Arachnida - spiders, scorpions, etc.
Merostomata - Horseshoe crabs, etc.
Pycnogonida - sea spiders 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 in the Cambrian era. Most of the marine chelicerates, including all of the eurypterids, are now extinct.

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 appendages on the segments of the prosoma are as follows:

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.

The Chelicerata are divided into three classes:

• Arachnida (spiders, scorpions, mites, etc.)
• Merostomata (horseshoe crabs, sea scorps - 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.

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 Merostomata, found from the Ordovician onwards. When young, these show a resemblance to the trilobites, suggesting a possible relationship between these two groups.