Let's see if you can solve a riddle.

What can be as small as the head of a pin, or as large as a desk, lives in the sea (but also sometimes in fresh water!), has no brain, but can kill a human? 

Do you have a guess?

That's right, it's a jellyfish! Despite the potential danger of these invertebrates, jellyfish have an almost hypnotizing, soothing quality about them when they swim in a tank, and can be very beautiful as well. 

Jellyfish (phylum Cnidaria) come in many different colors and sizes. Most everyone knows that they live in the oceans, but there are also some freshwater specimens as well - even in Oklahoma!

The squishy-looking "jelly" part of the jellyfish is called the bell, and it is mostly made up of water. There is a thin layer of skin on top of the bell, which is where the nervous system is located. But since jellyfish have no brain, they also don't have a central nervous system. Their nervous system is called a "nerve net." Jellyfish also do not have eyes, but some have ocelli, which can detect light. You might remember ocelli from other entries - many insects have ocelli instead of eyes as well. 

Not all jellyfish stings are harmful to humans - in fact, many are so slight that we would never even if a sting occurred. Jellyfish sting by a type of stinging cells that they have on their tentacles, called nematocysts. These are used to kill fish or other small animals that are unfortunate enough to get entangled. Jellyfish do not actively hunt their prey - they simply eat whatever swims through and is stung in their tentacles. They have a "mouth" under the bell, and a simple or "incomplete" digestive system in that they expell wastes through that same hole. Many other invertebrates have this same sort of digestive system. 

Large jellyfish populations can be a sign that the habitat they're living in is in trouble. Too many jellyfish can be a sign of overfishing. They can take over the ecosystem they're in, essentially choking out other marine life. 

Jellyfish washed ashore in CT

Isn't that one beautiful? It almost looks like a flower. My sister spied this one and took its picture for me.

Happy Friday everyone!