phylum: Limulus Polyphemus
Despite their name, horseshoe crabs are not true crabs. Unlike true crabs,
which have two pairs of antennae, a pair of jaws, and five pairs of legs,
horseshoe crabs lack antennae and jaws, and they have seven pairs of legs,
including a pair of chelicerae. Chelicerae are appendages similar to those used
by spiders and scorpions for grasping and crushing. In addition, horseshoe
crabs have book lungs, similar to spiders and different from crabs, which have
gills. Thus, horseshoe crabs are more closely related to spiders and scorpions
than they are to other crabs. Their carapace is divided into three sections:
the anterior portion is the prosoma; the middle section is the opithosoma; and
the “tail” is called the telson. Horseshoe crabs have two pairs of eyes located
on the prosoma: one anterior set of simple eyes and one set of lateral compound
eyes similar to those of insects. In addition, they possess a series of
photoreceptors (light-sensitive organs) on the opithosoma and telson
http://www.dnr.sc.gov/marine/mrri/acechar/specgal/crabshoe.htm
Despite their name, horseshoe crabs are not true crabs. Unlike true crabs,
which have two pairs of antennae, a pair of jaws, and five pairs of legs,
horseshoe crabs lack antennae and jaws, and they have seven pairs of legs,
including a pair of chelicerae. Chelicerae are appendages similar to those used
by spiders and scorpions for grasping and crushing. In addition, horseshoe
crabs have book lungs, similar to spiders and different from crabs, which have
gills. Thus, horseshoe crabs are more closely related to spiders and scorpions
than they are to other crabs. Their carapace is divided into three sections:
the anterior portion is the prosoma; the middle section is the opithosoma; and
the “tail” is called the telson. Horseshoe crabs have two pairs of eyes located
on the prosoma: one anterior set of simple eyes and one set of lateral compound
eyes similar to those of insects. In addition, they possess a series of
photoreceptors (light-sensitive organs) on the opithosoma and telson
http://www.dnr.sc.gov/marine/mrri/acechar/specgal/crabshoe.htm