Class Merostomata

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Subphylum Chelicerata

• Class Merostomata
• Class Pycnogonida

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Subphylum Chelicerata

• Defining characteristics
• Absence of antennae
• Contains three classes, most of the animals are
  terrestrial and freshwater
• Marine species are horseshoe crabs, sea spiders,
  and mites

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Chelicerate Segmentation

• Cephalothorax usually has 6 pairs of appendages
• First pair chelicerae
• Second pair pedipalps
• Both function as mouthparts
• The rest are walking legs

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Taxonomic Summary

• Phylum Arthropoda
• Subphylum Chelicerata
• Class Merostomata
• Class Pycnogonida

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Class Merostomata

• Defining characteristics
• Appendages on the abdomen are flattened and
  modified for gas exchange as book gills
• Terminal portion of body drawn out into an
  elongated spike
• Only 4 living species the horseshoe crabs

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Horseshoe Crab Habits

• The cephalothorax and abdomen are covered by a
  hard non-jointed carapace

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Merostomata Appendages

• Possess an anterior chelicerae and 5 pairs of
  walking legs
• In males the first pair of walking appendages are
  modified for grasping the female during
  reproduction
• All the legs (except the last pair) bear pinchers
  for manipulating food
• Walking legs bear gnathobases on basal part of
  legs, which aid in food manipulation

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Respiration

• The abdomen bears the book gills and is endowed
  with blood
• The blood or components of it are used in
  pharmaceutical research

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Reproduction
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Class Pycnogonida

• Defining characteristics
• Body is not divided into distinct regions
• Unique proboscis at the anterior end, with an
  opening at its tip
• Includes the sea spiders, several of which are
  common in our area
• Feed on soft bodied invertebrates, hydroids,
  bryozoan colonies, and sponges

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Sea Spiders

• Have a long proboscis for feeding and a digestive
  tract with gonads extending into the walking legs
• Difficult to discern the cephalothorax, and
  abdomen
• Anterior appendages may bear cheliceras that
  grasp pieces food

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Sea Spiders

• Ovigers structures used by males to carry eggs
  on the body
• When eggs are mature they are deposited in a
  hydroid colony