Lecture: Invertebrates/Vertebrates 4/16/26

Definition and Comparison:
  - Open Circulatory System:
    - Characteristics:
      - Present in arthropods (e.g., insects, arachnids, and crustaceans).
      - Consists of a heart but lacks blood vessels.
      - Hemolymph (the fluid equivalent to blood) bathes the organs directly in the body cavity.
  - Closed Circulatory System:
    - Present in vertebrates and some invertebrates.
    - Characteristics:
      - Comprises a heart and a closed network of blood vessels to circulate blood.
      - Blood is always contained within the vessels, allowing for more efficient transport of gases and nutrients.
Example Quiz Question:
  - What type of circulatory system do arthropods have?
    - Answer: Open circulatory system.

Subphylum Categories:
  - Hexapoda (insects).
  - Myriapoda (centipedes and millipedes).
  - Crustacea (crabs, lobsters, shrimps).
Characteristics of Insects (Hexapoda):
- Number of Legs: Six.
Identification of Myriapods:
  - Centipedes:
    - Less paired legs per segment (one pair).
    - Typically more toxic.
  - Millipedes:
    - Two pairs of legs per segment.
    - Usually safe and non-toxic.

Vertebrates:
  - Defined by the presence of a backbone.
  - Two main subtypes:
    - Chordates (vertebrates with a backbone).
    - Invertebrates (lack a backbone).
Early Evolution:
- Cambrian period around 530 million years ago marked the emergence of diverse life forms in the ocean.
- Vertebrates represent one of the most successful groups of animals.
Discussion:
  - Contrast between Chordates and Arthropods:
    - Both are successful; however, chordates exhibit more complexity due to the presence of a vertebral column (backbone).

Defining Features:
  - Bilaterians:
    - Organisms exhibit bilateral symmetry.
  - Deuterostomes:
    - Development processes where the mouth forms second (after the anus).
Genetic and Evolutionary Significance:
  - Phylum Chordata is home to species that display high levels of complexity and adaptability.
  - Approximately 52,000 species of vertebrates exist today.

Characteristics to Know:
  1. Presence of a notochord during embryonic development.
  2. Dorsal hollow nerve cord, which develops into the central nervous system (CNS).
     - Example Organs: Brain and spinal cord.
  3. Pharyngeal slits, which may develop into gills in aquatic species or evolve into throat structures in terrestrial species.
  4. Post-anal tail, present during embryological development, may not be retained in adults (e.g., humans vs. cats).
Notochord:
  - Longitudinal flexible rod that provides skeletal support during development, located between the digestive system and nerve cord.
  - In most vertebrates, the notochord is reabsorbed and replaced by the vertebral column.

Types of Germ Layers:
  - Ectoderm:
    - Develops into the outer covering of the organism (skin) and the central nervous system.
  - Mesoderm:
    - Develops into the body's muscles.
  - Endoderm:
    - Forms the digestive tract and organs associated with it.
Complete Digestive System:
- Characterized by a one-way gut system where food enters through the mouth and waste exits through the anus.
Symmetry:
- Majority of chordates exhibit bilateral symmetry.

Testable Characteristics of Chordates:
  - All chordates possess a notochord at some stage during development.
   - The distinction of nerve cords is crucial for understanding the central nervous system's development.
  - Pharyngeal slits can be transformed into specific respiratory structures in different species.
  - Importance of the post-anal tail during embryonic growth stages.
Comparative Anatomy:
- Phylogenetic relationships among chordates identified through embryological similarities.
Review for quizzes and lab exams emphasizing these characteristics thoroughly.

Further discussions on vertebrate evolution and classification will follow.
Next class will involve practical applications related to dissections and comparative anatomy.