They were sitting still at the bottom of a shallow tank.
The molted opisthosome of a Horseshoe Crab has a common slipper snail living on it.
I decided to add it also as an observation of the horseshoe crab, not just as I had done previously, as an observation of the slipper snail.
Merostomata is the name given to a grouping of the extinct Eurypterida (sea scorpions) and the Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.