Neuroscience Resources for Kids - Invertebrate NervousSystem

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

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

Invertebrates are useful animals to study because their nervous systemfunctions in basically the same way as that of vertebrates. Neurons in allanimals work using an electrochemicalprocess. Since the nervous system of invertebrates is less complex than that of vertebrates, it is easier to isolate and study neuralfunctions in these animals without backbones.

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

Animal NervousSystem Features/Behavior
Ameba/Paramecium
While the ameoba is asingle-celled animal, itdoes appear to be sensitive to the environment. This tiny animal movesaway 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 movingaway.
Euglena(flagellate)
Image courtesy of Biodidac
Euglena have an eyespotthat acts as a shield for a light sensitive receptor. This small animalcan detect the strength and direction of light. It prefers a locationwith moderate light and moves away from darkness and bright light. Euglena probably use this receptor to keep themselves in light which theyuse for photosynthesis. Euglena use photosynthesis for energy althoughthey can eat solid food (like animals) if they are kept in thedarkness.
Sponge
Image courtesy of Biodidac
Sponges are theonlymulticellularanimals without a nervous system. They do not have any nerve cells orsensory cells. However, touch or pressure to the outside of a sponge willcause a local contraction of its body.

Hydra
Thehydra has a nervous system characterized by a nervenet. 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 synapsewithin a nerve net. The nerve net is concentrated around the mouth. Unlike higher animals, the hydra does not have any grouping of nerve cellbodies. In other words, there are no ganglia.
The hydra does have specialized cells for touch and chemical detection.
Jellyfish
Like the hydra, thejellyfish has a nervoussystem characterized by a series of interconnected nerve cells (a nervenet). The nerve net conducts impulses around the entire body of thejellyfish. The strength of a behavioral response is proportional to thestimulus strength. In other words, the stronger the stimulus, the largerthe response.
Some jellyfish (for example, Aurelia) have specialized structurescalled "rhopalia". These rhopalia have receptors for:
light (called ocelli)
balance (called statocysts)
chemical detection (olfaction),
touch (called sensory lappets)
Shown tothe left is a statocyst.When the animal moves and body is tilted, the statocyst makes contactwith the cilium. When the cilium bends, it causes action potentials tofire in a nerve. This provides information to move muscles.
Anemone
Like the jellyfishand hydra, the anemone has a nerve net.
Flatworms(Planaria)
The nervous systemof the flatworm has anorganization different from the invertebrates describe above. It doeshave a nerve net, but these are connected by long nerve cords. These cordsare connected to cerebral ganglia located in the head region. The centralnervous system has been described as "ladder-like" because of the nervesconnecting the nerve cords.
Flatworms have "auricles" that project from the side of the head. Theseauricles contain chemoreceptors that are used to find food. Flatwormsalso have eyespots called "ocelli". The ocelli are sensitive to light andare connected to the cerebral ganglia. Generally, the flatworm avoidslight.
Earthworm
The nervous systemof the earthworm is "segmented" just like the rest of the body. The "brain" islocated above the pharynx and is connected to the first ventralganglion. 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 willstop eating and will not dig. Each segmented ganglion getssensory information from only a local region of its body andcontrols muscles only in this local region.
Earthworms have touch, light, vibration and chemical receptors all alongthe entire body surface.
Sea Star
("Starfish")
The nervous systemof the starfish is verysimple...there is no brain and there are not even any ganglia tocoordinate movement. The nervous system is characterized by a nerve ringthat surrounds the mouth. A radial nerve branches off of the nerve ringand extends to each arm. The picture on the left shows one of 3 nervenets 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 pigmentsthat allow the starfish to detect shadows and changes in the brightness oflight.
Snails
Thenervous system is characterized by6 ganglia. Some snails have chemosensors called "osphradia" in the mantlecavity. These osphradia are used to detect chemicals in the air or water.
Aplysia
(Sea Hare)

