Zoology: Chapter 2 — Non-Bilaterians: Porifera & Cnidaria

2.1 Phylum Porifera: The Parazoan Grade #

Sponges (Porifera) represent the most basal extant metazoan lineage, lacking true embryonic germ layers, true tissues, basement membranes, and nervous systems (cellular grade of organization). The sponge body is built around an intricate aquiferous water canal system that circulates immense volumes of water through microscopic incurrent pores (ostia) into an internal cavity (spongocoel), venting out a massive excurrent opening: the osculum.

2.1.1 Cellular Differentiation in the Mesohyl #

The sponge wall consists of two cellular layers sandwiching an extracellular gelatinous protein matrix called the mesohyl:

1. Pinacocytes: Flattened epithelial-like cells forming the outer dermal boundary (pinacoderm); capable of slow contraction to regulate body diameter.

2. Choanocytes (Collar Cells): Flagellated cells lining internal chambers, showing striking homology to unicellular choanoflagellates. The choanocyte features a central flagellum surrounded by a cylindrical collar of interconnected actin-supported microvilli. Undulation of the flagellum creates negative hydrodynamic pressure, driving water across the collar microvilli. Microvilli trap microscopic picoplankton and bacteria (0.2–2.0 µm) via mucopolysaccharide adhesion, ingesting particles by phagocytosis.

3. Archaeocytes (Amoebocytes): Pluripotent stem cells wandering through the mesohyl. They receive phagocytosed food vacuoles from choanocytes, digest nutrients intracellularly, differentiate into all other cell types (sclerocytes, spongocytes, collencytes), and yield gametes.

4. Sclerocytes: Mesohyl cells that secrete the inorganic mineral endoskeleton: crystalline spicules composed of calcium carbonate (CaCO₃, Calcarea) or hydrated amorphous silica (SiO₂, Hexactinellida and Demospongiae).

2.1.2 Canal Architectures: The Shift from Asconoid to Leuconoid #

To overcome the physical limitation of surface-area-to-volume ratio as body size expands, sponges evolved increasingly folded aquiferous designs:

• Asconoid (Simplest): Tubelike body; choanocytes line the entire central spongocoel. Size is strictly limited (<1–2 mm) because spongocoel water volume increases with the cube of the radius (r³), while choanocyte pumping surface area only increases with the square (r²).
• Syconoid: Body wall folds radially into horizontal flagellated canals; choanocytes are restricted to radial canals, while the central spongocoel is lined by pinacocytes.
• Leuconoid (Most Complex, 95% of living sponges): Body wall is extensively folded into thousands of microscopic, spherical flagellated chambers linked by a branched network of incurrent and excurrent canals. The spongocoel is lost or reduced to excurrent ducts. Leuconoid design achieves vast surface area and massive pumping efficiency (a 10-cm sponge can filter >1,000 liters of water daily).

   ASCONOID                   SYCONOID                       LEUCONOID
(Tubular spongocoel)    (Radial canal pockets)       (Branched chamber network)
  Ostia ──> Spongocoel    Incurrent ──> Radial Canal    Subdermal ──> Microscopic
         │                        │                            Chambers ──> Excurrent
         ▼                        ▼                                      │
      Osculum                  Osculum                                   ▼
                                                                      Oscula

2.2 Phylum Cnidaria: Tissue-Level Diploblasts #

Cnidarians (corals, sea anemones, jellyfish, hydroids) represent the earliest true Eumetazoa:

• Tissue Grade Organization: Possess true intercellular junctions (desmosomes) and a basal lamina separating two epithelial layers: outer epidermis and inner gastrodermis.
• Gastrovascular Cavity (Coelenteron): A blind, sac-like digestive space possessing a single opening acting as both mouth and anus. Digestion is two-phased: initial extracellular digestion within the cavity via gastrodermal enzyme secretion, followed by phagocytosis and intracellular digestion.
• Diffuse Nerve Net: Non-myelinated, non-polarized neurites forming an interconnected subepidermal nerve net. Synapses conduct action potentials bidirectionally across electrical and chemical gaps.
• Epitheliomuscular Cells: Epidermal cells featuring basal longitudinal myofibrils; gastrodermal nutritive-muscular cells contain circular myofibrils. Working antagonistically against the incompressible fluid of the coelenteron, they establish a versatile hydrostatic skeleton.

