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to Invertebrate Nervous System
Animal Nervous
System Features/Behavior
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Ameba/Paramecium
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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. |
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Sponge 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. |
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Hydra
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. |
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Jellyfish
Like the hydra, the jellyfish has a nervous system characterized by a series of interconnected nerve cells (a nerve net). The nerve net conducts impulses around the entire body of the jellyfish. The strength of a behavioral response is proportional to the stimulus strength. In other words, the stronger the stimulus, the larger the response. Some jellyfish (for example, Aurelia) have specialized structures called "rhopalia". These rhopalia have receptors for:
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Anemone |
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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 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. |
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Earthworm
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. |
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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. |
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Snails
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. |
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Aplysia
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.
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Bivalves
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Crab
The crab
has a condensed central nervous system consisting of several ganglia.
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Lobster
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.
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Insects
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 since 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.
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Octopus
The octopus has the most complicated brain of all the invertebrates. The octopus brain is estimated to have 300,000,000 neurons. These neurons 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. Octopi can perceive shape, color intensity and texture. Another difference is that the eye of the octopus has NO blind spot since 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. |
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.
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.
