Invertebrate

An invertebrate was an animal lacking a vertebral column (or spine). The group included all animals except fish, reptiles, amphibians, birds, and mammals.

During their attempt to teach an infant Changeling how to shapeshift, Doctor Mora suggested that he and Odo expose the Changeling to simple lifeforms such as algae, fungus, and then invertebrates. 

Arthropods

 * Arachnids
 * Insects
 * Lobster

Cephalopods

 * Cuttlefish
 * Octopus
 * Squid

Mollusk

 * Clam
 * Edosian slug
 * Jumbo Romulan mollusk
 * Jumbo Vulcan mollusk
 * Nautilus
 * Oyster
 * Scallop
 * The Aldebaran shellmouth may also have been a mollusk.

Urchins

 * Palamarian sea urchin

Worms

 * See: Worm