Life forms on a rock by the sea. Insects, sea shells and algae together. Seashore ecosystem and habitat. The term seashell usually refers to the exoskeleton of an invertebrate (an animal without a backbone), and is typically composed of calcium carbonate or chitin. Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods. Marine annelid worms in the family Serpulidae create shells which are tubes made of calcium carbonate cemented onto other surfaces. The shells of sea urchins are called tests, and the moulted shells of crabs and lobsters are called exuviae. While most seashells are external, some cephalopods have internal shells invasive nature.