Arachnida - spiders, scorpions, etc.|
Merostomata - Horseshoe crabs, etc.
Pycnogonida - sea spiders
The Chelicerata is a subphylum and one of the major subdivisions of the arthropods, including the arachnids, horseshoe crabs, and related forms. These mainly marine predators outlasted the now extinct trilobites, the common arthropod in the Cambrian era.
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:
The Chelicerata are divided into three classes:
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.
Most of the marine chelicerates, including all of the euryterids, are extinct. Only four species survive today.