Invertebrates

1. Basal Animals: Sponges and Cnidarians

Sponges (phylum Porifera) are the simplest animals — sessile filter feeders that lack true tissues. Water enters through pores, is drawn in by flagellated choanocytes, and exits through a large opening, the osculum; food is trapped and digested by cells. Support comes from spiny spicules and the flexible protein spongin. Cnidarians (jellyfish, sea anemones, corals, hydras) are diploblastic, radially symmetrical animals with true tissues. Their defining feature is the cnidocyte, a stinging cell holding a harpoon-like nematocyst used to capture prey. They have an incomplete gut — a single-opening gastrovascular cavity — and two body forms: the sessile polyp and the free-swimming medusa. Classes include Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa.

2. Lophotrochozoa: Flatworms, Mollusks, and Annelids

Lophotrochozoans are protostomes that possess a feeding structure (lophophore) or a trochophore larval stage. Flatworms (Platyhelminthes) are triploblastic, bilaterally symmetrical acoelomates with an incomplete gut. Free-living planarians (Turbellaria) regenerate remarkably; parasitic flukes (Trematoda) and tapeworms (Cestoda) cause human disease. Mollusks (Mollusca) are soft-bodied coelomates with a muscular foot, a visceral mass, a mantle that secretes a shell (in most groups), and a rasping radula. Classes include Gastropoda (snails, slugs), Bivalvia (clams, oysters, mussels), Cephalopoda (octopuses, squid — with complex brains and camera-like eyes), and Polyplacophora (chitons). Annelids (Annelida) are segmented coelomates with bristle-like setae; classes include Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches).

3. Ecdysozoa and Deuterostomia: Nematodes, Arthropods, and Echinoderms

Ecdysozoans grow by molting (ecdysis), shedding a tough outer cuticle. Roundworms (Nematoda) are abundant pseudocoelomates with a protective cuticle and a complete digestive tract (mouth and anus); many are free-living, others are parasites (hookworms, pinworms, the cause of elephantiasis). Arthropods (Arthropoda) are the largest phylum, defined by jointed appendages, a segmented body, and a hard chitinous exoskeleton that must be molted to grow. They have an open circulatory system in which hemolymph bathes tissues in a hemocoel. Subphyla include Chelicerata (spiders, scorpions), Myriapoda (centipedes, millipedes), Crustacea (crabs, shrimp), and insects (Hexapods). Echinoderms (Echinodermata) are deuterostomes — more closely related to chordates than to protostomes. Adults show pentaradial (five-part) symmetry and an internal skeleton of calcium-carbonate ossicles. Their hallmark is the water vascular system, a network of canals powering tube feet for movement and feeding. Classes include sea stars, brittle stars, sea urchins, sea cucumbers, and sea lilies.

How it works

1. Filter feeding in a sponge — water flows in through pores; choanocytes beat their flagella to create a current; food is trapped and water exits the osculum.
2. Stinging in a cnidarian — a cnidocyte is triggered by touch or chemicals, and its nematocyst fires a barbed thread that delivers venom and captures prey.
3. Mollusk locomotion and feeding — the muscular foot glides or burrows while the radula scrapes food into the mouth.
4. Arthropod growth — because the exoskeleton cannot expand, the animal secretes a new, larger one beneath the old, then molts the old shell and hardens.
5. Echinoderm movement — the water vascular system pumps water into tube feet, whose suction and expansion allow crawling and prying open prey.

Common confusions

Memory aids

Order the invertebrate ladder with "Some Children Playfully Make Amazing Nests And Eggs" — Sponges, Cnidarians, Platyhelminthes, Mollusks, Annelids, Nematodes, Arthropods, Echinoderms. For echinoderms, remember "water in the tubes, food on the move" — the water vascular system powers tube feet. For mollusks, remember "Foot, Mantle, Visceral mass, Radula" (FMVR).

Quick review

Topic Recap

• Invertebrates are animals without backbones and include the vast majority of animal species.
• Sponges are simple filter feeders without true tissues; cnidarians add tissues, radial symmetry, and stinging cells.
• Lophotrochozoans include flatworms (acoelomates), mollusks (foot, mantle, radula), and annelids (segmented).
• Ecdysozoans — nematodes and arthropods — grow by molting; arthropods are the most diverse phylum.
• Echinoderms are deuterostomes with pentaradial symmetry and a water vascular system.

Knowledge Check

1. Which cell type powers filter feeding in a sponge?
2. What is the defining stinging structure of a cnidarian?
3. Which phylum has a muscular foot, a mantle, and a radula?
4. Why must an arthropod molt in order to grow?
5. Which invertebrate phylum is a deuterostome and shares ancestry with chordates?

Answers and Rationales

1. Choanocytes — their flagella create the water current that brings food to the sponge.
2. The cnidocyte (with its nematocyst) — it fires a barbed thread to capture prey.
3. Mollusca — these three structures (plus the visceral mass) define the phylum.
4. Because its chitinous exoskeleton is rigid and cannot expand — it must shed the old shell and grow before the new one hardens.
5. Echinodermata — sea stars and relatives are deuterostomes, more closely related to chordates than to any protostome.