Invertebrates
Of the million or more animal species in the world, more than 98% are
invertebrates. Invertebrates don't have an internal skeleton made of bone.
Many invertebrates have a fluid-filled, hydrostatic skeleton, like a worm. Others have a hard outer shell, like insects and crustaceans.
There are many types of invertebrates. The most common invertebrates include
the protozoa, annelids, echinoderms, mollusks and arthropods. Arthropods
include insects, crustaceans and arachnids.
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