Cues / Keywords | Notes
Porifera | Cellular level; asymmetrical/radial; choanocytes (collar cells) in spongocoel; spicules ( / ) or spongin; water: ostia → spongocoel → osculum | Examples: Sycon (marine), Euspongia (bath sponge), Spongilla (freshwater)
Cnidaria | Tissue level; radial; diploblastic; cnidocytes with nematocysts; gastrovascular cavity (incomplete gut); polyp ↔ medusa polymorphism | Examples: Hydra (polyp only), Aurelia (jellyfish/medusa), Physalia (colonial siphonophore), Meandrina (brain coral)
Ctenophora | Tissue level; radial; diploblastic; comb plates (ctenes) for locomotion; bioluminescence; NO cnidocytes | Examples: Pleurobrachia, Ctenoplana
Platyhelminthes | Organ level; bilateral; triploblastic; ACOELOMATE; dorso-ventrally flattened; flame cells for excretion | Examples: Planaria (free-living), Taenia solium (tapeworm), Fasciola hepatica (liver fluke)
Aschelminthes | Organ-system level; bilateral; triploblastic; PSEUDOCOELOMATE; cylindrical body; complete gut | Examples: Ascaris (roundworm), Wuchereria bancrofti (elephantiasis), Ancylostoma (hookworm)
Annelida | Organ-system level; bilateral; triploblastic; COELOMATE; metameric segmentation; nephridia; closed circulation (in most) | Examples: Nereis (marine, polychaete), Pheretima (earthworm), Hirudinaria (leech — secretes hirudin)
Arthropoda | LARGEST phylum; bilateral; triploblastic; coelomate; jointed appendages; chitinous exoskeleton; open circulation (haemocoel); Malpighian tubules; compound eyes | Examples: Apis mellifera (honey bee), Bombyx mori (silkworm), Limulus (living fossil — Merostomata), Periplaneta (cockroach)
Mollusca | 2nd LARGEST phylum; bilateral; triploblastic; coelomate; mantle secretes shell; radula (except bivalves); open circulation EXCEPT Cephalopoda (closed) | Examples: Pila (snail), Pinctada (pearl oyster), Sepia/Octopus/Loligo (Cephalopoda — closed circulation)
Echinodermata | Radial (adult); bilateral (larva); triploblastic; coelomate; water vascular system; tube feet; calcareous endoskeleton; remarkable regeneration | Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber)
Hemichordata | Bilateral; triploblastic; coelomate; stomochord (NOT true notochord); gill slits | Example: Balanoglossus
Summary
Nine invertebrate phyla (Porifera to Hemichordata) show progressive complexity in body plan. The three most important features to classify any invertebrate phylum: symmetry, germ layers, and coelom type. Key exceptions: Cephalopoda have closed circulation (unlike other Mollusca); Echinoderm larvae are bilateral (unlike adults); Ctenophora have NO cnidocytes (unlike Cnidaria).