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#chelicerates

3 public questions tagged with this topic.

Animal Classification

Horseshoe crabs belong to class

Horseshoe crabs family Limulidae traditionally placed in class Merostomata along with extinct eurypterids, characterized by horseshoe-shaped prosomal carapace, compound lateral eyes, median ocelli, opisthosoma bearing book gills for aquatic respiration, and long telson spine for righting. Despite common name crab they are not crustaceans but marine chelicerates more closely allied to arachnids, molecular data nesting them within Arachnida as Xiphosura. Arachnida includes spiders and scorpions with book lungs, Insecta hexapods, Crustacea biramous appendages. Merostomate status historically reflects marine chelicerate identity with gill respiration distinguishing from terrestrial arachnids.

Ref: Brusca & Brusca, Invertebrates, 3rd ed., Merostomata Limulidae; NCBI Bookshelf, Horseshoe Crab Classification

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Animal Classification

The body of chelicerates is divided into

Chelicerate body divided into two primary tagmata rather than three in insects. Anterior prosoma or cephalothorax results from fusion of six segments, dorsal carapace covering eyes, chelicerae, pedipalps and four pairs of walking legs, concentrated sensory and locomotory function. Posterior opisthosoma or abdomen contains twelve segments often reduced, bearing genital aperture, respiratory organs like book lungs or book gills, spinnerets in spiders and telson in scorpions. Narrow pedicel in spiders separates prosoma and opisthosoma allowing flexibility. This bipartite prosoma-opisthosoma organization contrasts with head-thorax-abdomen pattern defining chelicerate tagmosis.

Ref: Hickman et al., Chapter 20: Chelicerata Body Division; Campbell Biology, 12th ed., Chapter 33

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Animal Classification

Chelicerates lack

Chelicerata represents arthropod subphylum containing arachnids, merostomes and pycnogonids, united by absence of antennae and presence of chelicerae as first appendage pair. Chelicerae may be jackknife or pincer-like for grasping or fang-like injecting venom as in spiders. Lacking antennae contrasts with Mandibulata which possess antennae for sensory functions, mandibles for biting and second antennae in crustaceans. Chelicerates rely on pedipalps and leg sensory hairs for chemoreception. Antennal absence therefore serves as key diagnostic character separating chelicerates from insects, crustaceans and myriapods in systematics and evolutionary morphology.

Ref: Brusca & Brusca, Invertebrates, 3rd ed., Chelicerata Characters; Campbell Biology, 12th ed., Chapter 33

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