Heres the underside of a horseshoe crab. Horseshoe crabs are an example of a design that has remained the same through millions of years. They are not technically crabs. They are in their own class,(Merostomata). The horseshoe crabs creep along the shallow bottom feeding on small invertebrates in the mud or grass flats. The long tail is used for flipping theselves back over when the surf tumbles them. They are harmless, but a little ominous looking with their wiggling legs and claws.