Nervous System of Hydra vs Planaria: A Net and a Brain

Hydra is a few millimeters long and has no brain. A planarian is a flat worm with a head, two eyes, and something that behaves like a brain. Both are simple animals. They are not equally simple.

This post compares their nervous systems directly. The difference between a loose web of nerves and a gathered bundle of them is one of the oldest and biggest steps in animal evolution.

Freshwater hydras. Each one is a tube with a ring of tentacles at the top and a sticky foot at the bottom — Image: Stephen Friedt, CC BY-SA 3.0, via Wikimedia Commons

Hydra: A Nerve Net With No Brain

Hydra belongs to the phylum Cnidaria. It is a small freshwater animal, usually under 10 mm long when stretched out. It is named after the many-headed beast of Greek myth, because cut one off and it grows back.

Its body is a tube. The outside layer is the ectoderm and the inside layer is the endoderm. Between them sits a gelatinous layer called the mesoglea. At the top is a mouth ringed with tentacles. At the bottom is a sticky basal disc that holds it onto a rock.

Its nervous system is a nerve net. This is a loose mesh of nerve cells spread through the body wall, all linked to their neighbors. There is no central gathering point and no brain at all. A signal entering the net spreads in every direction at once, and the whole animal is involved in the response.

A nerve net drawn out. Compare it with the ladder in a later picture: no center, no cord, just a mesh — Image: Room316, CC BY-SA 3.0, via Wikimedia Commons

This works because Hydra does not do much that needs directing. Its responses are whole-body: the whole tube contracts, the tentacles bend, the mouth closes. Touch, chemicals in the water, and light are all handled by sensory cells wired into the same net.

The stinging cell is a weapon, not a sense organ

This is worth correcting, because it is a common mix-up. The tentacles carry cnidocytes, stinging cells holding a capsule called a nematocyst. Inside the capsule sits a coiled thread, and a trigger hair called a cnidocil sits on the outside.

When something touches the trigger hair, the capsule bursts. The thread fires out, injects neurotoxins, and holds on. It works like a tiny harpoon. That is how Hydra catches prey and defends itself.

A cnidocyte is assembled inside the body wall and then moves out to the surface, because it cannot move once it is full — Image: Bruno Gideon Bergheim, CC BY-SA 4.0, via Wikimedia Commons

A fired nematocyst. The thread has shot out of the capsule and the barb at its tip is what holds on — Image: David D. Brand, public domain, via Wikimedia Commons

So the nematocyst is the weapon and the nerve net is the nervous system. They are different jobs. Each cnidocyte sits inside the net and is wired into it, but the cell that fires is not the cell that senses.

Planaria: A Brain, Two Cords, and a Ladder

Planaria are free-living flatworms of the phylum Platyhelminthes. They live in fresh water, in the sea, and on land. They are bilaterally symmetrical, which means the left side is a mirror of the right and there is a definite front end.

That front end carries the sensory organs, and it is where the nervous system gathers. Planaria have a bilobed cerebral ganglion, usually called the planarian brain. Two ventral nerve cords run from it down to the tail. Cross-connections, called commissures, link the two cords all the way along. The result looks like a ladder.

The planarian ladder. One brain at the front, two cords down the body, and cross-links between them — Image: Putaringonit, CC BY-SA 3.0, via Wikimedia Commons

A planarian. The blunt triangular head at the front is where all the sensors and the brain are — Image: Eduard Solà, CC BY-SA 3.0, via Wikimedia Commons

Compared with a net, this is a real upgrade. Signals can now be sorted before they are acted on. The brain receives input from the front and decides, rather than the whole body reacting to everything equally.

What the eyespots actually do

The head carries two ocelli, pigment cups that are sensitive to light. The older description of them as simple eyespots suggests they form pictures, and they do not. Each one is a cup of dark pigment with a clear opening. The animal compares how much light reaches each cup, which tells it where the light is coming from.

That is enough to steer away from bright light and to stay in the dark, which is where a planarian does best. Two projections called auricles sit at the base of the head and sense touch and chemicals in the water. Together, eyes and auricles give the planarian a forward-facing view of its surroundings.

A brain that flickers

One detail makes the case that this is a real nervous system rather than a bundle of wires. The planarian brain shows spontaneous electrical oscillations of its own, much like the background activity an electroencephalogram records in other animals. The activity happens without any input.

