Subphylum Chelicerata

• Named after the modification of chelicerae and the pedipalps
• Chelicerae


  first pair of appendages; feeding structures
• Pedipalps


  second pair of appendages; has various functions (e.g.) sensory, reproduction, etc.)
• Body segments

  cephalothorax or prosoma abdomen or opisthosoma (fused)
• Antennae absent
• Four pairs of walking appendages
• Class Merostomata

  • Aquatic chelicerates (sand burrowing horseshoe crabs)
  • Body segments: cephalothorax and abdomen
  • Dorsal side: prosoma covered by a carapace (hard shell); two lateral eyes, a caudal tail or telson
  • Ventral side: mout apparatus contain the chelicerae; attached to the cephalothorax are 4 pairs of appendages
  • Abdomen: two genital pores; six gill pairs
• Horseshoe crab (Dorsal view) Parts

  A) Prosoma

  B) Two lateral eyes

  C) Cephalothorax

  D) Caudal tail

  E) Telson

  
• Horseshoe crab (ventral view) parts

  A) Book Gills

  B) Genital Pores

  C) Chelicera

  D) Appendages

  
• Phylum: Arthropoda

  Subphylum: Chelicerata

  Class: Merostomata

  Order: Xiphosura

  Family: Limulidae

  Genus: Limulus

  
• Development of Merostomata

  Centrolecithal egg → cleavage → trilobite larva → from 13-14 instars → adult
• Habitat

  marine soft bottoms & shallow waters
• Prosoma covered by large horseshoe-shaped carapace (convex dorsally)
• 2 eye pairs: large dorsolateral pair; small dorsomedial pair; both on carapace
• Mouth is posterior to labrum (upper lip), with trisegmented chelicerae pair on each side
• 6pairs of cephalothoracic appendages

  First four pairs, fifth pair, chilaria
• Opisthosoma: unsegmented containing 6 short mobile spines at the posterior end, has 6 pairs of appendages with first pair forming the genital operculum
• Telson: long triangular spine-like tail or caudal spine (not a true telson for horseshoe crabs (lacks anal opening)
• Digestion

  extracellular, intracellular within the hepatic caeca
• Excretion & Osmoregulation

  4 pairs of coxal glands- Common saclike chambers, coiled tubules, bladder, and excretory pore (located at base of last pair of walking legs)
• Reproduction

  Dioecious with external fertilization