Imagecourtesy of BrainSurf
Theaplysia has severalganglia that are connected by long nerves. The cell bodies of someneurons are very large (1 mm in diameter). Neuroscientists like thesecells because they are easy to: 1) see 2) record action potentials3) inject chemicals.
Bivalves
(clams, scallops)
The nervous systemis comprised of 3 pairs of ganglia (cerebral, visceral and pedal) eachassociated with the esophagus, muscles close to the shell, and foot.
Crab
The crab has a condensed centralnervous system consisting of several ganglia.
Lobster
The lobster has abrain connected to a first ventral ganglion. This ganglion is locatedunder its stomach. A double nerve cord extends from the first ventralganglion to a series of paired segmental ganglia running through theentire body on the ventral side of the animal.
Insects
(suchas grasshoppers)
The grasshopper has abrain located betweenits eyes, just above the esophagus. The brain is connected to the 1stventral ganglion by a pair of ventral nerves that surround the gut. Thegrasshopper can do many things, like walking and jumping, WITHOUT itsbrain. The brain is used to relay sensory information to other parts ofthe body and to help with movement. The first ventral ganglion is usedprimarily to control movement of the mouth. The segmental gangliathroughout the length of the grasshopper are connected to the firstventral ganglion by a double nerve cord and serve to coordinate localactivities.
Insects have a compound eye containing many different units called"ommatidia". Each ommatidia is like an individual lens that samples asmall part of the visual field. There can be thousands of ommatidia in asingle insect eye. Science fiction/horror/monster movies that show aninsect that sees thousands of identical images of the ENTIRE visual fieldare WRONG -- an insect sees only ONE picture at a time since eachommatidia sees only a small part of the entire field. Some insects aresensitive to ultraviolet light and others can detect infrared wavelengthsof light.
Octopus
The octopus has themost complicated brain of all the invertebrates. The octopus brain isestimated to have 100,000,000 neurons. These neurons are arranged inlobes and tracts that are more specialized than simple ganglia. Anoctopushas a "good" memory and can also learn.
The eye of the octopus isvery similar to that of vertebrates in that it has a cornea, lens, irisand retina. It can also focus and form images. However, the octopus eyeis different from that of vertebrates in that it focuses light by movingthe lens closer and further away from the retina. The vertebrate eyefocuses by changing the shape of the lens. Octopi can perceive shape,color intensity and texture. Another difference is that the eye of theoctopus has NO blind spot since the nerve cells leave from the outside ofthe eyeball. The octopus also has a statocyst located next to the brain. The statocyst is used to detect changes in gravity and respond toacceleration.

Try this experiment using an earthworm.

Did you know?
The worlds largest invertebrate is the GIANT SQUID - (Architeuthis dux). The giant squid can grow up to 18m (59 ft) long and weigh up to 900 kg (1,980 lb).

For more information on invertebrates, see:

AmazingAnimal Senses
TheAplysia (sea hare) in neuroscience research
B-Eye -the eye of the honey bee
Classification of Invertebrates
CrayfishNervous System
Fly Brain - atlas and database
InsectAnatomy
Insecta
Insects
KeepingInsects and Spiders at School
Jellyfish
SeasideNeuroscience
UsingInsects in the Classroom
Wormland - the nervous system of worms
Worms

