“Sea Urchins Feed through Their Bottoms”. True or false?

"Sea urchins feed through their bottoms"... I have heard several times, including a similar remark from the film, Johnny English [DVD](upon hearing this, in a sushi bar, Rowan Atkinson's character then proceeds to spit out the sea urchin dish, in disgust).

This myth stems from the remarkably different anatomy, or body plan, of sea urchins (echinoids).

Sea urchins consist of a somewhat spherical body, covered in spines.  In most species, the mouth is on the lower or 'bottom' surface (known as the 'oral surface') — whereas its ‘true’ bottom or anus is on the opposite side of the organism, on its upper surface (‘aboral surface’).

Thus, sea urchins feed through their ‘bottom’ surface, but this includes their mouth, not their anus. So Rowan Atkinson’s character need not be so disgusted…

This unusual body plan (at least compared to our anatomy) appears to be an adaptation of the sea urchin to living on the ocean floor, so that its mouth is in contact with the substrate, allowing it to feed on algae or detritus.  As the anus is situated on its upper surface, waste does not therefore become mixed up in its food.

Read more about sea urchins...