Starfish 'arms' are actually extensions of their head, scientists say
Starfish may appear to have a plethora of limbs, but it turns out the creatures actually resemble something akin to a disembodied head.
Experts say it has long been a conundrum how starfish, sea urchins and other animals with a fivefold body plan, known as echinoderms, evolved from an ancestor with twofold symmetry – a body plan common today in animals including insects, molluscs and vertebrates.
“How any bit of an echinoderm related to any bit of any other organism in terms of its general body plan was really unclear,” said Dr Jeff Thompson, a co-author on the study at the University of Southampton.
The situation is further complicated by the fact echinoderms begin life as larvae with twofold symmetry, before metamorphosing into their adult forms.
Now researchers say the unusual adult body plan of starfish and other echinoderms does not arise as a result of the central part of the body, or trunk, giving rise to five identical parts.
“It doesn’t look like the trunk is there at all,” said Thompson.
https://www.theguardian.com/environment/2023/nov/01/starfish-arms-are-actually-extensions-of-their-head-scientists-say
This is crazy!
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There were once stemless, free-floating crinoids. All head and arms.