Mollusks first appeared in early Cambrian times, 650 million years ago. They are the second largest animal phylum, Mollusca (from Latin mollis, “soft”). The only larger animal phylum is Arthropoda. According to some estimates, there are approximately 100,000 mollusk species. All mollusks have soft, boneless bodies. Most have shells, though some shells are poorly developed and are even absent in some cases. Mollusk shells are like coats of armor.

Salem Press Encyclopedia of Science, 2023. 3p.