Chelicerata
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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 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) form the group Arachnomorpha.
[edit] 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 opisthoma (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 opisthoma, in contrast, are either absent or are reduced to their gill branch.
[edit] Classification
Lorryia formosa
The Chelicerata are divided into four classes:
- Arachnida (spiders, scorpions, mites, etc.)
- Xiphosura (horseshoe crabs)
- Eurypterida (sea scorpions, extinct)
- Pycnogonida (sea spiders)
The Pycnogonida actually show some strong differences from the body plan 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.
Sanctacaris, and perhaps the aglaspids, may also belong here. These are extinct forms found in Cambrian rocks. After them, the oldest group of chelicerates are the Eurypterida, found from the Ordovician onwards. When young, these show a resemblance to the trilobites, suggesting a possible relationship between these two groups.
