Overview
This lecture covers key characteristics, anatomy, reproduction, and classification of major invertebrate phyla: Porifera (sponges), Cnidaria (jellyfish, corals), and Ctenophora (comb jellies).
Invertebrates: General Overview
- Approximately 1.3 million animal species described, with 95% being invertebrates.
- Most animal species are aquatic due to the abundance of water on Earth.
Porifera (Sponges)
- Porifera means "pore bearing"; sponges are sessile (attached) animals with porous bodies.
- Choanocytes (collar cells) line sponges and aid in feeding by filtering water.
- Sponges lack true tissues and have two loosely associated cell layers separated by mesohyl (gelatinous layer).
- Main anatomical parts: spongocoel (central cavity), osculum (excurrent opening), porocytes (pore cells), epidermis, and choanocytes.
- Spicules provide support, made of calcium carbonate (calcareous) or silica (siliceous); commercial sponges have spongin fibers.
- Most are marine; some freshwater species exist (e.g., Spongilla).
- Sponges are filter feeders, likely evolved from colonial choanoflagellates.
- Amoebocytes (wandering cells) digest and distribute nutrients and form skeletal fibers.
- Most are hermaphrodites (monoecious); reproduce sexually (via amphiblastula larva) and asexually (regeneration).
Cnidaria (Jellyfish, Corals, Anemones)
- Cnidarians have cnidocytes (stinging cells), radial symmetry, and a gastrovascular cavity with a single opening.
- Exhibit diploblastic development (two germ layers: ectoderm and endoderm) with mesoglea (gelatinous layer) in between.
- Main body forms: polyp (sessile, asexual) and medusa (motile, sexual, e.g., jellyfish).
- Nerve net present for simple movement; no brain.
- Two main clades: Medusozoa (jellyfish stages—Hydrozoa, Scyphozoa, Cubozoa) and Anthozoa (only polyps—sea anemones and corals).
Medusozoa
- Hydrozoa: alternation of generations; dominant polyp stage (e.g., Hydra, Obelia, Physalia).
- Hydra: freshwater, solitary, only polyp stage, reproduces sexually and asexually (budding).
- Obelia: colonial, both polyp and medusa, external fertilization.
- Scyphozoa: "true" jellyfish, dominant medusa stage; e.g., Aurelia (moon jellyfish).
- Coastal species have small polyp (scyphistoma) stage; pelagic species lack this stage.
- Reproductive cycle includes planula larva, scyphistoma, strobila, ephyra (immature medusa).
- Cubozoa: box jellies (e.g., Chironex fleckeri), highly venomous.
Anthozoa
- Includes sea anemones and corals; only polyp form, no medusa.
- Corals secrete calcium carbonate exoskeletons (reef-building); corals are at risk due to environmental changes.
Ctenophora (Comb Jellies)
- Diploblastic, marine-only, resemble medusae but are a separate group.
- Move using eight ciliary (comb) rows called "ctenes."
- Use adhesive colloblasts, not stinging cells, to capture prey.
- Have a complete digestive tract (separate mouth and anal pore).
- Examples: genus Cestum (Venus’s Girdle), typical comb jellies (sea walnuts).
Key Terms & Definitions
- Sessile — fixed in place, not moving.
- Choanocyte — flagellated feeding cell (collar cell) in sponges.
- Mesohyl — gelatinous matrix in sponges.
- Spicule — supportive spike in sponges (calcareous or siliceous).
- Hermaphrodite/Monoecious — organism with both male and female reproductive organs.
- Cnidocyte — stinging cell unique to Cnidaria.
- Polyp — sessile, cylindrical Cnidarian body form.
- Medusa — free-swimming, bell-shaped Cnidarian form.
- Planula — ciliated larva of Cnidarians.
- Strobilation — process of asexual reproduction in jellyfish polyp stage.
- Colloblast — adhesive cell on Ctenophore tentacles.
- Ctene — fused cilia forming ciliary "comb" in Ctenophores.
Action Items / Next Steps
- Review lab specimens: Lucosolenia (sponge), Hydra, Obelia, Aurelia.
- Learn genus names and key characteristics for Porifera, Cnidaria, and Ctenophora.
- Prepare for questions on reproductive cycles and anatomy of each phylum.