Sand dollars are unique marine creatures, often discovered as bleached, disc-shaped “shells” on beaches. These are the hardened remains of a living invertebrate, closely related to sea urchins and starfish. When alive, sand dollars inhabit sandy or muddy ocean floors, often in large groups. Their structure and composition allow them to thrive through burrowing and filter feeding.
The Calcified Exoskeleton
The primary structural component of a sand dollar is its rigid skeleton, known as a test. This test is predominantly calcium carbonate, giving the dried remains their characteristic white appearance. It is an exoskeleton, formed from numerous fused plates, or ossicles. These interlocking plates create a sturdy, flattened, and often circular or oval structure that provides protection and support for the soft tissues within.
The arrangement of these plates forms a distinct five-fold symmetric pattern on the test. This intricate design contributes to the sand dollar’s structural integrity and reflects its internal organization. The test is porous, featuring tiny holes and grooves essential for the living animal’s biological functions. This structure remains after the sand dollar dies and its soft tissues decay, often bleaching before washing ashore.
External Features and Their Functions
The living sand dollar’s test is covered by a dense layer of tiny, flexible spines, giving it a velvety texture and colors ranging from gray to purple or brown. These spines are crucial for the sand dollar’s movement, allowing it to burrow into the sand and creep along the seabed. Fine, hair-like structures called cilia cover these spines, assisting in both locomotion and feeding.
A prominent external feature is the petaloid pattern, a five-part “flower” or “star” design on the upper surface of the test. This pattern is formed by specialized ambulacral areas, which contain pores through which modified tube feet extend. These tube feet are primarily used for gas exchange, allowing the sand dollar to breathe by absorbing oxygen from the surrounding water. Some sand dollar species also feature oblong holes or slits called lunules, which help equalize water pressure and prevent the animal from being swept away. Lunules can also aid in burrowing and food collection.
Internal Systems and Living Components
A living sand dollar contains various soft tissues and internal organs housed within its calcified test. Like other echinoderms, sand dollars possess a water vascular system, a network of fluid-filled tubes operating through hydraulic pressure. This system powers the tube feet for movement, feeding, and anchoring the sand dollar in its sandy habitat.
The sand dollar’s mouth is located on its underside, equipped with a complex chewing apparatus known as Aristotle’s lantern. This structure, composed of five tooth-like sections and associated muscles, grinds food particles such as algae and microscopic organisms. After digestion, waste is expelled through an anus located towards the rear of the body. The internal anatomy also includes a central nerve ring that coordinates its body functions.