A Hydra has no heart. It has no blood, no vessels, and no pump. It still moves oxygen, food, and waste to every one of its cells, and it does it with nothing but a thin body wall and water.
This post explains how that works, and why the size of an animal decides the whole question.
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| A Hydra is a tube about a centimeter long, with tentacles around one opening — Image: Przemysław Malkowski, CC BY-SA 3.0, via Wikimedia Commons |
The Problem: Moving Things Without a Pump
Transport means getting a substance from where it is to where it is needed. In a large animal that job is a whole organ system. Blood carries oxygen and nutrients down arteries and back up veins, and a heart provides the pressure that pushes it along.
A Hydra is not large. Its body is a tube, and a fully extended animal reaches up to about 10 millimeters. Sources give a column length of roughly 3 to 30 millimeters when extended, with a width near 1 millimeter. Older descriptions give a smaller figure, but even the largest numbers describe an animal you could cover with a fingernail.
At that scale, a pump would be pointless. Anything diffusing through the body reaches every cell in a fraction of a second, and the animal spends nothing on the machinery.
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| A living Hydra. Every part of the body surface sits against fresh pond water — Image: Corvana, CC BY-SA 3.0, via Wikimedia Commons |
The Body Plan That Makes Diffusion Enough
The reason diffusion works here is the shape of the animal. A Hydra has two cell layers, and each one is about a single cell thick.
- Epidermis, the outer layer, in contact with the pond water.
- Gastrodermis, the inner layer, lining the digestive cavity.
- Mesoglea, a jelly-like layer between them. In cnidarians it is not entirely cell-free; it holds a small number of amoeba-like cells, but it carries no vessels and no transport system of its own.
That thinness is the whole design. An outer cell is directly against the water outside. An inner cell is directly against the fluid in the stomach. Almost nothing in the animal is more than one cell away from a liquid it can exchange with.
A single Hydra holds 50,000 to 100,000 cells, built from three stem cell populations that keep renewing themselves. The number is not small, but the body is a tube, so the distance any molecule has to travel stays tiny.
Where Hydra sits in its phylum
Hydra belongs to the phylum Cnidaria, alongside jellyfish, sea anemones, and corals. Cnidarians are named for their stinging cells, the cnidocytes, which fire a harpoon-like thread at prey. The phylum is ancient. Fossils appear in rocks from about 580 million years ago, and a molecular clock puts the lineage even older.
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| Hydra is the small freshwater member of a phylum that is mostly marine — Image: Frédéric Ducarme, CC BY-SA 4.0, via Wikimedia Commons |
How Diffusion Actually Moves a Molecule
Diffusion is the spread of a substance from where it is concentrated to where it is less concentrated. The name comes from the Latin diffundere, which means to spread out.
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| Particles keep moving at random. Only the balance of that motion drags them downhill — Image: BruceBlaus, CC BY 3.0, via Wikimedia Commons |
Two details matter, and both are usually skipped.
First, molecules never stop moving and never move in one direction only. Every molecule in a gas or a liquid is jiggling, and it will randomly cross in whichever direction it happens to be heading. A molecule inside a Hydra will sometimes wander outward, against the gradient.
Second, what counts is the net movement. If more molecules cross inward than outward, the substance is moving inward, even though individual molecules are going both ways. Biologists say "net diffusion" precisely because they mean the difference, not the total.
Adolf Fick, a 26-year-old anatomy demonstrator in Zürich, published the first law of diffusion in 1855. He compared the spread of a substance to the conduction of heat, which had been described by Fourier in 1822, and to electric current, described by Ohm in 1827. Diffusing substances without bulk motion, Fick noted, is a different thing from the flow of a whole mass of liquid.
What a membrane lets through
A Hydra has no cell membranes it has to push material across, because its cells are exposed. Still, the rules are the same ones every cell obeys. Biological membranes are built from a phospholipid bilayer, which is hard for charged particles to cross. Small, nonpolar molecules such as oxygen and carbon dioxide pass straight through. Ions, proteins, and sugars do not, unless a channel protein lets them in.
