Chapter_32_Halo

Key Characteristics of Animals

• Animals are heterotrophs, meaning:

  • They obtain energy and nutrients by consuming other organisms.

  • They form tissues from layers of embryonic cells, including:

    • Mouth and anus structures for food processing.

    • Specialized muscle and nerve cells for movement and sensory detection.

Nutritional Modes

• Animals ingest food and digest it internally, in contrast to fungi, which digest food externally.

• Animals are categorized as heterotrophic eukaryotes (organisms that obtain food by ingestion).

Cellular Structure

• Animals are multicellular eukaryotes.

• Structural support is provided by proteins (like collagen) rather than cell walls.

• Unique nerve and muscle tissues define animal characteristics, and tissues are collections of similar cells functioning as a unit.

Reproduction and Development

• Most animals reproduce sexually, with diploid stages dominating their life cycles.

• Key developmental stages include:

  • Cleavage: Successive cell divisions without growth.

  • Formation of a blastula: A hollow ball of cells.

  • Gastrulation: Leads to the formation of a gastrula with distinct tissue layers.

Larval Stages

• Many animals have at least one larval stage, which:

  • Is sexually immature.

  • Differs morphologically and behaviorally from adults.

• After metamorphosis, larvae develop into juveniles resembling adults but are still immature.

Body Plans and Symmetry

• Animals can be characterized by their body plans, which consist of morphological and developmental traits.

• Symmetry types:

  • Radial Symmetry: Body parts arranged around a central axis (e.g., jellyfish). Typically sessile or planktonic.

  • Bilateral Symmetry: Body parts are arranged around two axes, leading to a defined left and right side. Often associated with active movement.

Tissue Organization

• Body plans may also vary according to tissue organization:

  • Diploblastic animals: Have only ectoderm and endoderm (e.g., cnidarians).

  • Triploblastic animals: Possess an additional mesoderm layer, resulting in more complex tissue and organ development.

Body Cavities

• Most triploblastic animals have a body cavity (coelom) for various functions:

  • Cushions internal organs.

  • Provides a hydrostatic skeleton for muscle action.

  • Allows for the independent movement of internal organs.

• Types of Body Cavities:

  • Coelom: A cavity surrounded by mesoderm tissue (e.g., earthworms).

  • Hemocoel: Body cavity filled with hemolymph, found in some organisms (e.g., arthropods).

  • Acoelomate: Animals without a body cavity, typically flat-bodied (e.g., flatworms).

Developmental Modes

• Development can be categorized into Protostome and Deuterostome:

  • Protostome Development:

    • Characterized by spiral and determinate cleavage.

    • Blastopore forms mouth.

    • Coelom formed by splitting masses of mesoderm during gastrulation.

  • Deuterostome Development:

    • Characterized by radial and indeterminate cleavage.

    • Blastopore forms anus.

    • Coelom formed by mesoderm buds from the archenteron.

Fate of the Blastopore

• The blastopore leads to the development of the archenteron and ultimately forms the digestive system:

  • In protostomes, the blastopore becomes the mouth.

  • In deuterostomes, the blastopore becomes the anus.

Evolutionary Relationships Among Animals

• Animal phylogeny is inferred from various sources such as:

  • Genomic data, morphological traits, rRNA genes, Hox genes, and protein-coding genes.

• Key points about living animals' relationships:

  1. All animals share a common ancestor.

  2. Sponges are the most basal animals.

  3. Eumetazoa includes all animals with tissues (excluding sponges).

  4. Most phyla belong to Bilateria.

  5. Three major clades of bilaterians are Deuterostomia, Ecdysozoa, and Lophotrochozoa.

Major Clades

• Deuterostomia: Includes hemichordates, echinoderms, and chordates (vertebrates).

• Ecdysozoa: Invertebrates that secrete exoskeletons and undergo ecdysis (e.g., nematodes and arthropods).

• Lophotrochozoa: Features such as lophophores and trochophore larvae characterize some members (e.g., molluscs, annelids).

Additional Notes

• The diversification of animals points to a complex evolutionary history, incorporating both morphological and genetic data to provide insights into the vast diversity found within the animal kingdom.

Learning objectives

1. Comparison of Radial and Bilateral Symmetry:

   • Radial Symmetry: Body parts are arranged around a central axis. Imagine a jellyfish; if you cut it in any direction through the center, the halves won't match (non-identical).

   • Bilateral Symmetry: Body can be divided into two mirrored halves along one plane (like a butterfly). A clear left and right side; when cut down the middle, both halves are the same (identical).

2. Three Main Tissue Layers:

   • Ectoderm: The outer layer, which develops into the skin and nervous system tissues.

   • Mesoderm: Middle layer that forms muscles, circulatory system, and the gut tract.

   • Endoderm: The innermost layer that develops into the digestive tract lining and organs.

3. Diploblastic vs Triploblastic Organization:

   • Diploblastic: Animals that have two tissue layers (ectoderm and endoderm) but lack mesoderm (e.g., cnidarians).

   • Triploblastic: Animals that possess three tissue layers (ectoderm, mesoderm, and endoderm) leading to more complex structures (e.g., arthropods, mammals).

4. Types of Body Cavities:

   • Coelom: A body cavity completely lined by mesoderm; provides cushioning for organs and a hydrostatic skeleton. Example: Earthworms.

   • Hemocoel: A body cavity filled with hemolymph (blood-like fluid), seen in some invertebrates like arthropods.

   • Acoelomate: Animals without a body cavity (e.g., flatworms) that are flat-bodied and have solid tissue.

5. Protostome vs Deuterostome Development:

   • Cleavage Patterns:

     • Protostome: Spiral and determinate cleavage (cell fates are set early).

     • Deuterostome: Radial and indeterminate cleavage (cell fates are not set early).

   • Coelom Formation:

     • Protostome: Coelom forms by splitting masses of mesoderm (schizocoely).

     • Deuterostome: Coelom forms from mesoderm buds off from the archenteron (enterocoely).

   • Fate of the Blastopore:

     • In Protostomes, the blastopore becomes the mouth.

     • In Deuterostomes, the blastopore develops into the anus.