Invertebrates are those animals without a backbone (spinal column). Invertebrates include animals such as insects, worms, jellyfish, spiders - these are only a few of the many types of spineless creatures.

Invertebrates have played an important role in discoveries about how the nervous system works. The squid, aplysia (sea hare), leech, horseshoe crab, lobster, and cockroach have all provided scientists with models by which to study the nervous system. The squid even helped win the Nobel Prize in Physiology or Medicine in 1963.

Invertebrates are useful animals to study because their nervous systems work the same way as that of vertebrates. Neurons in all animals work using an electrochemical process. It is easier to study the function of the more simple nervous systems of invertebrates.

Before reading about the nervous systems of some invertebrates, let's define a ganglion (plural is ganglia). A ganglion is a group or collection of nerve cell bodies.

AnimalNervous System Features/Behavior

Although the ameba is a single-celled animal, it does appear to be sensitive to the environment. This tiny animal moves away from light, but it has no photodetectors or eyes. The paramecium, another single-celled animal, also has no specialized sensory structures. However, it avoids cold, heat and chemicals by backing up and moving away.
Euglena (flagellate)

Image courtesy of Biodidac

Euglena have an eyespot that acts as a shield for a light sensitive receptor. This small animal can detect the strength and direction of light. It prefers a location with moderate light and moves away from darkness and bright light. Euglena probably use this receptor to keep themselves in light which they use for photosynthesis. Euglena use photosynthesis for energy although they can eat solid food (like animals) if they are kept in the darkness.

Image courtesy of Biodidac

Sponges are the only multicellular animals without a nervous system. They do not have any nerve cells or sensory cells. However, touch or pressure to the outside of a sponge will cause a local contraction of its body.

The hydra has a nervous system characterized by a nerve net. A nerve net is a collection of separate, but "connected" neurons. Neurons are connected by synapse. Communication between neurons can be in both directions at the synapse within a nerve net. The nerve net is concentrated around the mouth. Unlike higher animals, the hydra does not have any grouping of nerve cell bodies. In other words, there are no ganglia.

The hydra does have specialized cells for touch and chemical detection.


Like the hydra, the jellyfish has a nervous system characterized by a series of interconnected nerve cells (a nerve net). The nerve net surrounds the whole body of the jellyfish.

Some jellyfish (for example, Aurelia) have specialized structures called "rhopalia". These rhopalia have receptors for:

  • light (called ocelli)
  • balance (called statocysts)
  • chemical detection (olfaction),
  • touch (called sensory lappets)
Shown to the left is a statocyst. When the animal moves and body is tilted, the statocyst makes contact with the cilium. When the cilium bends, it causes action potentials to fire in a nerve. This provides information to move muscles.

Like the jellyfish and hydra, the anemone has a nerve net.
Flatworms (Planaria)

The nervous system of the flatworm has an organization different from the invertebrates describe above. It does have a nerve net, but these are connected by long nerve cords. These cords are connected to cerebral ganglia located in the head region. The central nervous system has been described as "ladder-like" because of the nerves connecting the nerve cords.

Flatworms have "auricles" that project from the side of the head. These auricles contain chemoreceptors that are used to find food. Flatworms also have eyespots called "ocelli". The ocelli are sensitive to light and are connected to the cerebral ganglia. Generally, the flatworm avoids light.


The nervous system of the earthworm is "segmented" just like the rest of the body. The "brain" is located above the pharynx and is connected to the first ventral ganglion. The brain is important for movement: if the brain of the earthworm is removed, the earthworm will move continuously. If the first ventral ganglion is removed, the earthworm will stop eating and will not dig. Each segmented ganglion gets sensory information from only a local region of its body and controls muscles only in this local region.

Earthworms have touch, light, vibration and chemical receptors all along the entire body surface.

Sea Star

The nervous system of the starfish is very simple...there is no brain and there are not even any ganglia to coordinate movement. The nervous system is characterized by a nerve ring that surrounds the mouth. A radial nerve branches off of the nerve ring and extends to each arm. The picture on the left shows one of 3 nerve nets that extend throughout the body.

Starfish have an interesting way of detecting light. They have "eyespots" at the tip of each arm. The eyespot contains light sensitive pigments that allow the starfish to detect shadows and changes in the brightness of light.


The nervous system is characterized by 6 ganglia. Some snails have chemosensors called "osphradia" in the mantle cavity. These osphradia are used to detect chemicals in the air or water.
(Sea Hare)

Image courtesy of BrainSurf

The aplysia has several ganglia that are connected by long nerves. The cell bodies of some neurons are very large (1 mm in diameter). Neuroscientists like these cells because they are easy to: 1) see 2) record action potentials 3) inject chemicals.
(clams, scallops)

The nervous system is comprised of 3 pairs of ganglia (cerebral, visceral and pedal) each associated with the esophagus, muscles close to the shell, and foot.

The crab has a condensed central nervous system consisting of several ganglia.

The lobster has a brain connected to a first ventral ganglion. This ganglion is located under its stomach. A double nerve cord extends from the first ventral ganglion to a series of paired segmental ganglia running through the entire body on the ventral side of the animal.
(such as grasshoppers)

The grasshopper has a brain located between its eyes, just above the esophagus. The brain is connected to the 1st ventral ganglion by a pair of ventral nerves that surround the gut. The grasshopper can do many things, like walking and jumping, WITHOUT its brain. The brain is used to relay sensory information to other parts of the body and to help with movement. The first ventral ganglion is used primarily to control movement of the mouth. The segmental ganglia throughout the length of the grasshopper are connected to the first ventral ganglion by a double nerve cord and serve to coordinate local activities.

Insects have a compound eye containing many different units called "ommatidia". Each ommatidia is like an individual lens that samples a small part of the visual field. There can be thousands of ommatidia in a single insect eye. Science fiction/horror/monster movies that show an insect that sees thousands of identical images of the ENTIRE visual field are WRONG -- an insect sees only ONE picture at a time because each ommatidia sees only a small part of the entire field. Some insects are sensitive to ultraviolet light and others can detect infrared wavelengths of light.


The octopus has the most complicated brain of all the invertebrates. The octopus nervous system has about 500,000,000 neurons, with two-thirds of these neurons located in the arms of the octopus. Neurons in the octopus brain are arranged in lobes and tracts that are more specialized than simple ganglia. An octopus has a "good" memory and can also learn.

The eye of the octopus is very similar to that of vertebrates in that it has a cornea, lens, iris and retina. It can also focus and form images. However, the octopus eye is different from that of vertebrates in that it focuses light by moving the lens closer and further away from the retina. The vertebrate eye focuses by changing the shape of the lens. An octopus can perceive shape, color intensity and texture. Another difference is that the eye of the octopus has NO blind spot because the nerve cells leave from the outside of the eyeball. The octopus also has a statocyst located next to the brain. The statocyst is used to detect changes in gravity and respond to acceleration.

Try this experiment using an earthworm.

Did you know?

  • The world's largest invertebrate is the GIANT SQUID - (Architeuthis dux). The giant squid can grow up to 18 m (59 ft) long and weigh up to 900 kg (1,980 lb).
  • Approximately 99% of the world's animals are invertebrates. (Turin, M.S. Aardvarks to Zebras, New York: Citadel Press, 1995)


For more information on invertebrates, see:

  1. Insect brains and animal intelligence
  2. Amazing Animal Senses
  3. Insect Nervous Systems
  4. Insects
  5. Jellyfish
  6. Using Insects in the Classroom


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