Answer:
Based on the description provided, Horace is most likely examining a sea cucumber because it uses tentacles for feeding.
Here's why:
Sea cucumbers are echinoderms that use tentacles around their mouth to sweep food into their digestive system. This matches the behavior described in the question.
Sea cucumbers also have tube feet, which they use for locomotion along the bottom of the tank, as mentioned in the question.
Brittle stars, while also echinoderms, do not use tentacles for feeding. Instead, they use their arms to capture food particles and bring them to their mouth.
Therefore, the correct answer is: a sea cucumber because it uses tentacles for feeding.