(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.

 

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Source:

 

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.