The "'Xiphosura "'are an chelicerates that includes a large number of extinct lineages and only four extant species in the family Limulidae, which includes the horseshoe crabs.
32.
The surviving marine species include the four species of xiphosurans ( horseshoe crabs ), and possibly the 1, 300 species of pycnogonids ( sea spiders ), if the latter are indeed chelicerates.
33.
Color vision requires a number of opsin molecules with different absorbance peaks, and at least three opsins were present in the ancestor of chelicerates and pancrustaceans; members of both these groups today possess color vision.
34.
Mandibulates also differ by having antennae, and also by having three distinct body regions : head, thorax and abdomen . ( The cephalothorax ( or prosoma ) of chelicerates is a fusion of head and thorax .)
35.
Although " sea spiders " are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans.
36.
Chelicerate head structures differ considerably from those of mandibulates ( i . e . insects, crustaceans and myriapods ); they possess eyes and a single pair of grasping appendages innervated from the brain, plus a labrum-like structure.
37.
The " stem-group arthropod " hypothesis is very widely accepted, but some trees suggest that the onychophorans may occupy a different position; their brain anatomy is more closely related to that of the chelicerates than to any other arthropod.
38.
However, a subsequent study using Hox gene expression patterns consistent with a developmental homology between chelicerates and chelifores, with chelifores innervated from a deuterocerebrum that has been rotated forwards; thus, the protocerebral Great Appendage clade does not include the Pycnogonida.
39.
""'Cheliceratichnus " "', meaning trace of a chelicerate arthropod, is an ichnogenus erected by Dalman and Lucas ( 2015 ) for a body imprint discovered in the Lower Jurassic East Berlin Formation of Holyoke, Massachusetts.
40.
Many Cambrian arthropods, including the trilobites themselves, possess a single pair of slender antennae, which have been equated with either the first or second antennae of the crustaceans; and either the chelicerae or the missing appendages of the supposedly reduced deuterocerebrum in chelicerates.
