AUXILIARY ORGANS — OPERCULUM. 
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Auxiliary and Protective Organs. 
Tliere are three ingenious, though dissimilar methods, hy which 
Univalves close the apertures of their shells, for the double purpose 
of protection against the intrusions of their enemies and to 
prevent the desiccation or evaporation of the natural moisture of the 
tissues of the body, during the mid-day periods of repose or the 
more extended dormancy of mstivation and hibernation, and also to 
guard against the extremes of cold, heat, and the injurious effects 
of climatal fluctuations generally. 
These anxiliaiy organs, known as the Operculum, the Clausium, 
and the Epiphragm, are formed in an analogous way to the shell 
itself, the secretion to form these different organs being ju'obably 
poured out as a gelatinous substance holding calcareous matter in 
solution, which hardens and crystallizes by exposure, the semi-Huid 
chitinous matter, which constitutes the investing and permeating 
organic framework, moulding itself around the growing crystals formed 
by the calcareous matter present in the solution. 
The Operculum {operio, to cover) is perhaps the most important 
and universal of these contrivances for closing the mouth of the shell, 
and is a special structure chiefly develoi)ed amongst the branchiferous 
Gastropods, but has been retained in some genera which have pro- 
bably become adapted to terrestrial life and aiirial respiration. 
It originates upon the embryo in the ovum, above the foot, and at 
the posterior end of the body, upon a restricted portion distinguished 
by its denser texture, which has been termed the operculigerous lobe, 
and presents, especially in genera foreign to our limits, very great 
diversity of structure, but our native operculiferous species are very 
few and do not exhibit the remarkable forms of many exotic species. It 
may consist of chitinous layers, as in Vimpam contecta and other 
species, but is sometimes strengthened by calcareous matter, as in 
Neritum JluviatiUs, etc., the inner surface being always marked by a 
muscular scar showing the point of attachment to the animal and shell. 
The operculum is usually, but not invariably, the size of the aper- 
ture it closes, and according to Mr. Kenneth McKean in Bpthinia 
tentacuhita fits the mouth of the shell so accurately that if the 
animal be killed with boiling water and allowed to dry up within the 
shell, and it be afterwards held in the warm hand, the operculum will 
by off with a considerable report owing to the expansion of the 
confined air or gas. 
