposi- 

 tion of single membraneous layers in folds or plaits, lying 

 more or less obliquely to the general surface. 



In the Chitonidce, Coat of mail Shells, the shell consists 

 of eight transverse plates, imbedded in the mantle ; in the 

 Limpets, the ordinary form is that of a cone. The 

 arrangement of the teeth is somewhat remarkable. 



The majority of Gasteropoda are furnished with a 

 shell, denominated spirivahe. The cause of this spiral 

 arrangement is said to be owing to the shape of the body 

 of the animal inhabiting the shell, which, as it grows, 

 enlarges its shell principally in one direction ; thus, of 

 course, making it form a spire, modified in shape according 

 to the degree in which each successive turn surpasses in 

 bulk that which preceded it. It would rather appear that 

 this is principally owing to the ciliary motion imparted to 

 the early stage of the embryo ; the first deposit of calca- 

 reous matter forming the axis, the tube continues to rotate 

 upon its axial pillar or columella, as it is called ; and by 

 reason of some other peculiar vital tendency, the shell is 

 gradually deposited in a series of cells ; thus enlarging its 

 conical form, and winding obliquely from right to left. 

 Every turn around the axis is termed a wliorl ; and when 

 the columella is hollow, it is said to be umbilicated. In 

 the spirivalve-shelled Gasteropoda, we find a difference in 

 structure between that part of the mantle which enve- 

 lopes the viscera, and which is always concealed within 

 the cavity of the shell, and the portion placed around its 

 aperture. 



The mouths of most Gasteropoda consist of a strong 

 muscular cavity, and a crescentic-shaped tooth-bearing 

 membrane, armed with sharp points, and separated by 

 semi-circular cutting spaces, admirably adapted for the 

 division of the food upon which they feed. Most of them 

 are beautiful objects for the microscope. 



Professor Huxley very properly objects to the use of the 

 commonly accepted term tongue for the tooth-bearing 

 membrane of the mollusca, and more appropriately desig- 

 nates it " the odontophore." 



" The odontophore consists essentially of a cartilaginous 



