University of the Punjab

Institute of Zoology
Submitted to: Dr. Zulfiqar Ali
Submitted by:
Rafia Ashraf (Z21-19)
Maheen Farooq (Z21-03)
Muhammad Saleem (Z21-35)
Topic:
“Nervous System of Invertebrates”
 Nervous System
“A nervous system is an organized collection of neurons that interact at points of contact called
synapses.”

Neurons
Neurons are the functional units of nervous system that are electrically excitable cells whose
membranes can generate and transmit changes of voltages that form a signal.

Electrochemical signals:
Once a neuron has been stimulated by some sort of stimulus, it generates an electric potential that
travels down the length of the cell. This is the 'chemical' part of electrochemical. The primary class of
signaling molecules are called neurotransmitters.

Nervous System of Invertebrates

❖ Phylum Porifera
The phylum name Porifera means pore-bearing. Sponges take their name from small holes that cover
their bodies.
There is no definite nervous system. However neurosensory and neuron cells are probably present
which seem to coordinate the flow of water.

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❖ Phylum Cnidaria
The name Cnidaria comes from the Greek word "Cnidus," which means stinging nettle.
The nervous system consists of one or more nerve nets or plexus. Information is likely to be
integrated in the sensory ganglia. These animals possess a central nerve net rather than a body
concentrating neurons. There is no central nervous system.
Cnidarians have nerve cells that are organized in a nerve net – a diffuse network where neurons
interact wherever they cross one another.
In nerve nets of cnidarians and ctenophores, conduction is un polarized. It can run in any direction
through the networks of neurons.

 Example:
 Aurelia aurita (Moon Jelly Fish):
Rhopalia (sensory organ) can be excited by external stimulation. They are connected to both the
motor nerve net (MNN) and separate diffuse nerve net (DNN) on the subumbrella. The MNN
selectively innervates the circular muscles while DNN selectively innervates the radial ones. The
muscles deform the jelly fish bell which interacts with the surrounding fluid. The muscles forces in
turn depend on bell shape.
Moon Jelly Fish
The bell is nearly relaxed. Rhopalia are clearly visible as bright spots on wedge- shaped sections of
the bell margin.

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 Hydra:
The nervous system of Hydra is composed of a few hundred to a few thousand neurons, depending
on the size of the animal. Two main types of neurons have been reported;
Sensory cells, exposed to the external or gastric environment. Ganglion cells, which form a two-
dimensional lattice known as nerve net. Hydra’s nerve net has actually two separate components:
one in the endoderm and one in the ectoderm. Morphology of both sensory cells and ganglion cells
can vary, in terms of the size of their cell bod and the ramification of their neurites. Functions of
nerve nets is poorly understood.

❖ Phylum Echinodermata:
Echinodermata are so named owing to their spiny skin. From the Greek, “echinus” meaning “spiny”
and “dermis” meaning “skin”
These animals have a circumoral nerve ring, which may be analogous to a brain. However, it may
be that most sensory input is integrated peripherally and that there is no need for a central brain
structure. Ectoneural subsystem containing the circumoral nerve ring and outer part of the radial
nerve cords includes sensory and motor components. Hypo-neural sub-stem, a thinner, inner layer
of the radial nerve cords thought to control locomotion. They have decentralized nervous system
that do not contain a centralized concentration of neurons.

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 Example:
 Asterias (starfish):
✓ Radial nerves run under the ambulaera (legs).
✓ Central nerve ring surrounds the gut.
✓ Nerve ring that encircles the mouth.
✓ Radial nerves coordinate the functions of the tube feet.
✓ Other nerve elements are also associated with body wall.

 Echinoidea (sea urchin):

The central nervous system of sea urchin larva, consists of two types of serotonergic neurons:
bottle shaped primary sensory neurons associated with long cilia of the apical tuft and small
interconnected serotonergic neurons that form a ganglion, along with support cells. The
peripheral nervous system of sea urchin larva is composed of neurons in the ciliary band, post-
oral neurons, and lateral ganglion.

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❖ Phylum Nematoda (Roundworm)
Roundworms are unsegmented worms that are also called Aschelminths. Phylum Nematoda
includes roundworms that are parasites of animals and plants some are free living. These
animals have highly centralized nervous system, with most neurons concentrated in several
anterior ganglia. There is a distinct brain region which forms a ring shape.
✓ Circumoral nerve ring, anterior ganglia.
✓ All nematodes have a major nerve cord running longitudinally along the ventral
midline of the body from head to tail.
✓ This ventral nerve cord contains cell bodies as well as processes, many of which
project into the nerve ring where they make and receive synapses with other
neurons.
✓ Circumoral brain or nerve ring composed of axonal and dendrite processes and
clusters of cephalic neuronal cell bodies.
 Example:
 Roundworm:
✓ Roundworm have a simple nervous system with a primitive brain.
✓ There are four nerves that run the length of the body and connected from the top to
the bottom of the body.
✓ Nerves branch from a circular ring which serves as the brain.

