The horseshoe crab Limulus polyphemus is an ancient genus that has changed little in over 250 million years. It is more closely related to chelicerates than crustaceans. Its body consists of a prosoma with six pairs of legs and two types of eyes, and an opisthosoma with six pairs of appendages that aid in respiration, reproduction, and locomotion. The first pair forms a genital operculum and the remaining five pairs are modified into gills. Horseshoe crabs are long-lived for invertebrates, with males maturing between 9-11 years and females between 10-12 years.

1. Limulus
 The horseshoe crab, Limulus polyphemus, is more closely related to chelicerates such as
spiders, scorpions, ticks and mites than it is to true crabs and other crustaceans.
 Horseshoe crabs are considered to be "living fossils" that have evolved little in the past
250 million years.
 Limulus is an ancient genus which has probably existed since the Silurian period (440 to
410 million years ago), and shows little morphological change from the now extinct
genus Paleolimulus that lived about 200 million years ago.
 This species is found in the Gulf of Mexico and along the Atlantic coast of North
America.
 Its body consisting of a prosoma or head region; and an opisthosoma or abdominal
region.
 The prosoma contains 6 pairs of legs, two types of eyes: two compound eyes, or
ommatidia, are located on either side of the head; and two simple eyes or ocelli, are
located in the center of the head.
 The opisthosoma contains an additional 6 pairs of appendages which aid in respiration,
reproduction, and locomotion.
 The first pair of abdominal appendages form a genital operculum which houses the
genital pores.
 The remaining 5 pairs of appendages are modified into a series of overlapping plates
which function as gills.

2.  Horseshoe crabs are long-lived and slow to mature in comparison to most other
invertebrate groups. Males reach sexual maturity between 9 - 11 years of age and females
between 10 - 12 years of age.