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| Gases cross the lipid layer freely. Charged particles need a channel — Image: BruceBlaus, CC BY 3.0, via Wikimedia Commons |
The Gastrovascular Cavity Does Two Jobs
Inside the tube is a single fluid-filled space, the gastrovascular cavity. Cnidarians have one opening, not two. The same mouth takes food in and lets waste out, and there is no separate anus anywhere on the animal.
That cavity is often described only as a stomach, which undersells it. It is a stomach, a delivery system, and a skeleton at once. When a Hydra closes its mouth, the cells around the opening fuse together and trap the water inside. The water then acts as a hydrostatic skeleton, like the air in a balloon, holding the body column firm instead of letting it collapse.
Getting the food in
Feeding starts with a harpoon, not with suction. Each tentacle is covered in cnidocytes, and each holds a capsule called a nematocyst with a coiled thread inside. When a cnidocyte is triggered, the thread fires out under pressure, either stinging the prey or coiling around it.
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| A capsule, a coiled thread, and a trigger hair. This is how food is caught before digestion even starts — Image: Antropoteuthis, CC BY-SA 4.0, via Wikimedia Commons |
The tentacles can reach four or five times the length of the body, and most of them join in on the attack within 30 seconds. Within two minutes the prey is being moved toward the mouth, and within ten minutes it is inside and digestion has begun. A Hydra can swallow prey more than twice its own size by stretching its body wall to do it.
Sending it out again
Once inside, the food is broken down in the cavity by enzymes from gland cells in the gastrodermis. The nutrients then reach the cells lining the cavity directly. Larger remains that cannot be digested are not carried away by diffusion at all. After two or three days the animal expels them through the mouth with a fast radial contraction of the whole body column.
Breathing and Excretion by the Same Route
Respiration works the way feeding does, in reverse. Oxygen dissolved in the pond water is more concentrated outside the animal than inside its cells, which are using it up. It diffuses inward through the epidermis. Carbon dioxide is being made faster inside than outside, so it diffuses outward.
Excretion partly follows the same road. Ammonia and other small waste molecules are small enough to pass straight out through the body surface. Anything larger leaves the way undigested food does, through the mouth.
This is why being thin is not a detail but a need. A thick body would put a cell deep inside, with nothing but tissue between it and the water, and diffusion would be too slow to keep it alive.
Water is a separate problem
Soluble waste and dissolved food are not the only things moving. Water has to move too, and it moves by osmosis rather than by plain diffusion. Osmosis is the net movement of solvent across a membrane that lets water through but holds back solute. A Hydra in fresh pond water is almost always in a dilute medium, so water tends to enter its cells and leave by the same outward route.
How a Sponge Solves the Same Problem More Simply
A freshwater sponge faces the same problem with even fewer moving parts. Sponges have no nerve cells and no muscle cells of any kind. They move water by using flagella.
Inside the sponge, thousands of cells called choanocytes beat a whip-like flagellum. The beating creates a current that draws water in through pores in the body wall, past the choanocytes, and out through a large opening at the top. Food and oxygen arrive in that current. Waste leaves in it.
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| Water enters small pores, is driven past the flagellated cells, and leaves through the top opening — Image: Philcha, CC BY-SA 3.0, via Wikimedia Commons |
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| A sponge in a river. It moves its own water with flagella and has no circulatory system at all — Image: Kirt L. Onthank, CC BY-SA 3.0, via Wikimedia Commons |
Compare the two designs. The sponge builds a pump out of thousands of beating flagella. The Hydra builds no pump at all and waits for molecules to wander the short distance on their own. Both work, because in both animals every cell is close to moving water.
