Chapter 2: Invertebrates: Porifera, Cnidaria, and Worms

Invertebrates — animals lacking a backbone — account for more than 95% of all known animal species. Among the most ancient are the sponges, cnidarians, and various worm phyla, which represent early evolutionary experiments in multicellular body plans. Studying them reveals the stepwise acquisition of tissues, body cavities, and bilateral symmetry.

Porifera: The Sponges

Sponges (Phylum Porifera) are the simplest multicellular animals. They lack true tissues and organs but possess specialized cells called choanocytes (collar cells) that beat flagella to draw water through pores (ostia) and expel it through a central opening (osculum).

Key features:

• Sessile (non-moving) as adults
• Skeleton of spicules (silica or calcium carbonate) or spongin fibers
• Capable of reaggregation — dissociated cells can reassemble into a functional sponge
• Reproduction: both sexual (larvae) and asexual (budding, fragmentation)

Cnidaria: Jellyfish, Corals, and Anemones

Phylum Cnidaria introduced true tissues and radial symmetry. The defining feature is the cnidocyte — a specialized stinging cell containing a nematocyst that fires a harpoon-like thread to immobilize prey.

Polyp vs. Medusa

Form	Shape	Movement	Example
Polyp	Cylindrical, attached	Sessile	Sea anemone
Medusa	Bell-shaped, free-swimming	Active	Jellyfish

Many cnidarians alternate between these forms (Aurelia aurita — moon jellyfish — goes through both stages). Corals exist primarily as colonial polyps, secreting calcium carbonate skeletons that build reefs.

Worms: Three Major Phyla

Platyhelminthes (Flatworms)

• Acoelomate (no body cavity), bilaterally symmetrical
• Includes free-living Planaria and parasitic tapeworms (Taenia) and flukes (Fasciola)
• Planarians can regenerate from tiny fragments — a famous model organism for regeneration research

Nematoda (Roundworms)

• Pseudocoelomate (fluid-filled cavity not lined by mesoderm)
• Most abundant animals on Earth — a single rotting apple may contain millions
• Caenorhabditis elegans is a landmark model organism (first animal with fully mapped connectome)
• Many are plant or animal parasites (Ascaris, hookworms)

Annelida (Segmented Worms)

Annelids introduced metameric segmentation — a body divided into repeated functional units — and a true coelom (fluid-filled cavity lined by mesoderm).

Earthworm (Lumbricus terrestris) anatomy highlights:

• Setae: bristles aiding locomotion
• Clitellum: reproductive band producing cocoons
• Closed circulatory system: blood moves through dorsal and ventral vessels
• Nephridia: excretory organs in each segment

Phylum	Body Cavity	Symmetry	Segmentation
Platyhelminthes	None (acoelomate)	Bilateral	No
Nematoda	Pseudocoelom	Bilateral	No
Annelida	True coelom	Bilateral	Yes

Key Checklist

• I can describe the filter-feeding mechanism of sponges using the terms choanocyte, ostia, and osculum
• I can distinguish polyp and medusa body forms in cnidarians and give examples of each
• I can compare the three worm phyla by body cavity type, segmentation, and representative species