They are, in fact, periodic precipitates of phosphate and carbonate of lime deposited in the colloidal organic substance of the mollusc.
"The Mechanism of Life"
Stéphane Leduc
He proved conclusively that snake-poison does not destroy protoplasm or interfere with infusorial life, that injected into the heart of a mollusc it causes an almost immediate cessation of its action, that hypodermic injections of it in fish produce contraction of the pigment cells and bleaching of the integuments, followed by asphyxial respiration, general paralysis and death.
"On Snake-Poison: its Action and its Antidote"
A. Mueller
The fact was at once noised abroad, and crowds visited his shop to listen to the sibilant mollusc, which not only whistled, but, it was said with some truth, drew the town as effectively as old Drury herself, on the other side of the court.
"The History of "Punch""
M. H. Spielmann