Annelids ? Arthropods? Internal and external segmentation


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  • Annelids ? Arthropods?
  • Internal and external segmentation
  • Paired, segmental appendages
  • Teloblastic growth
  • General arrangement of the nervous system
  • (ventral, ganglionated nerve cord, ladder like N.S.)
  • Phylum Onychophora
  • Phylum Onychophora :
  • 80 or so species of rare animals
  • 1.5 to 15 cm in length
  • Found in moist places, tropical and So. temperate regions
  • Distribution suggests they evolved before the Cambrian
  • Hallucigenia
  • Biogeography of Onychophora
  • Gondwana supercontinent
  • 500-200 mya
  • Characteristics
  • of the
  • Onychophora
  • No external segmentation
  • -Thin flexible outer cuticle - Growth by ecdysis
  • - Continuous muscles - Chitinous claws on leg tips
  • - Fluid filled body cavity - Chitinous jaws
  • - Appendages unsegmented - Open circulatory system
  • - Segmental excretory - Trachea, spiracular system
  • tubes like metanephridia
  • Annelid-Like Arthropod-like
  • PanArthropoda
  • According to this
  • cladogram are the Onychophora the missing link between arthropods and annelids??
  • Insecta
  • ~ 1 million
  • Chelicerata
  • (75,000)
  • Crustacea
  • 45,000
  • Myriapoda
  • (13,000)
  • Phylum probably contains 5 million species
  • of which 1 million have been described
  • Phylum Arthropoda
  • I. Extant Arthropod Diversity
  • Sub Phylum Mandibulata
  • Crustacea
  • Insecta
  • Myriapoda
  • Sub Phylum Chelicerata
  • Class Crustacea
  • Sub Class
  • Copepoda
  • Sub Class Cirripidea
  • Sub Class Branchiopoda
  • Class Insecta
  • Class Myriapoda
  • Sub Class Chilopoda
  • Sub Class Diplopoda
  • Sub Phylum Chelicerata
  • Class Merostomata
  • Class Pycnogonida
  • Class Arachnida
  • What might be advantages of
  • the arthropod exoskeleton?
  • Advantages:
  • Protection from injury and physiological stress
  • Barrier against osmotic and ionic gradients
  • Support
  • Attachment for muscles
  • Elaborated as a variety of structures: wings, bristles, antennas, eyes, auditory organs, outer shields, jaws, beaks, pincers, paddles
  • Most Significant Character: Exoskeleton
  • endocuticle
  • Chitin is high MW Nitrogenous
  • mucopolysaccheride
  • Epicuticle is of lipids and lipoproteins
  • Hardness from scleratized
  • proteins or from
  • calcium carbonate
  • in Crustacea
  • Fig 14.1
  • Limitations on movement? Arthro - Poda
  • Joints of un-scleratized chitin on the exoskeleton
  • Flexors and extensors from within the skeleton
  • But there are some constraints
  • Hydrostatic skeleton is of no use
  • Reduction of coelom: main body cavity is hemocoel which is part of an OPEN circulatory system
  • Blood flow:
  • tissue sinuses venous blood gills
  • aortas heart
  • But there are some constraints
  • How to increase body size?
  • Arthropod
  • Growth pattern
  • Tissue
  • growth
  • Growth is by molting (ecdysis) with tissue size gains occurring during the intermolt period
  • all arthropods
  • have “molt lines”
  • I.e. a line of
  • weakness
  • on the shell
  • III. Structural/neurobiochemical aspects of molting
  • Review of the molting process
  • Control of Molting: Crustacea (Insects Later)
  • X-organ produces the molting inhibition hormone
  • MIH stored in sinus gland
  • Y- organ in the head of the animal produces ecdysone
  • Ecdysone initiates molting
  • X organ
  • Sinus
  • gland
  • Control of Molting in Crustacea
  • Appropriate Stimulus
  • CNS inhibits MIH
  • Production by X-organ
  • Y-organ produces ecdysone
  • Drop in
  • Blood MIH
  • Molting Initiated
  • X organ exercises a negative control on molting
  • Segmentation- when pronounced it is generally considered an ancestral condition
  • Tagmosis - tendency to organize segments into regions having similar structure, function and appendages
  • Cephalization - in early arthropods the head was scarcely distinguishable; the development of a strong sensory, feeding cephalic region is a common theme in the various groups.
  • A=antenna
  • a=acron
  • C=chelicera
  • Ci= Chilarum
  • L = leg
  • Mnd =Mandible
  • Mx = maxilla
  • P= pedipalp
  • 0 = lost segments
  • The elaboration of the head has taken different courses in the major groups but are still considered to show serial homology
  • Ant
  • Leg
  • Leg
  • Leg
  • Leg
  • Leg
  • Leg
  • Ant
  • Leg
  • Leg
  • Leg
  • Leg
  • Leg
  • Leg
  • Ancestor
  • Trilobites
  • lost
  • Cheli
  • Pedi
  • L
  • L
  • L
  • L
  • Chelicerates Crustacea Insects
  • Ant1
  • Ant2
  • Mnd
  • Mx 1
  • Mx 2
  • Ant1
  • Mnd
  • Mx 1
  • Mx 2
  • -Protocerebrum and deutocerebrum also in Onychophora
  • -But tritocerebrum is a unique character of Arthropoda
  • -Position of cerebrums helps establish homology of segments and segmental appendages


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