Can a Piercer attack more than once?

From the 1st Edition Monster Manual:

…and when a living creature passes beneath their position above they will drop upon it in order to kill and devour it.

After they drop, can they attack again, or do they just lie there waiting to get skewered? And how do they get up on the ceiling anyway? The picture in MMI doesn’t show any little legs, or a mouth for that matter either.