List of Things That Have Shells

Mollusks

• Most of the seashells we know are part of a classification of animals collectively called "mollusks." Clams, mussels and the triton or trumpet shell are just some of the invertebrates that belong to the phylum Mollusca. These are creatures without any backbones that have proportioned bodies containing a head, visceral hump, mantle and foot. In most mollusks, hard shells are mounted on the visceral hump and house their internal organs. The mantle secretes a sheet of tissue made of calcium carbonate and other minerals that eventually becomes the shell during the mollusks' developmental process.

Crustaceans

• Crustaceans are considered the biggest collection of marine arthropods---or animals with segmentation. Lobsters, crabs, and shrimps are just some of the creatures that make up the approximately 30,000 species of crustaceans. All crustaceans have an external shell made of calcium and a protein called "chitin." The outer shell serves as protection and firm support for the muscle-attachments or joints, which allows crustaceans to move.

Turtles

• An animal that is one of the most famous shell-dwelling creatures is the turtle. Compared with the shells of mollusks and crustaceans, a turtle shell is a part of the animal's endoskeleton and is actually composed of living cells, nerves and blood vessels. The shell surfaces are considered epidermal structures made of the protein keratin. The exoskeletons are made of calcium phosphate and, just like human bones, are constantly growing.

Sea Urchins

• A sea urchin's shell is also known as a "test." The male shells have an attractive symmetrical design that serves as a tool for attracting females. The rigid shell is made of flat, connected calcareous ossicles, which split into 10 sections. Among these sections are five ambulacral plates, which have pierced holes where the tube feet stick out. The remaining plates without holes are called "interambulacral" areas.