Arachnida - spiders, scorpions, etc.|
Merostomata - Horseshoe crabs, etc.
Pycnogonida - sea spiders
In these the body is divided into an anterior prosoma (cephalothorax), composed of eight segments plus a presegmental acron that usually has eyes, and a posterior opisthoma (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 appendages on the prosoma are as follows:
- Chelicerae (pinchers)
- Legs or pedipalps
The Chelicerata are divided into three classes:
- Merostomata (horseshoe crabs, sea scorps - extinct)
- Arachnida (spiders, scorpions, mites, etc)
- Pycnogonida (sea spiders)
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.