Invertebrate

Explanation

Invertebrates are the type of animals that evolved without the vertebral column. The most common examples of invertebrates are the insects, worms, and some marine animals like crabs and star fishes. They also lack a skeleton made up of bone. Instead of a vertebral skeleton, the invertebrates possess exoskeletons and outer shells. Insects, being the largest class of invertebrates accounts for about 80 percent of the total species on Earth approximately.

How do invertebrates respond to stimuli?

Invertebrates do possess nuerons. So they do respond to external stimuli. The invertebrate nervous system is similar to that of the vertebrates and they do work the same way by sending signals electrochemically using the action potentials.