Animalia: The Basal Phyla

Cell Type: Eukaryotic and multicellular (aggregates).
Nutritional Mode: Chemoheterotrophic, deriving sustenance from ingestion.
Cellular Structure: Lacks cell walls; collagen holds cells together. Cell membranes are in direct contact with each other.
Mobility: Exhibits motility, characterized by active locomotion.
Reproductive Norm: Sexual reproduction predominates, whereby gametes arise through meiosis.

Eukarya: Opisthokonta forms the root of animal phylogeny, branching into:
- Choanoflagellates: Closest living relatives to animals.
- Porifera (Sponges): Represent a lineage that does not exhibit true tissue complexity.
- Other Animals: Including diverse lineages stemming from choanoflagellates.
Common Ancestor: An ancestral protist is considered the common ancestor of all animals.

Porifera: Paraphyletic group identified as basal within the animal kingdom, further categorized into:
- Calcarea: Sponges with calcium carbonate spicules.
- Silicea: Sponges with silicon dioxide spicules.
Ctenophora: Also known as comb jellies, described as more complex than sponges.
Cnidaria: Consists of tissues and exhibits more organizational complexity.

Characterization:
- Fundamental group based on the simplest body plan.
- Lacks true tissues or organs; asymmetrical.
- Size ranges from millimeters to meters.
- Cell Specialization:
- Choanocytes: Flagellated cells that facilitate water flow for feeding through phagocytosis.
- Amoebocytes: Motile cells that transport nutrients between cells and aid in digestion.
Feeding Mechanism: Primarily marine suspension feeders, being sessile.

Structure:
- Water is drawn in through the ostia (pores) and expelled through the osculum.
- Internally structured with a spongocoel (the central cavity).
- Makeup includes spicules and mesohyl containing amoeboid cells.
- Examples of specialized cells include choanocytes for capturing food particles via phagocytosis.

Origin: Emerged from colonial protists.
Development: Progressed from a ball formation (blastula) to a gastrula, allowing specialization and differentiation to occur in distinct cell layers (inner and outer).

Characterization:
- Possess a tissue level of organization signifying the onset of true multicellularity.
- Germ Layers:
- Ectoderm: Produces outer covering and forms parts of the nervous system.
- Endoderm: Forms the innermost layer lining the archenteron and gives rise to the digestive tract.

Characteristics:
- Known for being more complex than poriferans with two germ layers (diploblastic).
- Exhibits radial symmetry and a complete gut with both mouth and anus.
- Lifestyle: Marine organisms, feeding on plankton from surface to deep ocean habitat.
- Notable for their tentacles equipped with cilia forming combs.

Characteristics:
- Exhibits diploblastic development with distinct ectoderm (epidermis) and endoderm (gastrodermis).
- Contains specialized feeding cells known as cnidocytes.
- Displays an incomplete gut with only a mouth, facilitating both intra- and extracellular digestion.
- Organisms within this group exhibit radial symmetry and exist in two body forms: polyp and medusa.
- Metagenesis: An alternation of body forms between the polyp and medusa forms.

Medusa and Polyp Forms:
- Medusa: Free-swimming with umbrella-shaped body.
- Polyp: Sessile form, typically attached to substrates.
- Tissue Layers:
- Epidermis: Outer layer.
- Gastrodermis: Inner lining surrounding the gastrovascular cavity.
- Notable Examples:
- Cnidaria Classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (corals).

Hydrozoa: Demonstrates metagenesis, where one phase can be reduced or absent.
Scyphozoa and Cubozoa: Primarily free-swimming jellyfish with effective predatory capabilities and toxic properties.
Anthozoa (Corals):
- Exhibit a sessile lifestyle devoid of a medusa stage; notable for calcium carbonate skeleton formation.
- Thrives in shallow waters and often reliant on symbiotic relationships with dinoflagellate algae for nutrition through photosynthesis.
- Importance of coral reefs in marine ecosystems and challenges posed by global climate change, including coral bleaching phenomena.
- Research Implication: Invasive microbes have been shown to assist corals amidst global warming but may reduce their growth rate.

Key Basal Phyla: Includes Porifera, Ctenophora, and Cnidaria.
Porifera: Exhibits no symmetry, the absence of ontogeny, and lacks a tissue organization; characterized by the presence of choanocytes.
Ctenophora: Features radial symmetry, a complete digestive system, and is diploblastic.
Cnidaria: Also diploblastic with radial symmetry, possessing an incomplete digestive system supported by specialized cnidocytes; classes include Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (corals).