What Exactly is a Jellyfish?

What exactly is a jellyfish?

A jellyfish is a 'free-swimming marine coelenterate that is the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells' (http://www.merriam-webster.com/dictionary/jellyfish). This definition is very complicated and has many unusual words in it, a shorter definition by the same site is 'a sea animal that has a very soft body and that can sting'. Of course, put anything in such simple terms and it loses a lot of its value so in this post I will explain what a jellyfish is.

So what is a jellyfish REALLY?

Well, jellyfish are a complicated structure of organs just like other multi-cellular organisms. I have provided a cross-section to demonstrate its complexity and will briefly describe their evolution and dip slightly into their anatomy

http://www.pic2fly.com/Jellyfish+Body+Parts+Diagram.html

Evolution and Anatomy

It is estimated that jellyfish have been on the earth for more than 500 million years. This figure comes from an exiting discovery made in 2007.

In 2007, scientists discovered a 500 million year old jellyfish fossil and were able to use it to compare our modern day jellyfish to their ancient ancestors. Upon comparison, the fossil showed a typical bell shape and tentacles much like the species swimming around today. However upon closer inspection a shocking discovery was made. It turned out that either the complexity of modern jellyfish developed rapidly about 500 million years ago, or that jellyfish are even older and developed long before that time. This suggests that jellyfish could be close to even 600 million years old. That is quite a feat and they achieved it using their simple anatomic structure. (source: http://www.livescience.com/1971-oldest-jellyfish-fossils.html)

Anatomy

As shown in the diagram above, jellyfish have simple but effective cellular structures. I will explain the terms used in the cross section to further better your knowledge of the anatomy.

Mesenchyma

Now although it's name suggests something quite important and complicated, the mesenchyma is nothing more than a cellular structure that is transparent in most cases but sometimes has pigment to cause colour. This cellular structure is like a firm gel that formats the main body of the jellyfish's bell (body)

Gastrovascular Cavity/Gastrodermis/Gastric Filaments

The Gastrevascular Cavity is a large chamber in which food is digested. It can split into many canals and is commonly known as a 'blind gut' as food enters through the same place that the waste leaves.

 

the Gastrodermis is quite simply the layer of cells that act as the lining of the Gastrovascular Cavity.

 

Gastric Filaments are used to stun and kill living animals that are taken into the Gastrovascular cavity as food.

Gonads

To put it briefly, Gonads are a jellyfish's sex organs.

 

Next post:

Where to find jellyfish and why they found there...