Mollusca constitute the 2nd largest animal phylum
50,000 to 110,000 living spp.
Includes clams, snails, octopi.
Most have a shell consisting primarily of calcium set in a protein matrix. The shell is secreted by specialized tissue known as the mantle.
Shell consists of:
- Thin outer organic layer (periostracum)
- Thin, innermost calcareous layer (nacreous layer)
- Thick calcareous middle layer (prismatic layer)