Starfish, correctly known as sea stars, are marine invertebrates belonging to the phylum Echinodermata. This group is characterized by radial symmetry and a unique internal skeleton. Sea stars do not possess hair, which is a structure composed of the protein keratin found primarily in mammals. Instead, the sea star’s outer layer is covered by specialized skin and a complex internal framework.
The Surface Structure of a Sea Star
The body covering of a sea star consists of a thin, ciliated epidermis overlaying a thicker layer called the dermis. Beneath this outer skin lies a rigid internal structure known as the endoskeleton.
This supportive framework is composed of numerous small, interlocking plates made of calcium carbonate, which are called ossicles. These ossicles are embedded within the dermis, creating a protective, mesh-like lattice. Many of these calcareous plates extend outward to form fixed spines or tubercles, giving the sea star its characteristic bumpy appearance. This durable, mineral-based structure provides mechanical protection against predators and the harsh physical environment of the seafloor.
Specialized Appendages
The sea star’s surface is dotted with various minute projections that serve specific biological purposes. One specialized structure is the pedicellariae, which are tiny, jaw-like or pincer-like appendages. These structures move independently and function primarily to keep the body surface clean by removing debris, parasites, and small settling organisms. In some species, pedicellariae are strong enough to be used for defense or to capture very small prey.
Another distinct surface feature is the papulae, often referred to as dermal gills or skin gills. These are thin-walled, finger-like projections of the body cavity that extend through the spaces in the skeletal ossicles. The papulae are filled with fluid and provide a large surface area for gas exchange, allowing the sea star to absorb oxygen from the surrounding seawater. They also play a role in the excretion of waste products and can be retracted for protection.
Locomotion and the Water Vascular System
Sea stars move across the seabed using a unique biological hydraulic system called the water vascular system. This specialized network of fluid-filled canals powers the hundreds of small, flexible appendages known as tube feet, or podia, located on the underside of each arm. Seawater enters the system through a sieve-like plate on the top surface, called the madreporite, and is distributed through a series of canals.
Each tube foot is connected internally to a muscular, bulb-like sac called an ampulla. When the muscles of the ampulla contract, they force water into the attached tube foot, causing it to elongate and extend toward the substrate. The tube foot then attaches to the surface using a chemical adhesive, and the muscles within the foot contract to pull the animal forward.