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.

Do starfish have a brain?

<img src="https://faculty.washington.edu/chudler/gif/intop1.gif" width="75" height="46">

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), <a href="http://serendip.brynmawr.edu/~pbrodfue/leech_talk/outline1.html" dir="ltr">leech</a>,
horseshoe crab, lobster, and cockroach have all provided scientists with
models by which to study the nervous system. The squid even helped win
the <a href="http://www.nobel.se/medicine/laureates/1963/index.html" dir="ltr">Nobel Prize in
Physiology or Medicine in 1963</a>.

Invertebrates are useful animals to study because their nervous systems 
work the same way as that of vertebrates. <a href="https://faculty.washington.edu/chudler/cells.html" dir="ltr">Neurons</a> in all animals work using an <a href="https://faculty.washington.edu/chudler/ap.html" dir="ltr">electrochemical process</a>. 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. <img src="https://faculty.washington.edu/chudler/gif/intop4.gif" width="60" height="75"><img src="https://faculty.washington.edu/chudler/gif/intop5.gif" width="50" height="96"><img src="https://faculty.washington.edu/chudler/gif/intop6.gif" width="45" height="101">AnimalNervous
System Features/Behavior<td align="center" dir="ltr">
Ameba/Paramecium
</td><img src="https://faculty.washington.edu/chudler/gif/ameba.gif" width="74" height="67"><td>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.</td>Euglena
(flagellate)<img src="https://faculty.washington.edu/chudler/gif/euglena.gif" width="150" height="92">Image courtesy of Biodidac
<td>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. </td>Sponge<img src="https://faculty.washington.edu/chudler/gif/sponge1.gif" width="75" height="112">Image courtesy of Biodidac
<td>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. </td>Hydra<img src="https://faculty.washington.edu/chudler/gif/hydra.gif" width="125" height="161"><td dir="ltr">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 <a href="https://faculty.washington.edu/chudler/synapse.html" dir="ltr">synapse</a>. 
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.</td>

The hydra does have specialized cells for touch and chemical detection. 
<td>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. </td>

Some jellyfish (for example, Aurelia) have specialized structures
called "rhopalia". These rhopalia have receptors for:<UL><LI>light (called ocelli)
</LI><LI>balance (called statocysts)
</LI><LI>chemical detection (olfaction),
</LI><LI>touch (called sensory lappets)</LI></UL><img src="https://faculty.washington.edu/chudler/gif/stato.gif" width="241" height="144"><td> 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.</td>Anemone<img src="https://faculty.washington.edu/chudler/gif/anemoner.gif" width="125" height="131"><td>Like the jellyfish
and hydra, the anemone 
has a nerve net.</td>Flatworms
(Planaria)<img src="https://faculty.washington.edu/chudler/gif/flatworm.gif" width="60" height="216"><td>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.</td>
 
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. Earthworm<img src="https://faculty.washington.edu/chudler/gif/earthw1.gif" width="216" height="136"><td>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.</td> 

Earthworms have touch, light, vibration and chemical receptors all along
the entire body surface.Sea Star 
("Starfish")<img src="https://faculty.washington.edu/chudler/gif/starfner.gif" width="187" height="166"><td>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.</td>

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. Snails<img src="https://faculty.washington.edu/chudler/gif/snail.gif" width="130" height="60"><td>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. 
</td>Aplysia (Sea Hare)<img src="https://faculty.washington.edu/chudler/gif/aplysia1.gif" width="125" height="55"> Image
courtesy of BrainSurf
<td>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.</td>Bivalves (clams, scallops)<img src="https://faculty.washington.edu/chudler/gif/clam.gif" width="100" height="69"><td>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.
</td>Crab<img src="https://faculty.washington.edu/chudler/gif/crabner.gif" width="250" height="180"><td>The crab has a condensed central
nervous system consisting of several ganglia.</td>Lobster<img src="https://faculty.washington.edu/chudler/gif/lobster.gif" width="150" height="162"><td>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.</td>Insects (such
as grasshoppers)<img src="https://faculty.washington.edu/chudler/gif/insect.gif" width="216" height="216"><td>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.</td>

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. Octopus<img src="https://faculty.washington.edu/chudler/gif/octopus1.gif" width="150" height="103"><td>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. </td> 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. <img src="https://faculty.washington.edu/chudler/gif/pdficon.gif" width="28" height="31">

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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), <a href="http://serendip.brynmawr.edu/~pbrodfue/leech_talk/outline1.html">leech </a>, horseshoe crab, lobster, and cockroach have all provided scientists with models by which to study the nervous system. The squid even helped win the <a href="http://www.nobel.se/medicine/laureates/1963/index.html">Nobel Prize in Physiology or Medicine in 1963 </a>.

Invertebrates are useful animals to study because their nervous systems work the same way as that of vertebrates. <a href="https://faculty.washington.edu/chudler/cells.html">Neurons </a>in all animals work using an <a href="https://faculty.washington.edu/chudler/ap.html">electrochemical process </a>. 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.

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

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

<ul><li>light (called ocelli) </li><li>balance (called statocysts) </li><li>chemical detection (olfaction), </li><li>touch (called sensory lappets) </li> </ul>

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.

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

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.

Image courtesy of BrainSurf

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 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.

Neuroscience for Kids - Invertebrate Nervous System

Do starfish have a brain?

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