MadSci Network: General Biology

Re: Jellyfish

Area: General Biology
Posted By: Eli Hestermann, Grad student Biological Oceanography. Woods Hole
Date: Thu Apr 18 15:53:55 1996

Wow! Lots of questions here; let's see if I can help. Here are the answers, in order:

1. As you might guess, jellyfish are named that because they look and feel like they're made of jelly.

2. Jellyfish are clear because that makes them hard to see in the water. That makes it harder for predators to see and catch them.

3. Most jellyfish actually have two forms, called the polyp and the medusa. The polyp is attached to the bottom of the ocean, and the medusa grows on the end of a polyp and eventually comes off and swims away. A sea anemone is a good example of what the polyp form looks like. The medusa is what most people think of when they think about jellyfish. It is the form that swims.

4. A dead jellyfish can still sting, because stinging is like a reflex and it operates on its own.

5. Jellyfish are made of the same types of things we are: mostly proteins, sugars, and fats. They have two main layers, called the ectoderm (the outside layer) and the endoderm (inside) with a jelly-like material in between.

6. Jellyfish use tentacles to trap smaller animals and then drag them to the middle of the body and stuff them into a mouth-like opening. They digest the food inside a pocket in their body, and then spit the rest back out.

7. Jellyfish don't have brains. What they do have is a 'nerve net' that allows them to use all their muscles at the same time. This is how they swim. They don't have eyes like we do, but they can see light and dark.

8. Ctenophores, or 'comb jellies' are major predators of jellyfish. They are related to jellyfish and strain the water with tentacles that look like combs. This allows them to catch jellys.

I hope this answers your questions. Keep up your interest in the oceans!

Eli Hestermann
Woods Hole Oceanographic Institution

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