MOLLUSCA—CLASS III. GASTEROPODA. 3 
The shells of the Gasteropoda exhibit a considerable variety of form ; 
sometimes they are small, and attached to different parts of the body, 
and sometimes they are entirely concealed within the mantle for the pro- 
tection of the branchiz ; the majority of them, however, are spirivalve. 
Their progress of growth may be traced with considerable interest : 
after being ejected either from the parent or from the egg, the animals 
form an allotted number of volutions in order to complete their shell, 
which assumes various appearances at different periods of growth; it is 
curious to observe the true geometrical formation of the spines and 
varices, as also the frequent operation of absorption ; a remarkable eco- 
nomy is also exercised in the repair of accidental fractures. The outer 
surface of the shell is generally covered with a thick fibrous epidermis, 
and the inside of it lined with a highly-polished enamel; sometimes, 
however, when the mantle is expanded entirely over the shell, as in the 
Olives and Cowries, it is coated with enamel both inside and out. Most 
of the spirivalve Gasteropoda are furnished with a small accessory ap- 
pendage attached to the hinder part of the foot for the purpose of shut- 
ting themselves within the shell ; it has on this account been called the 
operculum, and is either horny or calcareous. Some naturalists attach 
an undue importance to this organ, adopting its variations as the rules 
of generic division ; but we have observed the opercula of mollusks with 
considerable attention, and feel assured that their varieties indicate no 
change of organization in the animal. 
The Gasteropoda may be described as having a distinct head situated 
at the anterior part of the body ; it is more or less prominent, and gene- 
rally provided with eyes and feelers ; the mouth is naked and protrudes, 
and is most frequently furnished with hard parts ; the mantle is variable, 
but generally free, and produces a shell varying considerably in form and 
structure. The branchiz, which are seldom symmetrical, are made sub- 
servient either to the respiration of air or water. The system of circu- 
lation is duplex, performed by an unilocular heart which is divided into 
two remote auricles. 
The shell of the Gasteropoda is either horny or calcareous, generally 
B2 
