Coral, sea anemones, jellyfish, and starfish have simple and decentralized nervous systems without brains. Snails have a more developed nervous system with a primitive brain divided into ganglia. Bivalves like mussels have ganglia and nerve cords. Lobsters have well-developed smell and taste, while crabs have large ganglia masses and ganglia down their bodies. Scorpions have a brain in their cephalothorax that controls sensory organs.

6. Coral and sea anemones contain the
simplest form of a nervous system. There
is no centralization of the nervous system;
therefore they don't have a brain. Neurons
communicate with one another whenever
they cross the path of another. This is
called en passant synapse. The neurons
send the information in almost every
direction and lay between the two layers of
each the coral and sea anemone.

9. Jellyfish, also, do not have a specialized nervous system, but they do have a nerve net.
The nerve net is found in the epidermis. The rhopalial lappet located around the
jellyfish’s body sends information from the nerve rings. Jellyfish are sensitive to light, but
they cannot see images. They use the ocelli to help determine which way is up by
detecting sunlight. The nerve net helps the jellyfish know when the prey or a predator
touches it on their oral arms.

12. Cephalization
Ganglia

16. The nervous system of Sea Star or Starfish is primitive and poorly developed. However,
they have eye spots that can detect light and help with orientation. However, they lack
the advanced nervous system of chordates.

19. Echinoids, also known
as sea urchins. Sea Cucumbers Sand Dollars
Sea urchins, sea cucumbers, and sand dollars are all part of the Phylum Echinodermata.
There is little knowledge of the nervous system of echinoderms. Without a brain, the
nervous system coordinates the movement of the tube feet and spines. The defense
mechanisms of echinoderms give evidence that the nervous system must be more
complex than it looks.

22. Cepaea hortensis
The center of the nervous system of snails is more
towards the head because that’s where all of the
animal’s sensory organs are located.
Snails have two pairs of tentacles; the upper pair
serve as two eyes.
Snails have a cerebral ganglia that is divided into 5
sections and that serves as a primitive brain that
makes snails capable of associative learning and
these different sections of the ganglia are connected.
Limpets central nervous system is coiled and
cephalized.

23. Most bivalves can perceive light by their late larval stages through their eyespots.
photoreceptors are reported in the pallial nerve.
Cilia perceive touch and vibrations through the water.
Cells along the edge of the mantel respond to light and touch (photoreceptors). Ganglia
are located above the mouth, in the digestive system, and in the foot. It’s connected by
two pairs of long nerve cords.

24. Bivalves are simple. They have 3 pairs of ganglia and two pairs of long nerve cords. The
cerebrophleural ganglia is located near the esophagus. It is connected to two nerve
cords, located closely to visceral ganglia that are located under the posterior adductor
muscle. Then they are connected to a second pair of nerve cords that carry nerve
signals to a pair of pedal ganglia near the foot.

25. Lobster

26. LOBSTERS: Lobsters have highly developed systems of both smell and taste. The first
antennae, act as the "nose" of the lobster. Hundreds of fine hairs cover the antennules
and are the actual organs of smell. In order for a lobster to be able to smell something,
or to be able to walk towards a smell, it has to constantly sample the chemicals in the
water to determine their changing concentration. Lobsters do this in the same way that
humans do - - they sniff. Sniffing is accomplished by flicking the antennules downward
quickly - - this removes the old water and replaces it with new water and a new odor
sample. Because lobsters have two antennules, they can determine the direction of the
smell by comparing the difference in concentrations between the two antennules.
The legs and mouthparts possess the taste organs, which are also hairs, but of different
shapes from those found on the antennules. Legs probe the sediment for food items and
pass these items to the mouthparts which provide the final determination of whether
something should be swallowed or not.

27. S
H
R
I
M
P
S

28. SHRIMPS: Nerves carry impulses from their eyes, antennules, and antennae to their
brain. Two nerve trunks run from the brain forming a singular ventral nerve cord.

29. CRAB

30. Calico Crab
Atlantic Ghost Crab
True Crabs:
True crabs have a massive amount of ganglia, which can also be identified as the
“brain.” They also have another mass of ganglia that passes down their body. True crabs
don’t have a true brain but ganglia which are in control of all of their body functions.

32. Horseshoe Crabs:
Horseshoe crabs
have a brain the is
around the mouth.
This brain controls
the eyes and many
different other
functions.

34. Scorpions are invertebrates in the class Arachnida. Within the cephalothorax they have
a "head" which includes a brain. Because they are invertebrates, they do not have a
spinal column or spinal cord. They do have sensory organs which connect to the brain
through neurons and nerves and nerve chords.