Starfish, correctly known as sea stars, are marine invertebrates belonging to the phylum Echinodermata, a group that also includes sea urchins and sea cucumbers. Given their unusual appearance, many people wonder about their internal workings. The question of whether a starfish possesses a beak—a hard, keratinous structure found in organisms like birds and cephalopods—is a common one. To understand the sea star’s methods for feeding and movement, one must look closely at the specialized structures that define this ancient group of animals.
The Anatomy of the Starfish Mouth
Sea stars do not possess a beak because their feeding apparatus is entirely different from the biting or tearing structures found in other marine predators. Instead, the sea star has a simple mouth opening located in the center of its underside, referred to as the oral surface. This opening is surrounded by a tough, flexible membrane called the peristomial membrane and is controlled by a sphincter muscle.
The mouth leads into a short esophagus and then into a two-part stomach, but it lacks the sharp, rigid, keratin-based components that constitute a beak. Sea stars do possess an endoskeleton composed of thousands of small, calcareous plates called ossicles, embedded in their body wall. These ossicles form a protective framework, but they do not function as a cohesive, jaw-like biting structure.
Some species have small, movable spines around the mouth area. These are primarily for protection and helping to manipulate food toward the central opening, not for slicing or crushing.
Unique Feeding Mechanisms
The sea star’s method of consumption relies on stomach eversion, a specialized adaptation for external digestion. When feeding on shelled organisms like clams or mussels, the sea star first uses its powerful tube feet to pry the bivalve’s shells apart. The force generated by the hydraulic action of the tube feet applies a steady pull that eventually overcomes the mollusk’s strong adductor muscle, creating a tiny gap.
Through this opening, the sea star then pushes its large, glandular cardiac stomach out through its mouth and directly into the prey’s shell cavity. The everted stomach begins to secrete powerful digestive enzymes onto the soft tissues of the bivalve, dissolving the prey externally. This external digestion allows the sea star to consume prey that is much larger than its small mouth opening would otherwise permit.
Once the prey has been liquified, the sea star retracts its cardiac stomach back into its body using specialized retractor muscles. The remaining digestion and nutrient absorption occur internally in the pyloric stomach and its associated pyloric caeca, which extend into each arm.
Locomotion and Unique Body Structure
The sea star’s body structure is fundamentally defined by its radial symmetry, a characteristic where body parts radiate from a central disk, typically in five sections, known as pentamerous symmetry. This body plan means the sea star lacks a true head or a centralized brain, instead coordinating its actions through a nerve ring surrounding the mouth and radial nerves extending into each arm. This decentralized nervous system allows the animal to move and respond to stimuli in a coordinated manner.
Locomotion is primarily powered by the water vascular system, a hydraulic network unique to echinoderms that uses seawater pressure to operate thousands of tube feet, or podia. Water enters the system through a porous plate on the upper surface called the madreporite, flows through canals, and is distributed to the tube feet.
Each tube foot consists of a muscular sac, the ampulla, which pushes water into the outer podium, causing it to extend and adhere to a surface. The tube feet are arranged in grooves along the underside of the arms and function not only for movement but also for sensation and gas exchange. By alternately extending and retracting these suckered appendages, the sea star achieves a slow but steady gliding motion across the seafloor, rocks, and prey.
The sea star’s skin contains tiny, thin-walled outgrowths called dermal branchiae, which function primarily as surfaces for respiration and waste excretion.