Coral and sea anemones contain thesimplest form of a nervous system. Thereis no centralization of the nervous system;therefore they dont have a brain. Neuronscommunicate with one another wheneverthey cross the path of another. This iscalled en passant synapse. The neuronssend the information in almost everydirection and lay between the two layers ofeach the coral and sea anemone.
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 thejellyfish’s body sends information from the nerve rings. Jellyfish are sensitive to light, butthey cannot see images. They use the ocelli to help determine which way is up bydetecting sunlight. The nerve net helps the jellyfish know when the prey or a predatortouches it on their oral arms.
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 lackthe advanced nervous system of chordates.
Echinoids, also known as sea urchins. Sea Cucumbers Sand DollarsSea 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, thenervous system coordinates the movement of the tube feet and spines. The defensemechanisms of echinoderms give evidence that the nervous system must be morecomplex than it looks.
Cepaea hortensisThe center of the nervous system of snails is moretowards the head because that’s where all of theanimal’s sensory organs are located.Snails have two pairs of tentacles; the upper pairserve as two eyes.Snails have a cerebral ganglia that is divided into 5sections and that serves as a primitive brain thatmakes snails capable of associative learning andthese different sections of the ganglia are connected.Limpets central nervous system is coiled andcephalized.
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). Gangliaare located above the mouth, in the digestive system, and in the foot. It’s connected bytwo pairs of long nerve cords.
Bivalves are simple. They have 3 pairs of ganglia and two pairs of long nerve cords. Thecerebrophleural ganglia is located near the esophagus. It is connected to two nervecords, located closely to visceral ganglia that are located under the posterior adductormuscle. Then they are connected to a second pair of nerve cords that carry nervesignals to a pair of pedal ganglia near the foot.
LOBSTERS: Lobsters have highly developed systems of both smell and taste. The firstantennae, act as the "nose" of the lobster. Hundreds of fine hairs cover the antennulesand 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 thewater to determine their changing concentration. Lobsters do this in the same way thathumans do - - they sniff. Sniffing is accomplished by flicking the antennules downwardquickly - - this removes the old water and replaces it with new water and a new odorsample. Because lobsters have two antennules, they can determine the direction of thesmell by comparing the difference in concentrations between the two antennules.The legs and mouthparts possess the taste organs, which are also hairs, but of differentshapes from those found on the antennules. Legs probe the sediment for food items andpass these items to the mouthparts which provide the final determination of whethersomething should be swallowed or not.
Calico Crab Atlantic Ghost CrabTrue 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 crabsdon’t have a true brain but ganglia which are in control of all of their body functions.
Horseshoe Crabs:Horseshoe crabshave a brain the isaround the mouth.This brain controlsthe eyes and manydifferent otherfunctions.
Scorpions are invertebrates in the class Arachnida. Within the cephalothorax they havea "head" which includes a brain. Because they are invertebrates, they do not have aspinal column or spinal cord. They do have sensory organs which connect to the brainthrough neurons and nerves and nerve chords.