The Chelicerates

Members of the Subphylum Chelicerata have no antennae and feeding appendages called chelicerae. Their body is divided into two main sections: an anterior cephalothorax and a posterior abdomen.

• Class Merostomata. This class is represented by only four species of animals commonly known as "horseshoe crabs". Horseshoe crabs are common inhabitants of the coastal marine waters off the North American Atlantic coast and along the Gulf of Mexico. They spend most of their time burrowing through soft mud looking for worms, mollusks, and other tasty tidbits. Examine a living or preserved specimen of the horseshoe crab, Limulus polyphemus. The anterior cephalothorax is covered with a horseshoe-shaped carapace, from which the animals derive their common name. Two large compound eyes and a pair of small simple eyes (at the anterior end of the median ridge) are on the dorsal carapace. The abdomen (posterior to the carapace) bears rows of short spines and a single large telson. Despite its formidable appearance, the telson is used neither for offense nor defense, and living animals can be safely handled. When annoyed, horseshoe crabs fold their abdomen under the carapace to protect the more delicate ventral appendages. Turn your animal over to expose the ventral surface. The smallest of the pincers are the chelicerae, feeding appendages that are characteristic of this subphylum. The mouth is located directly behind them. There are five pair of walking legs. Book gills (the page-like appendages on the abdomen) are used for respiration and swimming. The anus is just anterior to the telson.

• Class Arachnida. The arachnids are the most successful group in this subphylum. They include spiders, scorpions, mites, and ticks and were among the first animals to move onto land. Although this group is probably the most unpopular of all invertebrates, they are ecologically important (spiders, for example, devour more insect pests than do birds).

  • Scorpions: Scorpions are most easily recognized by the stinging barb at the tip of their abdomen. Gently remove a preserved scorpion from its vial. The body is divided into an anterior prosoma and the posterior abdomen. As in the horseshoe crabs, the prosoma is covered with a dorsal shield (the carapace). Two pincher-like appendages (the pedipalps) are used to manipulate prey (mostly insects and spiders). There are four pair of walking legs.
  • Spiders: The body is divided into an anterior cephalothorax and a posterior abdomen. From a dorsal view, eight eyes are clustered at the front of the cephalothorax (the eyes are the shiny black dots). On the ventral side note the fangs at the base of the movable chelicerae. Nearby you will find two small leg-like appendages (the pedipalps). In adult or penultimate ("teenage") males the last segment of the pedipalps is expanded into a copulatory organ. The pedipalps are followed by four pair of walking legs. The last segment of each leg is called the tarsus (it serves as a foot). Trace your way back toward the body and identify the tarsus, metatarsus, tibia, patella, femur, trochanter, and a small coxa at the base of the leg. The sternum is the flat shield between the eight coxae on the ventral surface. Identify the entrance to the book lungs (two slits on the anterior part of the abdomen) and the spinnerets (finger-like appendages at the posterior tip of the abdomen). Silk glands in the abdomen are connected to the spinnerets. Examine the other spiders on display.
  • Mites and ticks. The abdomen of mites and ticks has lost any external evidence of segmentation and is fused to the prosoma. The carapace therefore extends over the entire body. Mites are a group of small (<1 mm), mostly parasitic arachnids (although free-living mites can inhabit both terrestrial and aquatic ecosystems). Ticks, on the other hand, are usually larger and are obligate external blood-sucking parasites throughout their entire lives.