Such fascinating creatures! If you have missed it so far, don’t miss it now – the two-part series by Mark H on DailyKos:
Marine Life Series: Horseshoe Crab Basics
Marine Life Series: Horseshoe Crab Anatomy
One day when I find some time, I’ll have to write a long detailed post about the fascinating aspects of the circadian system and vision in the horseshoe crab (oh, some of which was done by Erik Herzog, so you know I like the stuff!).