Recording-2025-03-25T17:46:59.797Z

Overview of Animal Diversity

• All animals are recognized as part of a former monophyletic group.

• Evidence includes molecular data supporting common ancestry.

• Multicellularity evolved in the ancestral lineage of animals.

Basal Groups

• The simplest animal forms include basal lineages such as sponges, comb jellies, and cnidarians.

• Basal lineages form a smaller part of overall animal diversity, with bilaterians making up the larger portion

Bilateral Symmetry

• Defined as an organism being identical on both left and right sides when cut through the center; e.g., human body.

• Notable exceptions in adult morphology include jellyfish, which exhibit radial symmetry during their adult stage but show bilateral symmetry in larval stages.

Classification of Animals

• Recognized animal phyla range between 30-35 among taxonomists, focusing particularly on three basal groups and those in the bilaterian category.

• Major subgroups of bilaterians include:

  • Protostomes: First mouth animals

  • Deuterostomes: Second mouth animals

Protostomes vs. Deuterostomes

• Protostomes split into:

  • Lophotrochozoans: Includes mollusks (e.g., cephalopods, snails) and annelids (e.g., segmented worms)

  • Ecdysozoans: Includes arthropods (insects, crustaceans) and nematodes (roundworms)

• Deuterostomes are smaller than protostomes and include phyla like chordates (vertebrates, mammals)

Animal Characteristics

• Unique traits of animals include multicellularity, specialized cells, and collagen (type IV), which forms the basal lamina beneath epithelial cells.

• Early forms of animals showed evolution of contractile elements (myofibrils) but did not initially have true muscles.

• Animals share a reproductive characteristic of large eggs produced through meiosis, retaining nutrients while discarding polar bodies.

The Ancestral Lineage

• Evidence suggests that sponges are the most basal animal group, exhibiting characteristics that link them with choanoflagellates (unicellular ancestors to animals).

• Feeding cells in sponges (choanocytes) resemble choanoflagellates, indicating a common feeding strategy of generating water currents to capture organic particles.

Cellular Structures

• Tight junctions and septate junctions developed in early animal evolution for cellular adherence and communication, allowing for complex multicellular structures.

• Adherence junctions and desmosomes appear later in vertebrate evolution, allowing more specialized cell interactions.

Evolution of Tissue Layers

• Gastrulation marks the formation of tissue layers, either as an ancestral trait or as an evolutionary development in the metazoan lineage, excluding sponges to include all other animals.