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Exploring the NervousSystem

Animal	NervousSystem Features/Behavior
Ameba/Paramecium	While the ameoba is asingle-celled animal, itdoes appear to be sensitive to the environment. This tiny animal movesaway 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 movingaway.
Euglena(flagellate) Image courtesy of Biodidac	Euglena have an eyespotthat acts as a shield for a light sensitive receptor. This small animalcan detect the strength and direction of light. It prefers a locationwith moderate light and moves away from darkness and bright light. Euglena probably use this receptor to keep themselves in light which theyuse for photosynthesis. Euglena use photosynthesis for energy althoughthey can eat solid food (like animals) if they are kept in thedarkness.
Sponge Image courtesy of Biodidac	Sponges are theonlymulticellularanimals without a nervous system. They do not have any nerve cells orsensory cells. However, touch or pressure to the outside of a sponge willcause a local contraction of its body.
Hydra	Thehydra has a nervous system characterized by a nervenet. 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 synapsewithin a nerve net. The nerve net is concentrated around the mouth. Unlike higher animals, the hydra does not have any grouping of nerve cellbodies. In other words, there are no ganglia. The hydra does have specialized cells for touch and chemical detection.
Jellyfish	Like the hydra, thejellyfish has a nervoussystem characterized by a series of interconnected nerve cells (a nervenet). The nerve net conducts impulses around the entire body of thejellyfish. The strength of a behavioral response is proportional to thestimulus strength. In other words, the stronger the stimulus, the largerthe response. Some jellyfish (for example, Aurelia) have specialized structurescalled "rhopalia". These rhopalia have receptors for: light (called ocelli) balance (called statocysts) chemical detection (olfaction), touch (called sensory lappets) Shown tothe left is a statocyst.When the animal moves and body is tilted, the statocyst makes contactwith the cilium. When the cilium bends, it causes action potentials tofire in a nerve. This provides information to move muscles.
Anemone	Like the jellyfishand hydra, the anemone has a nerve net.
Flatworms(Planaria)	The nervous systemof the flatworm has anorganization different from the invertebrates describe above. It doeshave a nerve net, but these are connected by long nerve cords. These cordsare connected to cerebral ganglia located in the head region. The centralnervous system has been described as "ladder-like" because of the nervesconnecting the nerve cords. Flatworms have "auricles" that project from the side of the head. Theseauricles contain chemoreceptors that are used to find food. Flatwormsalso have eyespots called "ocelli". The ocelli are sensitive to light andare connected to the cerebral ganglia. Generally, the flatworm avoidslight.
Earthworm	The nervous systemof the earthworm is "segmented" just like the rest of the body. The "brain" islocated above the pharynx and is connected to the first ventralganglion. 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 willstop eating and will not dig. Each segmented ganglion getssensory information from only a local region of its body andcontrols muscles only in this local region. Earthworms have touch, light, vibration and chemical receptors all alongthe entire body surface.
Sea Star ("Starfish")	The nervous systemof the starfish is verysimple...there is no brain and there are not even any ganglia tocoordinate movement. The nervous system is characterized by a nerve ringthat surrounds the mouth. A radial nerve branches off of the nerve ringand extends to each arm. The picture on the left shows one of 3 nervenets 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 pigmentsthat allow the starfish to detect shadows and changes in the brightness oflight.
Snails	Thenervous system is characterized by6 ganglia. Some snails have chemosensors called "osphradia" in the mantlecavity. These osphradia are used to detect chemicals in the air or water.
Aplysia (Sea Hare) Imagecourtesy of BrainSurf	Theaplysia has severalganglia that are connected by long nerves. The cell bodies of someneurons are very large (1 mm in diameter). Neuroscientists like thesecells because they are easy to: 1) see 2) record action potentials3) inject chemicals.
Bivalves (clams, scallops)	The nervous systemis comprised of 3 pairs of ganglia (cerebral, visceral and pedal) eachassociated with the esophagus, muscles close to the shell, and foot.
Crab	The crab has a condensed centralnervous system consisting of several ganglia.
Lobster	The lobster has abrain connected to a first ventral ganglion. This ganglion is locatedunder its stomach. A double nerve cord extends from the first ventralganglion to a series of paired segmental ganglia running through theentire body on the ventral side of the animal.
Insects (suchas grasshoppers)	The grasshopper has abrain located betweenits eyes, just above the esophagus. The brain is connected to the 1stventral ganglion by a pair of ventral nerves that surround the gut. Thegrasshopper can do many things, like walking and jumping, WITHOUT itsbrain. The brain is used to relay sensory information to other parts ofthe body and to help with movement. The first ventral ganglion is usedprimarily to control movement of the mouth. The segmental gangliathroughout the length of the grasshopper are connected to the firstventral ganglion by a double nerve cord and serve to coordinate localactivities. Insects have a compound eye containing many different units called"ommatidia". Each ommatidia is like an individual lens that samples asmall part of the visual field. There can be thousands of ommatidia in asingle insect eye. Science fiction/horror/monster movies that show aninsect that sees thousands of identical images of the ENTIRE visual fieldare WRONG -- an insect sees only ONE picture at a time since eachommatidia sees only a small part of the entire field. Some insects aresensitive to ultraviolet light and others can detect infrared wavelengthsof light.
Octopus	The octopus has themost complicated brain of all the invertebrates. The octopus brain isestimated to have 100,000,000 neurons. These neurons are arranged inlobes and tracts that are more specialized than simple ganglia. Anoctopushas a "good" memory and can also learn. The eye of the octopus isvery similar to that of vertebrates in that it has a cornea, lens, irisand retina. It can also focus and form images. However, the octopus eyeis different from that of vertebrates in that it focuses light by movingthe lens closer and further away from the retina. The vertebrate eyefocuses by changing the shape of the lens. Octopi can perceive shape,color intensity and texture. Another difference is that the eye of theoctopus has NO blind spot since the nerve cells leave from the outside ofthe eyeball. The octopus also has a statocyst located next to the brain. The statocyst is used to detect changes in gravity and respond toacceleration.