Animals that don't have a backbone include insects, spiders, scorpions and cephalopods. Bivalves such as clams, mussels and oysters lack a backbone, as do jellyfish, starfish, sponges, sea cucumbers and sea pigs. Crustaceans such as shrimp, lobsters and crabs lack backbones, as do worms such as earthworms and leeches.
These animals are called invertebrates. Most animals are invertebrates. They are found in all kinds of habitats but are most abundant in the oceans.
Most invertebrates not only lack a backbone, but they lack bones of any kind. Insects, spiders and crustaceans have exoskeletons. Many of them must shed these exoskeletons from time to time to allow their bodies to grow. A sponge has a system of calcium carbonate spicules that give its body its shape.
Tunicates are unusual animals in that they have a larval stage where they have a notochord, a structure that resembles a backbone. However, they lose the notochord when they become adults.
Other invertebrates undergo complete metamorphosis. This is where the larval form of the animal looks and behaves very differently from the adult form. The nymphs of dragonflies, for example, are aquatic and lack wings. The caterpillars of butterflies resemble fat worms. Caterpillars are voracious, but their adult versions sip nectar and other fluids or don't feed at all.