Feeding legs (cirri) of an acorn barnacle. 40x magnification. Darkfield microscopy.

Barnacles are crustaceans that secrete their shell (made of calcite) by means of a cement gland that then attaches them to a substrate. The shell is composed of six plates and each barnacle has six pairs of modified legs, or cirri. Most of the body remains inside the shell but since barnacles are filter feeders, they extend their cirri (that are covered in sensory hairs; pictured in this print) into the water to feed on plankton and particulate matter. Most barnacles live in the intertidal zone, which is a pretty badass place for any marine organism to live because the temperature, water level, and wave conditions are variable. They are well-adapted to these changes and when the tide goes out and conditions get dry, they are able to close some of their plates, shielding the body from the hot sun and predators. This description is specific to acorn barnacles but there are other types, like goose barnacles, that have slightly different anatomical features. Barnacles can also be used as indicator species to measure the bioaccumulation of harmful chemicals or heavy metals in marine and estuarine environments.