GASTEKOPODA. 11 



1. Is developed on the embryo long before it is 

 hatched. 



2. That it is placed on and covers a peculiar part 

 of the body, which bears the same relation to it as 

 the part of the body called the mantle, bears to the 

 part usually called the shell of these animals ; and it 

 is formed, and increases in size, by an opercular man- 

 tle in the same way as the shells are. 



3. That the operculum is more or less conical, and 

 is increased in size by the addition of new matter to 

 the inner surface, and especially to the part of it near 

 the margin, the new matter either forming more 

 or less complete rings round the nucleus (or first- 

 formed part) when it is called annular, and is homo- 

 logous to the simply conical shells, as the Patella ; 

 or the new matter is deposited almost entirely on 

 one edge of the nucleus, when the operculum forms 

 a more or less elongated cone, which when long is 

 generally twisted round an imaginary axis (like a 

 spiral shell), the broad part of the cone being next 

 the edge of the opercular mantle which generates 

 the new matter for enlarging its size, as the mouth 

 of the shell is on the outer edge of the mantle of the 

 univalve shell. 



4. That the operculum is attached to the animal 

 by means of one or more muscles which, as in inequi- 

 valved bivalved shells, pass from the larger valve, or 

 shell, to the smaller one, or operculum. 



5. The operculum as it increases in size is gra- 

 dually moved on the end of the muscle ; the many- 

 whorled opercula of the trochi revolve as many 

 times on the end of the muscle as the many-whorled 



