Phylum Porifera (Sponges) Overview

Phylum Porifera (Sponges)

• Sponges are the most primitive multicellular animals, belonging to Phylum Porifera.
• Porifera means "pore bearers."
• Sponges (around 15,000 species) are mostly marine, with a few freshwater species.
• They are sessile (immobile).
• Their cells lack specialized tissues or organs.
• Most are asymmetrical, some are radially symmetrical.
• Their bodies have many pores (ostia) through which water flows.
• Digestion is intracellular.
• Respiration and excretion occur through simple diffusion.
• The central cavity is called the spongocoel or atrium, which opens to the outside through the osculum.
• The spongocoel is lined with choanocytes (collared flagellated cells).
• Reproduction is both asexual and sexual.

Asexual Reproduction

• External budding: Outgrowth at the base, continuous growth, and detachment to a new surface.
• Regeneration: Fragmentation and regrowth of lost parts.
• Internal budding (gemmules formation in freshwater sponges): Specialized structures containing archaeocytes, secreted in a protective coat, and can survive harsh conditions.

Sexual Reproduction

• Development is indirect, with a larval stage (e.g., amphiblastula larva).
• Fertilization occurs after sperm and egg are released.

Sponge Body Wall Structure

• The body wall is pierced by incurrent openings (ostia), connecting to the central cavity.
• The body wall has a gelatinous material called mesenchyme (mesoglea) containing:
  • Skeletal components (spicules - calcareous or siliceous or spongin fibers).
  • Amoebocytes.
• Outer layer (epidermis): Composed of pinacocytes, which are flattened cells forming the outer layer; and choanocytes (collar cells) lined along the spongocoel, with flagella that create water flow.
• Inner layer (gastrodermis): Contains amoebocytes for intracellular digestion.

Sponge Types

• Ascon type: Simplest type, thin body wall with a central spongocoel. Water flows directly from ostia to spongocoel to osculum.
• Sycon type: Body wall is folded horizontally, creating inhalant and flagellated canals. Water flows through these canals to the spongocoel.
• Leucon type: Body wall is highly folded with many small, rounded flagellated chambers. Water flows through complex canal systems.

Phylum Cnidaria

• Cnidarians are aquatic animals (mostly marine, some freshwater).
• They are solitary or colonial.
• They are sedentary or free-swimming.
• They exhibit radial symmetry.
• Most are carnivores.
• The mouth is surrounded by tentacles with stinging cells (cnidocytes).
• Tentacles help capture food, ingest it, and defend.
• Body wall has two layers: ectoderm and endoderm separated by mesoglea.
• Gastrovascular cavity (coelenteron): Central cavity with a single opening for intake and waste removal.
• Digestion is extracellular and intracellular.
• No respiratory, circulatory, or excretory systems.
• Sense organs include simple eyespots, ocelli, and statocysts.
• Nervous system consists of nerve nets.

Cnidarian Morphology

• Polyp: Sessile, cylindrical body form.
• Medusa: Free-swimming, bell-shaped body form.
• Reproduction is both asexual (budding and regeneration) and sexual (gametes).
• Life cycle often includes metagenesis (alternation of generations) between polyp and medusa stages.

Cnidarian Locomotion

• Various methods exist to achieve movement, including looping, somersaulting, climbing, inverting, and floating.

Polymorphism

• Polymorphism in Cnidarians refers to the occurrence of multiple morphological types within a single species, like the polyp and medusa.
• Polyp stage is often associated with feeding, attachment, and asexual reproduction.
• Medusa stage is associated with sexual reproduction.