Class: Merostomata
The most common type of horseshoe crab has the scientific classification of Limulus polyphemus.
The head and thorax of the horseshoe crab are fused together to make the cephalothorax. The body is covered with a hard, thick shell. The abdomen is protected by a narrow shell that is hinged from the main shell at the front and ends in a long, sharp spine.
"The animal has a large compound eye on each side of the cephalothorax, two pairs of smaller, simple eyes between the compound eyes, and five light-receptive organs beneath the shell."(1) Its mouth is located in the middle of the underside of the cephalothorax. A pair of pincers (chelicerae) for seizing food are found on each side of the mouth. The horseshoe crab has six pairs of walking legs. "The underside of the abdomen bears six additional pairs of appendages; the first covers the genital opening, the other five are modified as gills."(2) It feeds on small invertebrates that are buried beneath sand or mud. "In late spring the female lays eggs in the intertidal zone of bays and estuaries." (3)
A Horseshoe Crab found near Sengacontacket Pond in Edgartown, MA
Horseshoe crabs are one of the oldest classes of marine arthropods. They are related to extinct water scorpions. They only inhabit certain areas of the ocean--one of them is the North Atlantic where they are fairly common.
(1)"Horseshoe Crab," Microsoft Encarta 96 Encyclopedia. 1993-1995 Microsoft Corporation. Funk & Wagnalls Corporation.
(2) Ibid.
(3) Ibid.