Chelicerata, phylum of the living horseshoe crabs (class Merostomata), the arachnids (class Arachnida), and the sea spiders (class Pycnogonida). Chelicerates are characterized by the absence of antennae and jaws and the presence of feeding structures (chelicera), which are modified pincerlike appendages used mainly for grasping and fragmenting food. Nearly all the merostomates are extinct, the only living representative being Limulus, the horseshoe crab (subclass Xiphosura) and its relatives, which inhabits the soft bottom mud of shallow, coastal seas. Merostomates have five or six gills, which have been modified from body appendages, and a long tail (telson) lacking appendages. The extinct giant water scorpions (subclass Eurypterida) belong to this class. Members of class Pycnogonida are commonly known as sea spiders. These exclusively marine carnivores are spiderlike in appearance and range in body length from 4/100 in. (1 mm) to 4 in. (5 cm); the leg spread is sometimes over 2 ft (61 cm). Most sea spiders have four pairs of legs. They feed with a sucking proboscis on algae and other invertebrates and are found in oceans all over the world. The largest class of chelicerates, class Arachnida, includes orders Aranaea (spiders), Acarina (ticks and mites), Opiliones (daddy longlegs, or harvestmen), and Scorpionida (scorpions), among the most important. Arachnids are predominantly terrestrial, and most are carnivorous, with the digestion of prey starting outside the body. The body is composed of an unsegmented anterior region (prosoma), with a pair of chelicera, a pair of leglike appendages (pedipalps), four pairs of walking legs, and a posterior region (opisthosoma); it is equipped with book lungs or tracheae, for respiration. Arachnids are an ancient group, their fossil records dating back to the Carboniferous period.

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, including the arachnids, horseshoe crabs, and related forms. These mainly predatory arthropods ultimately outlasted the now extinct trilobites, the common marine arthropod of the Cambrian era. Most of the marine chelicerates, including all of the eurypterids, are now extinct. The chelicerates and their closest fossil relatives (mostly originally included in the Xiphosura) are grouped together with the trilobites to form the taxon Arachnomorpha.

Characteristics

In the Chelicerata, the body is divided into an anterior prosoma (or cephalothorax) composed of eight segments plus a presegmental acron and a posterior opisthosoma (or abdomen) composed of twelve segments plus a postsegmental telson. As in other arthropods, the mouth lies between the second and third segments, but whereas in other groups there is usually a pair of antennae on the last preoral segment, here there are none. The prosoma usually has eyes. The first two segments of the prosoma bear no appendages; the third bears the chelicerae. the fourth segment bears legs or pedipalps, and all subsequent segments bear legs.

The chelicerae, which give the group its name, are pointed appendages that grasp the food in place of the chewing mandibles most other arthropods have. Most are unable to ingest anything solid, so they drink blood or spit or inject digestive enzymes into their prey. The legs on the prosoma are either uniramous or have a very reduced gill branch, and are adapted for walking or swimming. The appendages on the opisthosoma, in contrast, are either absent or are reduced to their gill branch.

Classification

The Chelicerata are divided into five classes:

• Arachnida (spiders, scorpions, mites, etc.)
• Xiphosura (horseshoe crabs)
• Eurypterida (sea scorpions, extinct)
• Chasmataspidida (chasmataspids, extinct)
• Pycnogonida (sea spiders)

The Pycnogonida actually show some strong differences from the body plan as described above, and it has been suggested that they represent an independent line of arthropods. They may have diverged from the other chelicerates early on, or represent highly modified forms. Sometimes they are excluded from the Chelicerata but grouped with them as the Cheliceriformes. The name Merostomata should be avoided because in all recent cladistic hypotheses it refers to a paraphyletic group composed by the Xiphosura + Eurypterida.

The Burgess shale animal Sanctacaris, and perhaps the aglaspids, have also been suggested as belonging here, but this is now disputed. These are extinct forms that arose in the Cambrian; the aglaspids are believed to have died out during the Silurian. After The oldest group of unequivocal chelicerates are the Pycnogonida, found from the late Cambrian onwards.