The lecture focuses on deuterostomes, particularly echinoderms and basal chordates.
Distinction between protostomes and deuterostomes:
Protostomes: First invagination in the embryo develops into the mouth.
Deuterostomes: First invagination becomes the anus; the mouth develops from the second invagination.
Importance of studying echinoderms for understanding chordate development.
Echinoderms
Examples: Starfish, sea urchins, and sea cucumbers.
Sea Cucumber: Elongated body; mouth and anus at opposite ends.
Sea Urchin: Ventral structure.
Starfish: Often mistaken for having five-fold symmetry but actually exhibit bilateral symmetry.
Chordates
Sister group to echinoderms.
Includes basal chordates and vertebrates.
Basal Chordates
Tunicates: Start life as typical chordates but later embed into substrates, resembling sponges.
Amphioxus (Cephalochordate): Small organisms with a head region.
Craniata
Includes hagfish and vertebrates.
Hagfish (Hyperartia):
Lack lower jaw and limbs.
Possess slime glands for defense (similar to the character "Slimer" from Ghostbusters).
Lampreys:
Have eyes and oral discs with teeth.
Evolving fins for improved swimming.
Nathostomata
Vertebrates with jaws (biting mouths).
Basal Group: Chondrichthyes
Includes cartilaginous fish like sharks and rays.
Lack bones; strong supportive structures are unnecessary in aquatic environments.
Fish with Bone
Ray-finned Fish (Actinopterygii):
Common fish such as tuna and salmon.
Includes seahorses.
Lobe-finned Fish (Sarcopterygii):
Ancestors to terrestrial vertebrates.
Includes lungfish and terrestrial vertebrates (mammals, reptiles, birds, amphibians).
Lungfish (Dipnoi)
Three types based on geography: Australia, South America, Africa.
Have muscular limbs; some capable of terrestrial movement.
Possess bone in fins, unlike ray-finned fish.
Evolution to Terrestrial Vertebrates
Terrestrial vertebrates evolved from sarcopterygian ancestors.
Example: Fossil fish from the Devonian period.
Bone structure in fins:
Single large bone connects to the body.
Pair of bones, followed by smaller bones and five digits.
This bone pattern is inherited by modern terrestrial vertebrates.
Conclusion
Evolutionary homology in limb structure can be traced back to these sarcopterygian ancestors, illustrating the continuity from aquatic to terrestrial life forms.