Specimen | Arthropods Lec Manual

• Subphylum Trilobitomorpha

  Class : Trilobita - refers to transverse trilobation of the dorsal body
• body of Trilobites : oval , flattened with dorsal cuticle

  forms are dictated by the mode of existence that may be burrowing , benthic , crawling , and planktonic swimming
• 3 body divisions of trilobites

  • cephalon
  • thorax / trunk
  • pygidium
• Cephalon of trilobites - fusion of ofacron and 4 post-oral segments ; bears a ventral mouth, a pair of ventral antennae, distal tips which bifurcates
• thorax / trunk of trilobites - segments are separated , each bearing a pair of biramous appendages ; with telopodite (inner walking leg) & pre-epipodite (outer gill bearing branch)
• pygidium of trilobites - fused segments with reduced appendages: possesses lateral lobes & middle axial lobe arising from 3 longitudinal divisions made by a pair of furrows from anterior to posterior
• development of trilobites : 3 larval stages

  • protaspia - planktonic with carapace
  • meraspia - carapace , pygidium and some developing trunk appendages present
  • holaspia - adult features are evident
• Subphylum Chelicerata

  body division - cephalothorax or prosoma , abdomen or opisthosoma

  antennae are absent

  chelicerae : first pair of appendages , feeding structures

  pedipalps : second pair of appendages, perform various functions

  four pairs of walking appendages
• Class Merostomata

  aquatic chelicerates

  5 or 6 pairs of abdominal appendages : modified as gills and by a spike-like telson at the end of the body
• Digestion of Xiphosura

  Extracellular

  Intracellular within the hepatic caeca
• Excretion & osmoregulation of Xiphosura

  4 pairs of coxal glands – common saclike chambers - coiled tubules =>bladder =>excretory pore at the base of the last pair of walking legs
• development of xiphosura

  centrolecithal egg ==> cleavage ==> trilobite larva==> from 13-14 instars ==> adult
• Order Eurypterida

  extinct giant arthropods that existed from the Cambrian to the Permian period

  aquatic , probably attained the largest size of any of the arthropods

  smaller cephalothorax

  division of abdomen : pre abdomen (mesosoma) and post abdomen (metastoma)

  six pairs of gills , first pair forms the operculum
• Class Arachnida

  largest and most important of the chelicerate classes

  early arachnids: aquatic contemporary living arachnids: terrestrial
• Entailed morphological and physiological changes of Class Arachnida

  • epicuticle became waxy
  • arachnid book lungs and trachea developed from book gills
  • appendages became better adapted for terrestrial locomotion
  • development of silk in spiders , pseudoscorpions ; poison glands in scorpions, spiders, and pseudoscorpions
• Class Arachnida , body divided into :

  prosoma : unsegmented, covered dorsally by a solid carapace; ventral surface covered by coxae of the appendages

  abdomen : segmented , divided into a preabdomen and a postabdomen
• Appendages of Class Arachnida

  arise from the prosoma

  pair of chelicerae ( for feeding )

  pair of pedipalps

  4 pairs of lungs
• respiration of Class Arachnida

  book lungs and/or trachea
• circulation of class Arachnida

  heart => dorsal anterior and posterior aorta => small arteries => tissue spaces or sinuses => large ventral sinus => book lungs => venous channels => pericardium => ostia => heart
• Excretion of Class Arachnida

  coxal glands : thin -walled spherical sacs along the sides of the prosoma that collect wastes from the surrounding blood

  malpighian tubules : one or two pairs of slender tubes that arise from the posterior of the mesenteron ; wastes pass across its thin syncytial walls and then out into the intestine

  nephrocytes : large phagocytic cells that are localized in clusters in certain parts of the prosoma and abdomen
• nervous system of Xiphosura

  • brain
  • protocerebrum : contains the optic centers and optic nerves
  • tritocerebrum : collar or ring surrounding the esophagus since ganglia originally located in the thorax and abdomen have migrated anteriorly and fused with the subesophageal ganglion
• sense organs of Arachnida

  • sensory hairs : simple innervated setae
  • trichobothria : very long , fine hairs
  • eyes : photoreceptors
  • slit sense organs : slit-like pits that detect slight changes in the tension of the exoskeleton and also to detect sound vibrations
• Reproductive System of Arachnida

  dioecious

  single or paired gonads in abdomen

  ducts exit through genital orifice

  sperm transfer is primitive and indirect through spermatophores
• prosoma of Order Scorpiones - covered by a single carapace, long abdomen in a stinging apparatus
• chelicera of Order Scorpiones : small, triarticulate, chelate, project anteriorly from the front of the carapace
• pedipalps of Order Scorpiones : enlarged, form a pair of pincers for capturing prey
• abdomen of Order Scorpiones : 7 segmented preabdomen, 5 narrow segments of postabdomen
• eyes of Order Scorpiones : 2 to 5 pairs that are small lateral along the anterior lateral margin of the carapace
• gas exchange of Order Scorpiones : book lungs
• excretion of Order Scorpiones : 2 pairs of malphigian tubules and a single pair of coxal glands
• nervous system of Scorpiones : distinct nerve cord with 7 unfused ganglia
• feefing of Order Scorpiones : predaceous
• Order Pseudoscorpions - lack the long abdomen and sting
• Pedipalp of Order Pseudoscorpions : with poison glands
• Feeding of Order Pseudoscorpions : feed on small arthropods such as collembolands and mites
• eyes of Order Pseudoscorpions - 1 or 2 at each anterior lateral comer or may be absent
• gas exchange of Order Pseudoscorpions : trachea system through 2 pairs of spiracles on the ventral side of 3rd and 4th abdominal segments
• excretion of Pseudoscorpions : coxal gland
• sense organs of Pseudoscorpions : indirect eyes, tactile, and trichobothria, and lyriform organs