The Two Side by Side

	Hydra	Planaria
Phylum	Cnidaria	Platyhelminthes
Body symmetry	Radial	Bilateral
Nervous system	Nerve net, no center	Brain, two cords, cross-links
Sensory organs	Sensory cells in the net	Ocelli and auricles on the head
Stinging cells	Present, on the tentacles	Absent
Gas exchange	Through the body wall	Through the body wall
Circulatory system	None	None
Digestive tract	One opening, in and out	One opening, branched intestine

The pattern is not that Hydra is a small planarian. They are on different branches of the animal tree, and each arrangement suits its own body. A net suits a radially symmetrical tube that reacts as one piece. A gathered system suits a creature that moves its front end first and needs to know what is in front of it.

What both of them share

Neither animal has a circulatory system or a respiratory system. Both take in oxygen straight through the body wall, because both are small and flat enough for diffusion to keep up. Both have a digestive tract with a single opening, so what goes in and what comes out share the same door.

Both also regenerate. Hydra grows back a lost head or a lost foot. Planaria can rebuild most of the body from a small piece, and they do it using neoblasts, stem cells that stay available throughout life and supply whatever cells are needed.

Where This Trend Goes Next

Once nerve cells start gathering at the front of the body, they keep gathering. An earthworm has a ladder of the same general kind as a planarian. A human has a brain and a spinal cord, with nerves running out to everything else.

An earthworm has the same ladder idea, plus a ganglion in each segment. The cord is segmented — Image: Looie496, public domain, via Wikimedia Commons

Where the trend ends: a central brain and spinal cord, with a separate peripheral network reaching every part of the body — Image: Laura Guerin, CK-12 Foundation, CC BY-SA 3.0, via Wikimedia Commons

None of this is a ladder of superiority. A nerve net is not a broken brain. It solves the problems of a radially symmetrical animal completely. The centralized system solves a different set of problems, and animals with those bodies got much better at moving forward through complicated space.

How a signal actually travels

In both animals a signal crosses from one nerve cell to the next across a tiny gap called a synapse. In a chemical synapse the first cell releases a chemical into the gap, and the next cell picks it up. That chemical is a neurotransmitter.

A chemical synapse. The signal jumps the gap as a chemical rather than travelling inside one long wire — Image: Looie496 and The US National Archives, public domain, via Wikimedia Commons

Nerve tissue first appeared in worm-like animals roughly 550 to 600 million years ago. Hydra sits below that line, which is why it never grew a brain.

Key Takeaways

• Hydra has a nerve net: a loose mesh with no brain and no central point.
• Planaria has a bilobed brain, two ventral nerve cords, and cross-links that make a ladder.
• Nematocysts are weapons, not sense organs. Hydra senses with cells in its nerve net.
• Planarian ocelli are pigment cups. They read the direction of light, not images.
• The planarian brain shows its own background electrical activity, like an EEG.
• Neither animal has a circulatory system, a respiratory system, or a second opening for the gut.
• Both regenerate. Planaria use neoblast stem cells to do it.
• A net suits a radial tube that reacts as one piece. A ladder suits an animal that moves head-first.

Frequently Asked Questions

Is a nerve net a simple brain?

No. A brain needs a gathering point where signals are combined and decisions are made. A nerve net has no such point. It is a different solution to the same problem, and it works well for the body it serves.

Why do planaria need a brain when Hydra does not?

Because of body shape. Hydra is a tube with radial symmetry and reacts as one piece, so equal treatment from all sides suits it. A planarian moves head-first, so the sensors at the front matter more than the ones at the tail. A brain lets it favor them.

Do planaria really learn?

They can be trained to associate a stimulus with a reward, and they retain some of what they learned. Their simple anatomy, fast regeneration and reliable behavior made them a standard choice for studying memory and regeneration for decades.

What is the difference between an ocellus and an eye?

An ocellus is a simple light-sensitive cup. It tells the animal which direction light is coming from. An eye forms a focused image and has lenses, chambers and a retina. Planaria have the first, not the second.

Can a planarian regrow a whole animal from a piece?

A planarian can rebuild most of its body from a small fragment, and some species can regenerate from a piece containing no brain at all. The supply of neoblast stem cells is what makes this possible.

Is centralized nervous system always better?

No. It costs more to build and maintain. A net is cheaper and is the right answer for simple radial animals. Centralization pays off when an animal has to move through a complex environment with a definite front end.

Sources: Wikipedia articles on Hydra, cnidocyte, nematocyst, nerve net, planarian, triclad, neoblast, regeneration in biology, nervous system, neuron, chemical synapse, ganglion and the earthworm nervous system. Images: Wikimedia Commons, with authors and licenses noted in each caption.