Most animals (~97%) lack a vertebral column. Major invertebrate phyla:

• Porifera (sponges): no true tissues, filter-feeders, sessile
• Cnidaria (jellyfish, corals): radial symmetry, stinging nematocysts, two tissue layers
• Platyhelminthes (flatworms): bilateral symmetry, acoelomate, includes tapeworms and planarians
• Nematoda (roundworms): pseudocoelomate, abundant in soil; C. elegans is a model organism
• Mollusca (snails, clams, octopus): soft-bodied, many with shells, muscular foot
• Annelida (earthworms, leeches): segmented body, true coelom
• Arthropoda: greatest phylum by species count; exoskeleton, jointed appendages; insects, arachnids, crustaceans

Vertebrates belong to phylum Chordata (with notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail at some stage). Vertebrate classes:

• Fish (Agnatha, Chondrichthyes, Osteichthyes): aquatic, gills
• Amphibia: semi-aquatic, moist skin, external fertilisation generally
• Reptilia: amniotic egg, scales, ectothermic
• Aves (Birds): amniotic egg, feathers, endothermic, wings
• Mammalia: hair/fur, mammary glands, endothermic, most viviparous

All vertebrates share bilateral symmetry, cephalization (concentration of sense organs at head), and internal skeleton.