🌊✨️ At the beach of Baltrum, the sea washed up the delicate skeleton of Echinocardium cordatum, a heart urchin. Its sharp spines are gone, leaving only the pale, fragile shell with fine patterns.

🫧🤍 The heart urchin lives under the sea, buried in soft sand or mud, up to 15 centimeters deep. It uses its small spines to dig and feeds by filtering tiny food particles like the decomposing remains of organisms and plants (detritus) from the sand around it.

👽🪸 Sea urchins may look very alien at first glance, yet they are actually more closely related to us humans than to snails or insects. This is because sea urchins – just like starfish – belong to the echinoderms.

🪞🤔 If you only look at their appearance, you might think: Wait a minute! Starfish and sea urchins look nothing like most animals. Many animals – including humans – have two mirror-image halves (bilateral symmetry). Echinoderms, however, are radially symmetrical, usually with five identical sections. That might make it seem like they’re even less related to us than snails or insects, which also have two symmetrical sides.

🧬🔬 But appearances can be misleading. Echinoderms, like all vertebrates, belong to the deuterostomes. Snails and insects, on the other hand, are part of the protostomes. A key clue: echinoderms are bilaterally symmetrical in their larval stage. It’s only later in life that they develop their characteristic radial symmetry.

👄♻️ The fundamental difference between protostomes and deuterostomes shows up very early in embryonic development: in deuterostomes, the first body opening (the blastopore) becomes the anus, and the mouth forms later. In protostomes, it’s the other way around – the blastopore develops into the mouth.

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