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What are Jellies?

by AMNH on

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What exactly is a jelly?

It's is a general term for any kind of transparent, gelatinous (or jellylike) animal that floats in the ocean.

Multi-tentacled jellyfish floats undersea.
The Crown Jellyfish is one of many amazing species you can encounter in The Jelly Dome.
Courtesy of D. Keats/Flickr

Jellies belong to two different groups, cnidarians and ctenophores, and while members of the two groups may sometimes look alike, they are not all closely related. 

A jellyfish with tentacles extending from an umbrella-like structure floats undersea.
The Pacific Sea Nettle is part of the group known as cnidarians, or true jellyfish, that you can see in The Jelly Dome.
Courtesy of HRae/Wikimedia Commons

Hundreds of jelly species live in oceans around the world, from shallow bays to the deep sea. Some even live in fresh water. The most common jellies are true jellyfish (cnidarians) and comb jellies (ctenophores).

Most jellyfish have long stinging tentacles and have oral-arms that help catch and eat food. Comb jellies have oval bodies lined with rows of fluttering cilia. Instead of stinging, they use their tentacles to pull prey into their large mouths.

The Arctic comb jelly is bubble-like and translucent in form; additionally, it is faintly bioluminescent with an oil slick-like rainbow effect.
The Arctic Comb jelly is a ctenophore, and strange among this group in its preference for dwelling in cold waters. 
Courtesy of K. Raskoff/Wikimedia Commons

Whether they’re cnidarians or ctenophores, jellies have bodies that are made of two transparent layers—an outer one for protection and an inner one for digesting food. Between the two layers, you’ll find nothing but a watery gel—in fact, their bodies are more than 95% water! Aside from these few parts, there’s not much more to them. These amazing animals get along with no bones, no head, no legs—not even a brain!