His kind get the tube portion buried and then secrete a mucus which binds the sand into a protective tube that just incidentally contains stinging cells like those on the tentacles, protecting the animal from other under the sand residents such as bristleworms. Then the head, with the tentacles, projects above the sand and captures various prey. At night, the head is extended quite a distance; during the day, not so much, plus he's likely to hide under the sand inside the tube he's constructed at times during the day — just tired, I suppose. And of course, when I say "he" and "his", I am not being literal.

#aquarium

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