invertebrates – an animal lacking a backbone or spinal column

To remember the meaning of invertebrates, use the following mnemonic:

We would have to insert this backbone to be a vertebrate (invertebrate) instead the snail could not stand up.

Invertebrates are animals that do not have a backbone or spinal column, making up about 97% of all animal species on Earth and representing the most diverse group in the animal kingdom. The enormous group includes insects, spiders, crabs, worms, jellyfish, snails, octopuses and many other animals that vary dramatically in size, shape, habitat, and lifestyle. 

Instead of an internal skeleton made of bone or cartilage (like vertebrates) invertebrates may have external skeletons (exoskeletons) like insects and crustaceans or no skeleton at all like jellyfish. 

Invertebrates generally have simpler nervous systems than vertebrates, though some like octopuses show surprising intelligence, and many specialised sensory organs that can detect things humans cannot, such as magnetic fields.