Inside barnacles lives an animal that is a specialized crustacean, closely related to shrimp and crabs. Despite their hard, shell-like exterior, barnacles are not mollusks but crustaceans with a body plan somewhat like an upside-down shrimp inside the interlocking calcareous plates

. The barnacle's body lies inside the shell, typically on its stomach, with long, feathery thoracic limbs called cirri that extend out to filter plankton and other microscopic food particles from the water. These cirri are used to sweep food toward the barnacle's mouth inside the shell

. Barnacles have no true heart but pump blood through a sinus near the esophagus, and they absorb oxygen through their cirri and body surface instead of gills. They have minimal segmentation, with the body divided mainly into head and thorax, and a vestigial abdomen

. Some barnacles are free-living suspension feeders attached to hard surfaces like rocks, ships, or even animals such as whales and turtles. Others, like the Rhizocephala group, live as internal parasites inside crabs, altering their hosts' behavior and appearance

. In summary, inside the barnacle's hard shell lives a shrimp-like crustacean with specialized feeding legs (cirri), adapted to a sessile life filtering food from water, with some species even parasitic inside other crustaceans