3- Jellyfish : Body Anatomy

To understand how these intriguing bodies of mostly water live, we must get our basics right. So this post will focus on how Jellyfish are fundamentally arranged according to nature.

The body of a jellyfish remains more or less the same across different species. It has three basic components:

1. Dome-Shaped main body
2. Tentacles
3. Oral Arms

source: http://portphillipmarinelife.net.au/species/7762

(Now, if you are like me, you would have probably never heard of the word “oral arms”. Turns out, it is a very common to mistake these long projections called are oral arms for tentacles.)

Let’s understand these components in more detail now.

1. Dome-Shaped main body- This is also called the bell or the exumbrella. The underside of the exumbrella is called the subumbrella. The edge or rim of the bell is called the bell margin.

In those jellyfish species that have eyes, the eyes are typically located along the Bell Margin. The underside of the bell contains the mouth of the jellyfish called an orfice. Orfice is the only opening in jellyfish and serves as the following:

1. Mouth
2. Anus
3. Allows passage of sperms and eggs into the water.
4. Serves as the gastrovascular cavity. This means that it functions as a gullet, stomach, and intestine all in one. The gastronomic cavity extends and branches around the bell and forms various branches and canals. These radial canals (as they radiate outwards) and their various branches eventually meet the edge of the bell where they unite to form a thick canal called a circular canal. Circular Canal runs around the circumference of the jellyfish.

source: http://www.biozoomer.com/2014/05/jellyfish-nutrition.html

2.  Tentacles-  Their functions include:

1. Paralysing the prey or predators
2. Aid Movement
3. Aid capturing the prey

The number of tentacles and their lengths in a jellyfish varies in different species.

3. Oral Arms- Thicker strands that look like tentacles surround the orfice or mouth. These are called oral arms and are usually always four in number. Their function:

1. Capture of Prey- Oral Arms hold the stinging cells which inject venom and thus, as the name suggests, aid in the capture of prey. These along with the tentacles help transport the food to the mouth.
2. Reproduction- These are of utmost importance when it comes to reproduction. Jellyfish are hermaphrodites, which means that they produce both eggs and sperm. Moreover, they reproduce both sexually and asexually! To add to their amazement, the fertilized eggs or zygotes may develop inside or outside a female’s bell shaped body. Inside the body, the eggs are kept in brood pouches in the oral arms and are then released at a particular stage. This internalisation is a bit rare because most jellyfish eggs develop outside the body.  If the gametes fuse outside the jellyfish’s body, the egg formed floats in water and latches onto a solid surface to develop into a polyp and slowly an entire jellyfish.  (Find more about this in the next few posts about reproduction in jellyfish)