Sand dollars are captivating marine animals often discovered on beaches, recognized by their flattened, disc-shaped remains. These distinctive structures, known as tests, are the internal skeletons of once-living creatures. This exploration will clarify why sand dollars are indeed members of the echinoderm phylum, a diverse group of marine invertebrates.
Defining Echinoderms
Echinoderms are a phylum of marine invertebrates characterized by several distinctive features. They exhibit radial symmetry, typically pentamerous, meaning their body parts are arranged in five sections around a central axis. While their larval stages exhibit bilateral symmetry, adult echinoderms undergo a transformation to develop this radial body plan.
Echinoderms also possess a water vascular system, a network of fluid-filled canals that perform multiple functions. This hydraulic system aids in locomotion, feeding, gas exchange, and sensory perception. Water enters this system through a sieve-like plate called the madreporite.
Echinoderms also possess an internal skeleton, or endoskeleton, composed of calcified plates called ossicles. These ossicles are embedded in the skin and can be fused to form a rigid structure, as seen in the tests of some species. The name “echinoderm” itself, meaning “spiny skin,” refers to these calcium carbonate elements, which can sometimes bear spines.
Sand Dollars: A Clear Case of Echinoderm Identity
Sand dollars distinctly exhibit the defining characteristics of echinoderms, confirming their classification within this phylum. Their rigid, disc-shaped body is a clear example of an echinoderm endoskeleton, formed by the fusion of numerous calcified ossicles. This “test,” often found washed ashore, is the animal’s internal skeletal structure, which is covered by living tissue and tiny spines when the animal is alive.
Their five-part radial symmetry is evident in the distinctive, petal-like design on their upper surface. This pattern consists of five paired rows of pores, through which specialized tube feet extend, primarily for gas exchange. Live sand dollars are typically covered in short, movable spines that give them a velvety texture and aid in burrowing and crawling through sand.
Sand dollars also possess a water vascular system, a hallmark of echinoderms, which is crucial for their movement and feeding. This system uses hydraulic pressure to extend and retract their tube feet, allowing them to move across the seafloor and collect food particles. Their mouth is centrally located on their flattened underside, where cilia and tube feet work to transport food, such as tiny crustaceans and algae, towards it.
Diverse Members of the Echinoderm Phylum
The phylum Echinodermata encompasses a wide array of marine animals beyond sand dollars, all sharing radial symmetry, a water vascular system, and an endoskeleton. Sea stars, also commonly known as starfish, are perhaps the most recognizable echinoderms, typically displaying five arms radiating from a central disc. They use their numerous tube feet for movement and to grasp prey.
Brittle stars resemble sea stars but have a more distinct central body disc and slender, flexible arms. These arms allow them to move quickly across the seafloor or cling to structures. Sea urchins, close relatives of sand dollars, have a more spherical body covered in long, movable spines that assist in locomotion, defense, and sensing their environment.
Sea cucumbers, while appearing worm-like, still exhibit the pentamerous body plan, often having five rows of tube feet. Their bodies are soft, with ossicles present as small, disconnected pieces within their skin. Feather stars and sea lilies, categorized as crinoids, are among the more ancient echinoderms, often having feathery arms for filter feeding. These examples highlight the diversity within the echinoderm phylum.