Most organisms of this phylum have 3 layers on their exterior, including the periostracum and prismatic layer. Many use byssal threads to attach to a hard surface. Certain classes use a crystalline style to aid in digestion. 
Most organisms of this phylum have 3 layers on their exterior, including the periostracum and prismatic layer. Many use byssal threads to attach to a hard surface. Certain classes use a crystalline style to aid in digestion. The Aplacophora class secretes calcium containing spicules into its epidermis to aid in protection. Also covered with the nacreous layer, its larval form is generally known as the veliger. For ten points, name the phylum containing cephalopods, gastropods, and bivalves.
Most organisms of this phylum have 3 layers on their exterior, including the periostracum and prismatic layer.
{Mollusca}