Bio Review Notes #88
Lower chordates

The Chordata include animals with a notochord, a dorsal hollow nerve cord, gill glits, and many embryological similarities linking them with echinoderms. Chordates include tunicates, sea lancets, fishes, amphibians, reptiles, birds, and mammals.

 Notochord: A stiff, flexible rod, forming the body axis. When muscles contract, it allows bending but prevents the body from collapsing like an accordion. In embryos, it induces the nervous system to form above it.

Gill slits: Openings from pharynx to either side, just behind mouth.

Hemichordata: Acorn worms and their relatives. All of them filter feed. Some use gill slits; others use tentacle-like feeding structures. Related to Chordata, but now usually treated as a separate phylum.

Phylum Chordata: Animals with a notochord, a series of gill slits, and a dorsal, hollow nerve cord developing from a neural tube. These traits may occur in larval stages, not always in adults.

Deuterostome characteristics: Embryological similarities shared by chordates, hemichordates, and echinoderms:
  • Radial cleavage: The 8-celled stage has 2 tiers of 4 cells each, with each cell directly below or above another.
  • Indeterminate cleavage: Cells separated in early embryonic stages can develop into an entire embryo.
  • Deuterostome condition: The embryo's blastopore becomes the posterior (tail) end. (Remember that in molluscs, arthropods, and other protostome phyla, the blastopore becomes the mouth.)
Urochordata (tunicates or "sea squirts"): An actively swimming larva with well-developed notochord and nerve cord undergoes metamorphosis into a filter-feeding adult. The adult usually passes large amounts of water through a large gill basket.

Cephalochordata (sea lancets or amphioxus): Small, thin animals that filter feed by passing water through many gill slits. A notochord extends the entire length of the animal, including the head.

Vertebrata (vertebrates): Animals with a vertebral column or backbone that functionally replaces the notochord in adults, and a braincase that encloses and protects the brain. Examples: fishes, amphibians, reptiles, birds, and mammals.


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