Origin of chordates

Deuterostome:
1. anus develops from blastophore
2. radial cleavage
3. indeterminate cells
4. anus formed first then mouth second

Hemichordata:
1. worm-like benthic dwellers
2. marine
3. body plans

  • solitary species (acorn worms Enteropneusta)
  • filter feeding colonial species (Pterobranchia)

What makes them not a chordate?:
1. have a dorsal nerve cord
2. gill slits


Only have 2 out of 5 chordate features

Phylum Chordata:
1. highly diverse and successful phylum
2. includes lampreys, fish, amphibians, reptiles, birds & mammals

  1. most complicated mechanisms of self maintenance than any other group in the animal kingdom
  2. structural body plans retains many features of more advanced invertebrates

5 features of Chordates:
1. notochord
2. dorsal nerve chord
3. pharyngeal gill slits
4. post-anal tail
5. endostyle

Notochord:
1. skeletal rod for body support
2. extends length of body
3. axis for muscle attachment
4. stiff longitudinally
5. flexible from side to side
6. located on dorsal side of gut

Dorsal nerve cord:
1. tubular (hollow)
2. located on dorsal side of notochord
3. anterior end becomes enlarged to form the brain

Pharyngeal gill slits:
1. perforations leading to pharynx to outside
2. paired on either side
3. originated as adaptations for filter feeding
4. later developed as primary respiratory organ (i.e. gills in fish) or as other body structures

Endostyle:
1. groove in the pharynx floor
2. ciliated
3. secretes mucous
4. moves food to stomach
5. in humans, becomes the thyroid
6. lampreys have both endostyle and thyroid

Post-anal tail:
1. tail structure after end of digestive tract
2. part of skeletal support, muscles and nervous system
3. aids locomotion/agility
4. balance

Other features:
1. bilaterally symmetrical
2. segmented muscles
3. closed circulatory system with ventral heart
4. pronounced cephalisation
5. complete digestive system

Terminology:
1. Chordate: notochord at some stage in life
2. Protochordata / acraniata: no cranium
3. Craniata: has a cranium
4. Vertebrata: has (or had) vertebrae
5. Agnatha: craniata without jaws
6. Gnathostomata: craniata with jaws
7. Pisces: jawed vertebrate with appendages in the form of fins
8. Tetrapoda: jawed vertebrate with 4 limbs
9. Amniota: vertebrates whose embryos develop in fluid-filled sac

Acraniate chordates (Protochordates):

Urochordata (sea squirts):
1. aquatic (~2000 species)
2. microscopic to several centimetres
3. non-living "tunic" surrounds animal
4. urochordata ('tail chordates')
5. larvae mobile, adults usually sessile
6. larvae have notochord, hollow dorsal b=nerve cord, pharyngeal gill slits & post-anal tail
7. adults retain only endostyle and pharyngeal gill slits


Sessile filter feeders:
1. beating of cilia 'sucls'water into siphon
2. water passes into ciliated pharynx
3. endostyle runs along side pharynx
4. produces mucous to trap food particles
5. food moved by cilia to stomach for digestion
6. indigestible waste out through anus with excess water

Reproduction:
1. hermaphroditic
2. each animal contains a single ovary and single testis
3. germ cells carried into atrial cavity where fertilisation occurs

3 Orders:

  • Doliloda: 1-2 cm long, siphon at either end, 8-9 strands of circular muscles and sexual and asexual reproduction
  • Salpida: solitary or colonial, oozoid is asexual and makes chain 10's -100's, blastozooid (colony) reproduces sexually and one of the fastest growing multicellular animals
  • Pyrosomida

Cephalochordata:
1. elongate & laterally compressed
2. <10 cm in length
3. aquatic (shallow costal waters)
4. free swimming, but benthic dwellers
5. notochord & nerve cord along entire body which persist through life
6. use gill slits for filter feeding
7. some cephalisation, but not well-developed

Filter feeding:
1. water enter mouth driven by cilia
2. food particles trapped in pharyngeal slits in mucous (from endostyle)
3. then cilia move food into gut
4. filtered water passes into atrium and then exits via atriopore
5. closed circulatory system

Reproduction:
1. dioecious
2. gametes released by atrium
3. external fertilisation

Craniata / Vertebrata (fish, amphibians, reptiles, birds and mammals):
1. large & highly successful sybphyla (~570000 species)
2. notochord, dorsal nerve cord, pharyngeal gill slits & post-anal tail present at some stage
3. notochord usually replaced by spinal column of vertebrae in adults
4. vertebrae surround dorsal nerve cord
5. living endoskeleton of bone / cartilage
6. complex muscle system

Features:
1. distinct head with highly advanced sense organs (eyes, nose, ears)
2. usually two pairs of joimted appendages
3. elaborate nervouse system for sensory, motos & organ function
4. efficient respiratory system consisting of gills, moist skin or lungs
5. efficient closed circulatory system with heart (2-4 chambers) and blood containing haemoglobin

Are hagfish vertebrates?:
1. lack all traces of vertebrate
2. lack the pineal eye and eye muscles
3. no Mauthner neurons. in brain stem
4. heart pumpong is intrinsic with no coordination
5. blood system has a few immune reactions done by vertebrates
6. no spleen
7. different arrangement of cranial nerves
8. no renal collecting ducts in kidney
9. no osmoregulation


They are VERTEBRATES due to genetics. Lampreys and hagfish are more closely related to each other than either is to the jaws fishes.