Hemichordates and Chordates

Hemichordates
1. e.g. Acorn worms & Pterobranchs
2. Marine and intertidal. Mostly dioecious (seperate sexes
  1. Fragile and live in burrows/tubes
3. Previously subphylum of chordates
4. Physiology
  1. Gill pores & dorsal nerve chord
  2. Notochord (not homologous)
  3. Buccal diverticulum
  4. Open circulatory system
  5. Glomerulus (unique)
5. Class 1: Enteropneusta
  1. Deposit/suspension feeders
  2. Asexual and sexual reproduction
  3. Some TORNIA larval stage
    1. Eggs hatch into swimming larvae which later metamorphose into acorn worms
6. Class 2: Pterobranchia
  1. Small and colonial
  2. Lophophore like feeding structure
  3. Asexual reproduction in most species
Chordates and their Origin
1. Defining Characteristics:
  1. 1. Notochord (stiff rod)
  2. 2. Dorsal hollow nerve chord (runs along length of body)
  3. 3. Post anal tail (muscular, skeletal elements. Propulsion in aquatic forms)
  4. 4. Pharyngeal gill slits (allow water entering mouth to exit through body)
2. Other (some shared) Characteristics
  1. Cephalisation (brain)
  2. Segmented & regionally differentiated
  3. Bilateral symmetry
  4. Ventral heart (closed circulation)
  5. Blood from heart to anterior, ventrally
  6. Generally active
3. Possible ancestors
  1. 1. ANNELID WORMS?
    1. FOR
      1. Bilateral symmetry
      2. Segmented
      3. Active
      4. Cephalisation
      5. Longitudinal nerve chord
    2. AGAINST
      1. Flip over - Relocation of mouth and anus unlikely
      2. Solid ventral nerve chord (opposed to dorsal)
      3. Segmentation complete
      4. No notochord and no gill slits
    3. REJECT
  2. 2. ECHINODERMS?
    1. FOR
      2. Although echinoderms are sessile, larval forms are motile. Chordates evolved through paedomorphosis
      3. Larval forms became sexually mature
      4. Marine
    2. AGAINST
      1. Paedomorphosis doubted
      2. Gutmann's (1981) "Chordates into Echinoderms" theory
      3. Recent molecular evidence: only a subset of genes persist from tunicate larvae to adult (head without trunk
    3. REJECT
  3. 3. HEMICHORDATES?
    1. FOR
      1. Bilateral symmetry
      2. Motile
      3. Progressive alteration of adult
    2. AGAINST
      1. Forms such as Branchiostoma close to main lineage, but Tunicates are distant
4. ORIGIN
  1. Major unsolved problem
  2. Evolved from ancestral deuterosome
CHORDATES: PHYLUM
1. Sub-phylum: Urochordata
  1. Tunicates (Sea Squirts)
    1. Ascidiaceans
      1. All oceans, all depths
      2. Solitary, social, compound
    2. Larvaceans
      1. Solitary, luminescent, planktonic
      2. Retain larval features, paedomorphosis
    3. Thaliaceans
      1. Include free floating salps
      2. Tropical/subtropical waters
      3. No free swimming larval stage
    4. Mostly sessile and colonial. Larval stage bears chordate characteristics
    5. Settles, releases adhesive, radical metamorphosis. Tail and notochord reabsorbed. Sheds outer layer of cuticle, nervous system degenerates.
2. Sub-phylum: Cephalochordata
  1. Lancelets - Branchiostoma
    1. ~29 species (all marine
    2. up to 5cm long
    3. Larval stages have chordate characteristics
    4. Seperate sexes (dioeceous)
    5. Live buried in sand
    6. Ciliary mucus suspension feeders
      1. Buccal cirri: prevent sediments and other large particles
      2. Hatschek pit: mucus secretion structure near mouth
    7. Dorsal and ventral fin like structures
3. Sub-phylum: Vertebrata
  1. Vertebrates!
    1. See vertebrates mind map

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Hemichordates and Chordates

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