In the water, a jellyfish is elegant and mysterious. Out of the water, it is a blob. The reason is simple: a jellyfish is about 95 percent water, and only about five percent of its body is solid matter.
A jellyfish. Credit: NOAA.
A simple body
Jellyfish have no brain, no blood and no heart. Their bodies are made of three layers:
| Layer | What it is |
|---|---|
| Epidermis | the outer layer |
| Mesoglea | a thick, elastic, jelly-like middle layer |
| Gastrodermis | the inner layer |
Instead of a brain, an elementary nervous system called a nerve net lets a jellyfish smell, detect light and respond to other stimuli. A single digestive cavity works as both stomach and intestine, with one opening that serves as both mouth and anus.
Built in a circle
Jellyfish belong to the phylum Cnidaria, along with sea anemones, sea whips and corals. Like every member of that group, a jellyfish's body parts radiate from a central axis. This radial symmetry lets it detect and respond to food or danger coming from any direction.
About the sting
Jellyfish can sting with their tentacles. How bad a sting is varies, but in humans most jellyfish stings cause only minor discomfort.
Sources
Based on "What are jellyfish made of?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
ライセンス： CC0 1.0（パブリックドメイン） · 出典 oceanservice.noaa.gov
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