Echinoderms are a diverse group of marine animals that include starfish, sea urchins, sand dollars, sea cucumbers and crinoids. They are characterized by their radial symmetry, spiny skin, water vascular system and calcareous endoskeleton. But which group of animals is most closely related to them? The answer may surprise you.
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Echinoderms and Chordates: A Common Ancestry
The most closely related group to the echinoderms is the chordates, which include vertebrates such as fish, amphibians, reptiles, birds and mammals, as well as invertebrates such as tunicates and lancelets. Both echinoderms and chordates belong to the superphylum Deuterostomia, which means that they share a common pattern of embryonic development. In deuterostomes, the first opening that forms during gastrulation becomes the anus, while the second opening becomes the mouth. This is opposite to the pattern seen in protostomes, such as arthropods and mollusks, where the first opening becomes the mouth and the second opening becomes the anus.
Deuterostomes also share other features, such as a coelom (a fluid-filled body cavity), a dorsal nerve cord, pharyngeal slits (openings in the throat region) and a post-anal tail (an extension of the body beyond the anus). These features are more evident in chordates than in echinoderms, but they can be traced back to their common ancestor.
Echinoderms and Arthropods: A Superficial Similarity
At first glance, echinoderms may seem more closely related to arthropods than to chordates. Arthropods are also a diverse group of animals that include insects, spiders, crustaceans and centipedes. They are characterized by their bilateral symmetry, segmented body, jointed appendages and exoskeleton. Both echinoderms and arthropods have hard external structures that protect their soft internal organs. However, these structures are made of different materials: echinoderms have an endoskeleton made of calcium carbonate plates called ossicles, while arthropods have an exoskeleton made of chitin.
Echinoderms and arthropods also have different modes of locomotion. Echinoderms use tube feet, which are extensions of their water vascular system that can create suction and movement. Arthropods use muscles attached to their jointed appendages to move around. Furthermore, echinoderms and arthropods have different nervous systems. Echinoderms have a simple radial nervous system that consists of a nerve ring around the mouth and nerve cords radiating into each arm. Arthropods have a more complex bilateral nervous system that consists of a brain and a ventral nerve cord with ganglia (clusters of nerve cells) in each segment.
Conclusion
Echinoderms are more closely related to chordates than to arthropods based on their embryonic development, body plan and molecular evidence. Although echinoderms and arthropods may look similar in some aspects, they have evolved independently from different ancestors. Echinoderms and chordates share a common deuterostome ancestor that lived about 570 million years ago during the Cambrian period.
