Jellyfish Inside Out (Excerpts)

Have you ever found a jellyfish washed up on the beach? There’s not much left to this animal when it’s out of the water and has begun to dry up, is there? Just a jelly goo. That’s because, unlike a fish, a jellyfish as no body skeleton to cling to, no tough layer of scales to hold it together. Unlike a fish, a jellyfish has no heart, no brain, no blood.

A jellyfish is a simple animal made up of three main parts: some tentacles, a mouth and a bell—that gooey, umbrella-shaped body. And it’s 95% water! With only two thin layers of cells and some gel between, both holding it all together, the jellyfish is pretty fragile.

At the Monterey Bay Aquarium we group all gelatinous animals under the term of jelly. Comb jellies, gooseberry jellies and Portuguese man-o-war look like jellies. They have the same gel-gooey look, and some of them can sure sting you, but they differ from jellyfish in interesting ways. On the following pages are the four kinds of gelatinous animals we’ll be talking about in the book. The diagrams will show you the differences between them.

Although it’s not the brainiest creature in the sea, the jelly isn’t totally without resources. Nerve cells signal the muscles in the bell that food or danger is nearby. The jelly can then move away from danger or toward the food source. Sensors around the rim of the bell let the jelly know if it’s heading up or down, so it can correct its position as it needs to.

Jellies don’t have real eyes either, just eye spots. While these eye spots don’t form images they might help jellies detect food and danger. When a jelly needs to travel toward the light, the eye spots help it find its way. The box jelly is an exception to this. It has amazingly complicated eyes that can actually form images!