❖ Phylum Platyhelminthes:
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek platy, meaning
"flat" and helminth-, meaning "worm") are a phylum of relatively simple bilaterian,
unsegmented, soft-bodied invertebrates. A well-developed nervous system is present in
Platyhelminthes. These animals have a cephalized nervous system that consists of either a

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 simple nerve networks or head ganglion that acts as brain usually attached to the longitudinal
nerve cords that are interconnected across the body by transverse branches.
✓ Many major sensory structures are found in head since it is a region of the body that
first samples the environment during forward locomotion.
✓ The concentration of sensory receptors has led to a concentration of neurons in the
head region (cephalization), resulting in a centralized brain structure.
✓ This neuron receives and integrates signals from sensory structures located both on
the head and on other parts of body. At least six types of superficial sense organs are
differentiated.
✓ The brain also likely directs and controls the behavior of many effectors.

 EXAMPLES:
 Dugasia (Planaria):
✓ Planarians are considered to be among the most primitive animals with developed
central nervous system (CNS). The planarians CNS is located on the ventral side of the
body, and composed of a mass o cephalic ganglion in the head region and a pair of
ventral nerve cords (VNC).
✓ From cerebral ganglions (brain), nerve cords extend the length of the body. Planaria
are richly supplied with sensory receptors. Single sensory cells in the nerve plexuses
are widely scattered over the organism.
✓ In addition to CNS, planarians have a developed peripheral nervous system (PNS)
consisting of subepidermal and submuscular neural networks that are clusters of
sensory neural cells with free nerve endings.

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 Taenia (Tapeworm):
✓ The nervous system of tapeworm is very simple consisting of light sensitive pigment-
cup eyespots (either single or in groups) connected to a cluster of
✓ Nerve cells (brain) in the head and ventral, longitudinal nerve cords.
✓ The ocelli (pigment-cup eyespots) are sensitive to light and send signals to cerebral
ganglia (brain). Generally, flatworms avoid light.

❖ Phylum Annelida:
The annelids (Annelida from Latin anellus, "little ring"), also known as the ringed worms or
segmented. Annelids have well developed central nervous system. It comprises of a simple brain
and a solid double, longitudinal, ventral nerve cord. Nerves arise in each segment from the nerve
cord.
✓ Standard annelid brain is ring-shaped with two cerebral ganglia.
✓ In annelids except some members, brain cannot be divided into fore, mid and hind
segments because these divisions are difficult or impossible to distinguish in most
annelids, particularly the sedentary species.
✓ In most annelids, two cerebral ganglia (bundles of nerve cells) form a primitive
bilobed brain, from which sensory and motor nerve fibers lead to other areas of
body.
✓ The most primitive annelids have a pair of ventral nerve cords joined by transverse
connectives; the most advanced firms have the cords fused to form a single cord. A
ganglionic swelling of the cord is found in each body segment, with the most anterior
ganglion, being the most prominent. Two of five pairs of lateral nerves leave each
ganglion to innervate the body wall of that segment.
✓ Another plexus, called the enteric or sympathetic system, is found in the wall of gut.
✓ The brain of active, predatory polychaetas (a class of marine worms) is complicated.
✓ Studies of nervous system of annelids sow certain behavioral capabilities including
perception, motor coordination and learning.

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 EXAMPLES:
 Earthworm:
✓ The nervous system of an earthworm is composed of a primitive brain of fused
ganglia, a ventral nerve cord, and peripheral nerves. Impulses from
✓ earthworm’s sensory cells are transmitted by the peripheral nerves to certain parts of
body and proper responsive movements are coordinated.
✓ Their nervous system is connected to a variety of sensory cells that are capable of
detecting different environmental factors. Earthworms also have a developed sense
of touch, taste and can perceive the amount of moisture in surrounding coil.
✓ All nerves arise from central nervous system supply to various parts of the body. All
nerves present in earthworm are of mixed type as they contain both afferent and
efferent nerve fibers and adjustors (association neurons). From subpharyngeal
ganglia, 3 pairs of nerves arise.

 Leech:
✓ The central nervous system of leech consists of head ganglia, 21 body ganglia, and 7
fused tail ganglia. The ganglia are joined by connectives that consist of two large
lateral bundles of nerve fibers and a thin medial connective called Favre’s nerve.
✓ Each segmental ganglion contains about 400 neurons and is linked to its neighbors by
thousands of axons that form the connectives.
✓ The other types of cells in leech ganglia are two connective glial cells that surround
the axons, a neuropil giant glial cells that ensheathe the cell bodies of neurons.
✓ In the adult CNS, microglial cells are small resident cells evenly distributed in the
ganglia (more than 10000 for each one) and in the connectives (2000 for each one).

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 ✓ Leeches are reported to have about 400 neurons in each of the 21 ganglia which
make up their CNS, giving an estimated total of 8400 neurons in their entire CNS.

