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Insects and Cephalopods
- Evolved features with similar purposes as chordates.
- Structure Comparison: Notochord's role in various species.
Notochord Functionality
- Acts like a backbone, providing:
- Stiff base and structure.
- Facilitates movement.
- Evolved characteristics in organisms:
- Insects and cephalopods do not possess a notochord but have:
  - Exoskeleton (Insects)
  - Internal shell (Cephalopods)
- In humans, it evolves into the spinal disc.

Four Derived Characteristics of Chordates
1. Notochord
- Functions as a skeletal structure during development.
1. Post-anal Tail
- An extension past the anus; develops in humans but regresses during gestation.
1. Dorsal Hollow Nerve Cord
- Forms a thickening of the ectoderm; becomes the spinal cord in humans.
1. Pharyngeal Slits
- Openings in the pharynx; serve as a filter-feeding organ in aquatic organisms.

Functionality of Pharyngeal Slits
- Enable water to flow in and then be squeezed out.
- Facilitates oxygen exchange across gills, supporting filter-feeding behavior in fish:
- Allows ingestion of water with phytoplankton & dissolved oxygen without swallowing it.
Developmental Outcomes of Arches
- First Arch: Becomes the jaw.
- Second Arch: Becomes the hyoid bone, part of the mandibular arch.
- Function: Supports eating, swallowing, and breathing.

Why are Hemichordates Considered a Link?
- Acorn worms display:
- Post-anal tail.
- Pharyngeal slits.
- Lack a true notochord, possessing instead a rigid array of cells.
- Contain both dorsal hollow nerve and ventral solid nerve cords.

Tunicates as Chordates
- Considered chordates due to previous mobility in juvenile stage (neotony).
- Urochordata identified as the basal group in chordate evolution.

Jaws developed from the first two pharyngeal arches.
Evidence:
- Agnatha (jawless fish) have nine pharyngeal slits.
- Gnathostomes (jawed fish) have seven pharyngeal slits.

Dunpheostis's expansion limited by food scarcity leading to reproduction challenges and competition with smaller fish.

Function: Detects water movements and nearby organisms.
Adaptations in sharks and rays involve ampullae of Lorenzini for sensing electrical currents in the water.

Lung Evolution: Originated as buoyancy organs in fish.
- In lungfish (Diphyidae): Air sacs evolve into esophagus connected to lungs.
- In ray-finned fish: Evolved swim bladders for gas exchange, helping to forage at greater depths.

Adaptive Value of the Amniotic Egg: Provides a secure environment allowing for development beyond aquatic dependencies.
Individual Sacs:
1. Amnion: Protects embryo and prevents desiccation.
2. Allantois: Serves as a waste disposal and respiratory functions.
3. Chorion: Allows gas exchange and nutrient transportation.

Indicator Species Definition: Species that indicate environmental conditions.
Amphibians as Indicators:
- Sensitive to toxins, their mortality signals pollution levels, e.g. frogs signaling poor water quality.

Duplication of Hox Genes: Increases structural and genetic complexity, facilitating the development of new features and morphological diversity.
Hox Genes: Control body growth and determination of cell specialization.

Categories of Warm Blooded and Cold Blooded:
- These terms are versatile and often oversimplify species classifications.
Defined Terms:
- Endotherms: Generate internal body heat (e.g., mammals).
- Ectotherms: Rely on external heat sources (e.g., reptiles).
- Homeotherms: Maintain stable internal temperature (e.g., mammals).
- Poikilotherms: Body temperature varies with the environment.

Colonization Features:
- Fur: Provides insulation.
- Milk: Supports offspring development.

Definition: Structure connecting the mother and embryo.
Connection to Amniotic Egg: Provides nutrients similar to yolk in eggs but allows for gradual nutrient transfer.
Adaptive Value: Offers stability and portability during development, unlike fixed egg structures in reptiles.