Lecture 14: Chordates

Phylum Chordata

1. Notochord: rod that supports the body

2. Dorsal Hollow Nerve Cord: develops into the brain & spinal cord

3. Pharyngeal grooves: openings near throat area (fish=gills)

4. Endostyle / Thyroid gland: trap / maintain food

5. Post-anal tail

Dorsal hollow nerve cord characteristics

• dorsal to the GI tract

• anterior end enlarged to form brain

• in vertebrates: nerve cord passed through protective neural arches, brain enclosed by protective bone

Pharyngeal slits, grooves & pouches

slits = openings that lead from pharyngeal cavity to outside (gills)

grooves = inpocketing of ectoderm

• amniotes (birds)

pouches = outpocketing of endodermal lining of pharynx

Subphylum Urochordata

• invertebrates

• Tunicata

  • 90% in class Ascidiacea (sea squirts)

• body covered by protective test ( tunic )

• adults are sessile

• filter feeders w/ incurrent & excurrent siphons

Subphylums of Phylum Chordata

Invertebrates:

• Urochordata = tunicates

• Cephalochordata = lancelets

Vertebrata = fish, amphibians, reptiles, birds, & mammals

how does the endostyle work ?

1. secretes continuous sheet of mucus around the inside of pharynx (mucus traps food particles before they exit stigmata)

2. The mucus w/ food is rolled into rope & sent to esophagus

Cephalochordates

Lancelets

• no chambered heart

• water enters mouth, passes pharyngeal slits & exits through atriopore

• food trapped in mucus → intestine

Garstang’s hypothesis

Chordates evolved from a larval stage of tunicate-like animals (like sea squirts) that stayed in larval form instead of transforming into adults = Paedomorphosis

• Neoteny: body grows slower than normal, reproductive age reached before becoming adult, stays in larva form even when mature

• Progenesis: reproductive organs mature early, larva stops growing & never becomes an adult; can reproduce.