❖ Phylum Arthropoda:
✓ The Arthropoda (Greek word mean jointed appendages, segmented body).
✓ Arthropods have well developed nervous system.
✓ The nervous system of arthropods consists of a ventral nerve cord, an anterior brain
and several ganglia.
✓ Ganglia can act as control center.
✓ Different groups of arthropods exhibit different ganglia.
✓ The nervous system of arthropods is similar to annelid worms from which arthropods
evolved.
✓ The most common sensory receptors in arthropods are the cuticular hairs many of
which are the mechanoreceptors which are sensitive to touch, vibration, water
currents or sound waves; some hairs are chemoreceptors which detect odors or
chemicals.
✓ The nerve cells and axons are larger in diameter. And they carry maximum
information.
 Examples:
 Grasshopper:
✓ The central nervous system of grasshopper consists of brain and set of seg mental
ganglia that together make up the ventral nerve cord.
✓ The brain is divided into specialized area:
✓ Protocerebrum (controlling vision)
✓ Deutocerebrum (processing antenna signals)
✓ Stomatognathic system (regulates digestion)
✓ There are 1 million neurons in grasshopper.
✓ The optic lobe of the grasshopper lies in the “central brain” near the eye and
processes visual signals.

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❖ Drosophila melanogaster (Fruit Fly)
✓ Adult and larval fruit fly nervous system develop from the same set of stem cells
known as neuroblasts.
✓ The first phase occurs in the embryo to create neurons of the larval nervous system.
✓ The second phase occurs in the larva to create neurons of the adult nervous system.
✓ Fruit fly brain contains 100,000 neurons.
✓ The Drosophila central nervous system is composed of a bilaterally symmetrical brain
with two cell types, neurons and glia, both originating from progenitors named
neroglioblasts.

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❖ Phylum Mollusca:
✓ Mollusca is the division of invertebrate animals with soft, unsegmented bodies, no
jointed legs, and commonly covered by hard shells.
✓ The nervous system in Mollusca present numerous diversities. It exhibits gradual
coming up of complexities from simple to complex. The nervous system of Mollusca
seems to be wide range of complexity but Mollusca have a set of local brain.
✓ There central nervous system consists of three pairs of ganglia, the cerebral, pedal
and pleural ganglia. These ganglia are connected by connectives. The connectives are
the cerebro-pedal, cerebro-pleural and pleural-pedal.
✓ Theses connectives innervate the head, mouth and associated sense organs.
✓ From dorsal cerebral ganglia, two pairs of longitudinal nerve cords arise; A pair of
lateral nerve cords, often forming pleural ganglia (which innervate the mantle). And a
ventral pair of pedal nerve cords often forming pedal ganglia (which innervates the
foot)
Classification Of Mollusca:
• GASTROPODA:
✓ Gastropods possess extensive peripheral nervous system containing axons efferent
and afferent to the central ganglia and also large number of peripheral neurons
located within different organs.
✓ The nervous system of gastropods consists of a series of paired ganglia connected by
major nerve. It is sometimes called ganglionic.
✓ It is more complex than roundworm and flatworm.
✓ They have tentacles containing sensory organs located on other head.

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 Example:
 Snails:
✓ They have highly centralized nervous system.
✓ Snails do not have brain but their nervous systems are centralized to a certain extent
and they have analogous structures which are several pairs of ganglia (clusters of
neurons) connected by a nerve cord.
✓ They have clusters of ganglia (bundles of nerve cells) which in many species are fused
into a single organ.
✓ They have highly developed nervous systems to solve complex problems.
✓ Another example of gastropods is slug.

❖ Bivalvia (Pelecypod):
✓ Bivalves have a simple nervous system with usually three sets of ganglia connected
by nerve fibers.
✓ Ganglia are the clusters of nerve cells that form simple nerve centers distinct from
the brain.
✓ Bivalves do not have brain, but they have nervous system that reacts to stimulus.

 Example:
 Anodonta (Freshwater Mussel):
✓ The nervous system of mussel consists of paired cerebro-pleural ganglia (CPG)
connected to one another but cerebro-pleural commissures (CPG) and to the pedal
ganglia (PG) via the cerebral-pleural-pedal connectives (CPPC) and to the paired

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 visceral ganglia (VG) via long cerebral-pleural visceral connectives (CPVC).
✓ Their nervous system is incredibly rudimentary and has no centrality (meaning they
have no brain), and they are incapable of forming thoughts or experiencing pain.

❖ Cephalopoda:
✓ Cephalopods have the most complex nervous system of the invertebrates.
✓ Their brain-to-body-mass ratio fall a between that of endothermic and exothermic
vertebrates.
✓ The brain is protected in a cartilaginous cranium.
✓ Cephalopods have developed an expanded and centralized CNS that allows amazing
behavior and complex cognition.

 Example:
 Octopus:
✓ Octopus has the most complex central nervous system of all the cephalopods, with
almost 40 different lobes dedicated to a wide range of functions and substantial axial
nerve cords in its arms.
✓ Octopus have an extensive nervous system, with over 500 million neurons, similar in
number to that of a dog. But unlike dogs and other vertebrates, where the majority
of neurons are in the brain, over two thirds of the
✓ octopuses’ neurons are located within their arms and body.
✓ An octopus has more neurons in their peripheral nervous system (PNS) than in their
brain. PNS neurons could participate in forming cognitive networks with the central

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 brain in the same way that the cerebellum is now thought to contribute to
mammalian cognition.
✓ Octopus have nine separate brains that are connected to and communicate with
each other

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