Calcification and calcareous deposits are extremely common in many pathological conditions.
The old idea of the circulating blood being supersaturated with lime salts which in some way had first become liberated from atrophying bones, and then deposited, to form calcified areas in different tissues will have to be given up, as there is no evidence that this " metastatic " calcification ever takes place.
At the present day both experimental and histological investigations seem to indicate that in the process of calcification there is a combination of the organic substances present in degenerated tissues, or in tissues of low vitality, with the lime salts of the body.
In other Cheilostomes the amount of calcification may be much less, the supporting skeleton being largely composed of the organic material chitin.
- Shell of Aspergillum vaginiferum to show the original valves a, now embedded in a continuous calcification of tubular form.