(horseshoe crabs, eurypterids) The Merostomata includes two rather different groups of marine organisms, the eurypterids and the horseshoe crabs. Eurypterids are now extinct; they lived 200 to 500 million years ago. Some were huge, reaching a length of 3 m. Their morphology suggests that they fed on a variety of kinds of foods. Some may have been amphibious, emerging onto land for at least part of their life cycle. The horseshoe crabs are an ancient group, but only 5 species exist today. They feed on small invertebrates. Horseshoe crabs are often used as laboratory animals by physiologists.

Members of this class have a large shield that covers the cephalothorax. The compound eyes are reduced. The second pair of appendages, the pedipalps, resemble walking legs. They have a long, spike-like appendage called a telson that projects from the rear of their bodies. Respiration is via book gills.


Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.

Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.


Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.