The Transport Routes in One Table
| Substance | Route | Mechanism | Driven by |
| Oxygen | inward through the epidermis | diffusion | more oxygen outside than inside |
| Carbon dioxide | outward through the epidermis | diffusion | more carbon dioxide inside than outside |
| Digested food | from cavity fluid to gastrodermis cells | diffusion | concentration gradient across the inner layer |
| Water | in and out across the cell membrane | osmosis | solute concentration difference |
| Ammonia | outward through the epidermis | diffusion | concentration gradient |
| Undigested remains | out through the mouth | muscular contraction | radial squeeze of the body column |
| Body support | trapped cavity water | hydrostatic skeleton | water held in a closed cavity |
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means here |
| Diffusion | the net movement of molecules from where there are more of them to where there are fewer. Only the imbalance moves anything |
| Gastrovascular cavity | the single internal space of a Hydra. It digests the food, delivers it to the body wall, and holds the body firm |
| Mesoglea | the jelly layer between the two cell layers of the body wall. It sits between cells rather than being a third layer |
| Epidermis | the outer cell layer of the body wall. Oxygen and carbon dioxide cross it by diffusion |
| Gastrodermis | the inner cell layer, lining the gastrovascular cavity |
| Osmosis | the movement of water across a membrane, from the side with more free water to the side with less |
| Flagellum | a whip-like tail. A sponge beats flagella to drive water through its body |
| Radial contraction | a squeeze of the body column that pushes undigested remains out through the mouth |
Terms used in this article, defined as they are used above.
Key Takeaways
- Hydra has no heart, no blood, and no vessels. Its whole transport system is diffusion into and out of a body wall two cells thick.
- The body reaches up to about 10 millimeters extended, with a width near 1 millimeter. Anything that small is within diffusion range of every cell.
- Diffusion is net movement. Molecules scatter randomly in both directions, and only the imbalance moves the substance.
- The gastrovascular cavity is a stomach, a delivery system, and a skeleton at the same time. Trapped cavity water holds the body firm.
- Oxygen and carbon dioxide cross the epidermis by diffusion. Ammonia leaves the same way.
- Undigested remains are not carried by diffusion. A radial contraction of the body column pushes them out through the mouth.
- A sponge solves the same transport problem with flagella and no nerves or muscles. Thinness is the shared need.
Frequently Asked Questions
Does a Hydra really have no circulatory system?
Correct, and that is the point of the animal. It has a gastrovascular cavity, which distributes digested food, but no vessels, no heart, and no blood. Everything moves by diffusion or by water pressure.
How big is a Hydra?
A fully extended animal reaches up to about 10 millimeters, with a body column commonly given as 3 to 30 millimeters and a width near 1 millimeter. Very small, and small enough that diffusion keeps up.
Why is diffusion faster in small animals?
Diffusion is slow on its own, but time to cross scales with the square of distance. Halve the distance and molecules arrive four times faster. In a millimeter-wide body that is no time at all.
What is the difference between diffusion and osmosis?
Diffusion is the general spread of a substance down its own concentration gradient. Osmosis is specifically the movement of water across a membrane that passes water but not solute. Both produce net movement from higher concentration to lower.
What is the mesoglea for?
It is the jelly-like layer between the two cell layers. In a Hydra it carries no vessels and does not transport anything. In jellyfish it is thick and springy, and it is the only supporting structure the animal has.
How does a Hydra get rid of waste?
Two ways. Small dissolved waste such as ammonia diffuses out through the body surface. Larger indigestible remains are pushed out through the mouth by a rapid contraction of the body column.
Why does a Hydra have only one opening?
Cnidarians have a single opening that serves as both mouth and exit. It is not always open. When a Hydra is not feeding, the cells around it fuse shut, which is also how the trapped water inside holds the body column rigid.
Sources: Wikipedia articles on Hydra (genus), Cnidaria, gastrodermis, basal lamina, diffusion, osmosis, mesoglea, and the freshwater sponge Spongilla lacustris, with Fick's laws as described in the diffusion article. Images: Wikimedia Commons, with authors and licenses noted in each caption.
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