Sand dollars are the durable skeletal remains, or tests, of a marine invertebrate often found scattered on sandy beaches. These disk-shaped objects are not shells but are specialized, flattened sea urchins. They belong to the class Echinoidea of the phylum Echinodermata, which also includes starfish and sea cucumbers. The familiar bleached disk is the sturdy framework of what was once a complex, living animal.
Biological Identity and External Features
The living sand dollar is a burrowing organism adapted for life on the sandy seafloor. Its body is covered in countless tiny, velvety spines, which are often reddish-brown, purple, or green. These coordinated spines allow the animal to crawl and funnel food particles toward its mouth, located on the center of its underside. The upper surface features a distinctive five-petaled pattern, known as the petaloid ambulacra. These five paired rows of pores are perforations in the endoskeleton that serve primarily for gas exchange, functioning like gills.
The Internal Ossicles (The “Doves”)
When a dead sand dollar’s test is broken open, five small, hard, white fragments may fall out, often referred to as “doves” by beachcombers. These pieces are scientifically known as ossicles, small skeletal elements that form the animal’s jaw apparatus. This complex structure is called Aristotle’s lantern, a feeding mechanism used for grinding tiny organisms and detritus. The five ossicles represent the pyramids of the lantern, the main supporting elements of the jaws. This popular association with peace has led to common folklore surrounding the sand dollar.
Composition and Formation of the Test
The sand dollar’s test is an endoskeleton, an internal structure covered by the skin and spines of the living animal. This rigid structure is composed of numerous calcium carbonate plates, or ossicles, that are tightly fused together. This composition creates a durable yet lightweight framework, adapted for its burrowing lifestyle. The test is a mosaic of plates arranged in a five-part, radially symmetric design. Some species have specialized slots called lunules that pass through the test. These lunules help anchor the animal by allowing water to flow through its body, preventing it from being swept away by strong currents.
From Living Creature to Beach Relic
The transition from a living organism to a pale beach souvenir begins after the animal dies on the ocean floor. The soft internal tissues decay, and the tiny, colorful spines detach from the test surface. The remaining calcium carbonate skeleton is often washed ashore by the waves. Exposure to sunlight and the elements bleaches the skeleton white, revealing the iconic shape and petal pattern. The discovery of a pristine white sand dollar indicates that the creature is no longer alive, having left behind its structural framework.