animals

Nutritional Mode

• Animals are classified as heterotrophs, meaning they must consume food to obtain nutrients.

• The majority of animals ingest food through their mouths.

• Certain species, such as tapeworms, absorb nutrients directly from their environment without a mouth.

Cell Structure and Specialization

• Animals are multicellular organisms classified as eukaryotes, meaning their cells have a nucleus.

• Unlike plant cells, animal cells do not have cell walls.

• Structural proteins, particularly collagen, are critical for holding animal bodies together.

• Unique to animals, nervous and muscle tissues serve as defining characteristics.

• Tissues consist of groups of similar cells functioning together as a unit.

Reproduction and Development

• Sexual Reproduction: Most animals reproduce sexually, where the diploid stage is predominant.

• Cleavage: Following fertilization, a zygote undergoes cleavage, which is rapid cell division leading to a multicellular structure known as a blastula.

• Gastrulation: The blastula undergoes further development to form a gastrula, consisting of various embryonic tissue layers.

Developmental Stages

• Most animals pass through at least one larval stage, which is morphologically distinct and sexually immature.

• Larvae eventually undergo metamorphosis to transition into juveniles, which resemble adults but are not yet sexually mature.

Body Plans and Symmetry

• Coelom Formation:

  • Coelom is a fluid-filled body cavity found in many triploblastic animals.

  • Animals can be classified based on their body cavity:

    • Coelomates: Have a true coelom derived from mesoderm.

    • Pseudocoelomates: Have a body cavity derived from mesoderm and endoderm.

    • Acoelomates: Lack a body cavity; fully packed with tissue.

• Symmetry Types:

  • Animals are categorized by body symmetry:

    • Radial Symmetry: No distinct left/right or front/back; for example, jellyfish.

    • Bilateral Symmetry: Defined dorsal/ventral and anterior/posterior sides; often more complex organisms like humans.

Tissue Organization

• Tissue Layers:

  • Animals differ in the organization of tissues:

    • Ectoderm: Outer layer, becomes the skin and nervous system.

    • Mesoderm: Middle layer, becomes muscles and most organs.

    • Endoderm: Inner layer, forms the digestive tract.

• Germ Layers and Tissue Diversity:

  • Sponges and similar organisms lack true tissues.

  • Diploblastic animals (e.g., cnidarians) have two layers (ectoderm and endoderm).

  • Triploblastic animals include a mesoderm layer, leading to the development of more complex body plans.

Coelom Types

• A true coelom is present in coelomates, providing compartments for organ systems and development.

• Pseudo-coelom: A body cavity found in pseudocoelomates, serving similar functions but derived from both mesoderm and endoderm.

• Acoelomate: Animals without a body cavity, leading to specialized structures closely surrounding organs.

Developmental Styles

• Protostome vs. Deuterostome Development:

  • Protostome Development: Cleavage is spiral and determinate; the blastopore develops into the mouth.

  • Deuterostome Development: Cleavage is radial and indeterminate; the blastopore becomes the anus, allowing for more flexible early development.

• Coelom Formation Differences:

  • In protostomes, mesoderm solid masses split to form the coelom.

  • In deuterostomes, mesoderm buds from the archenteron wall to form the coelom.

Phylogeny of Animals

• Five key points regarding the relationships among animals:

  1. All animals derive from a common ancestor.

  2. Sponges are basal animals, representing early forms.

  3. Eumetazoa ("true animals") encompasses those with true tissues.

  4. Most animal phyla belong to the Bilateria clade.

  5. Three major clades of bilaterian animals are primarily invertebrates, except for Chordata, which includes vertebrates.

Clades of Bilaterians

• Deuterostomia: Includes echinoderms and chordates, exhibiting diverse forms among both vertebrates and invertebrates.

• Lophotrochozoa: A group of bilaterian invertebrates that possess unique structures like lophophores for feeding or undergo trochophore larva stages.

Summary of Body Plan Characteristics

• Acoela as basal bilaterians exhibits primitive body plans.

• Metazoan Classification: Key characteristics include true tissues, bilateral symmetry, and diversified body plans within three germ layers, leading to complex development.