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No Backbone
How is a fish different from a spider? Beyond the environmental factors, the primary difference is that a fish has a well-developed spine and a spider does not. Invertebrates either have a body that is an amorphous, fluid-filled membrane (protozoa, worms, sea cucumber) or a body with an exoskeleton or carapace (starfish, spider, beetle). Without a spinal cord, invertebrates have unique neurological systems. Newcastle University in the United Kingdom uses locusts and honeybees as models for studying how the brain and nervous systems function.
Multicellular Organisms
Invertebrates are multicelled animals. A sea sponge or worm may look like a blob of slime. However, an invertebrate has different types of cells that work together for the good of the animal. Conversely, a slime mold is a colony of individual cells with no differentiation or specific function. Additionally, invertebrates are animals that reproduce sexually with a male and female rather than asexual propogation, mitosis or spores.
Heterotrophic Metabolism
Invertebrates, like all animals, cannot make their own food. Invertebrates are heterotrophs, meaning they must consume organic matter and convert it to energy. At the opposite end of the spectrum are autotrophs, which make their own food. Plants are autotrophic, using sunlight and photosynthesis to manufacture the energy they need to survive.
Major Phyla
Invertebrates are animals. They are complex multicellular creatures that reproduce sexually and require food to live. However, according to the Smithsonian National Zoological Park, "of some 30 phyla representing 95 to 99 percent of the planet's animal species, the only thing they have in common is what they lack: a backbone." The invertebrate phyla fall into six major groups: Porifera or sponges; Cnidaria including jellyfish and coral; Worms, Mollusca ranging from snails to the octopus; Arthopoda, the largest group which includes most insects and spiders; and Echinodermata, the spiny creatures of the ocean.