2.2.3 Cnidocyte Architecture and Nematocyst Discharge #

The synapomorphy of Cnidaria is the cnidocyte—a specialized stinging cell concentrated along tentacles:

🔬 Key Illustrated Scientific Figure

Full-Screen Zoom

Click to enlarge on this page

Inside the cnidocyte sits an extraordinary intracellular organelle: the nematocyst. It consists of a protein capsule sealed by a lid (operculum), containing an inverted, coiled, barbed hollow tubule bathed in a high concentration (>2 M) of amino acids, peptides, and poly-gamma-glutamate bound with calcium cations (Ca²⁺).

🔬 Key Illustrated Scientific Figure

Full-Screen Zoom

Click to enlarge on this page

The Discharge Biophysics:

1. Mechanical triggering of the sensory cnidocil (coupled with chemical detection of prey mucus) activates calcium channels.

2. Influx of free water causes instantaneous osmotic pressure to spike to over 150 atmospheres (15 MPa) within the capsule.

3. The operculum ruptures, and the barbed hollow tubule evaginates explosively inside-out at accelerations reaching 40,000× g, penetrating prey cuticle in less than 700 nanoseconds.

4. Neurotoxins (pore-forming cytolysins and ion-channel blockers) are injected through the hollow shaft, inducing instant paralysis.

2.3 Metagenesis: Polyp and Medusa Dimorphism #

Cnidarians display two basic morphological body forms:

• Polyp: Cylindrical, sessile body; mouth and tentacles directed upward; asexual vegetative reproduction.
• Medusa: Bell- or umbrella-shaped, motile planktonic body; mouth and manubrium directed downward; sexual gametic reproduction.

In the class Hydrozoa (exemplified by Obelia), these phases alternate in a life cycle termed metagenesis (distinct from plant alternation of generations because both phases are diploid, 2n):

1. The sessile colonial hydroid consists of feeding polyps (gastrozooids) and reproductive polyps (gonozooids).

2. Gonozooids asexually bud off microscopic, free-swimming medusae via strobilation.

3. Medusae produce haploid eggs and sperm through meiosis; external fertilization yields a ciliated, free-swimming planula larva.

4. The planula settles on the benthos and metamorphoses into a new founder polyp, establishing a colonial hydroid via asexual budding.

2.4 Key Takeaways & Exam High-Yield Summary #

• Sponges have no tissues: Choanocytes maintain water flow; archaeocytes distribute nutrients and differentiate into gametes.
• Nematocyst Discharge: One of the fastest biomechanical events in biology (>40,000× g), driven by osmotic release and water influx.
• Metagenesis is not Alternation of Generations: In Obelia, both the polyp and the medusa are diploid (2n). Meiosis occurs exclusively during gamete formation in the medusa.
• Siphonoglyph: Ciliated groove in the pharynx of Anthozoans that circulates water into the gastrovascular cavity even when the mouth is closed.

2.5 Review & Self-Assessment Questions #

1. Why cannot a large sponge body be constructed with an asconoid aquiferous architecture?

• Answer: As volume increases cubically (r³), the spongocoel surface area lined by choanocytes increases only quadratically (r²). In a large asconoid sponge, choanocytes would be physically incapable of moving the stagnant volume of water inside the cavernous spongocoel, leading to metabolic hypoxia and starvation.

2. Explain the structural difference between the hydrozoan velum and the cubozoan velarium.

• Answer: The hydrozoan velum is a true inward muscular shelf containing ectodermal circular muscles that narrows the bell aperture to increase jet propulsion efficiency. The cubozoan velarium is an analogous structure formed by an internal fold of the bell margin containing branched gastrovascular diverticula.