- The trilobites comprise a major class-level clade of extinct marine arthropods characterized by calcareous, multisclerite, dorsal exoskeletons.
- In these specimens the external surface of the exoskeleton is present, unless complete or partial exfoliation is noted.
- Lobsters, like all animals with exoskeletons, periodically shed their armor as they grow.
- The hard parts may be mineralized, as in the shells of bivalves, or composed of organic material, such as the chitin that makes up the exoskeleton of arthropods.
- The advantage of an internal skeleton is that it allows the animal to grow much larger than is the case with the arthropod type exoskeleton.