Let's pay some attention to the Paracentrotus lividus stony urchin, dark coloured individuals of which can be easily confused with omnipresent Arbacia lixula by a casual inspection.

Below: Calcareous skeleton of Paracentrotus lividus (usually coloured from orange and pink to red) features a flattened shape, wide stomatic opening (not seen on the shot) and, comparing to other species examined here, less fragile edges of the aboral opening.

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