The super-phylum Radiata describes a unique and interesting group of animals. In this video lesson you’ll identify various animals that belong in this classification as well as describe the characteristics that make them so identifiable.
Whenever I visit the aquarium, one of my favorite exhibits to see is the room with all the jellies. There are moon jellies, upside-down jellies, neon-colored jellies, big ones, small ones..
.Jellies are really cool, but did you know they are also related to other interesting animals, like corals and sea anemones? They all belong to the Animal sub-kingdom Eumetazoa in a super-phylum called Radiata.This classification gets its name from the radial symmetry that the animals have.
Just like a bike wheel, the symmetry of the body is the same as it comes out from the center.The animals of Radiata also share a simple body structure that has true tissues. These are specialized tissues and cells. In animals such as humans, these may be as complex as internal organs and connective tissues like ligaments. In Radiatans, they are simple forms of tissues and nerves.
The animals of Radiata belong to one of two phyla: Cnidaria or Ctenophora. The Ctenophora are the comb jellies, while the Cnidarians are all of the other jellies, corals, and anemones you are familiar with. They are different enough to warrant belonging to separate phyla, but they do share a few similarities that make them all Radiatans. Let’s first look at their similarities and then examine what makes Cnidarians and Ctenophores different from each other.
Shared Characteristics of Radiata
The Radiata body structure is made of two layers of tissue: the external epidermis and the internal gastrodermis.
‘Epidermis’ means ‘outer skin’ in Greek, and this is much like your skin, also an epidermis! ‘Gastrodermis’ means ‘stomach skin,’ much like your gastrointestinal tract is your stomach inside your body. Unlike us, though, Radiatans don’t actually have a stomach – they simply don’t need one. In fact, these animals have only one opening where everything happens. Here, food comes in, and then later waste goes out!In between the inner and outer ‘skins,’ Radiatans have a middle layer called the mesoglea (‘meso’ means ‘middle’). This layer makes up most of the body and is mainly composed of water. It also provides some skeletal structure to the animals.
If you looked at Ctenophores and Cnidarians together you might not be able to determine what is different about them.
But Ctenophora (the comb jellies) have a unique feature that really sets them apart – their combs!The combs on comb jellies are plates of fused cilia that propel the jellies through water like little oars. This movement produces an amazing effect. When you see comb jellies moving through the water they look like miniature rainbows swimming around. This happens because of the way the light hits the combs as they move.Comb jellies are also different because they don’t sting. The stinging tentacles of Cnidarian jellies are used to catch prey, but comb jellies do this with cells that stick instead of sting.
Most people are pretty familiar with Cnidarians because they are such a unique group of animals.
Nothing else looks like quite like them!Cnidarians have two distinct forms. The medusa form is what most people imagine when they think of a typical jelly: the rounded top with a mouth and tentacles underneath. The other form is the opposite of the medusa.
This is called the polyp form and is where the tentacles and mouth are facing upward. This is what most people imagine when they think of a typical sea anemone.As you have seen with most jellies, medusae are free to float around the water at will. They look like swimming umbrellas as they move around. Polyps, on the other hand, are stuck in place. You’ve never seen an anemone floating around in the water because they are anchored down in one location.
Some Cnidarians will exist as a polyp before maturing into a medusa. Others will only exist as a medusa, while some only exist as polyps. Corals are examples of polyp-only Cnidarians, but they take it one step further and excrete a hard external skeleton, which is why they look like rocks along a reef.As mentioned before, Cnidarians are the ones you want to stay away from because these will definitely cause you pain! Cnidarians have special stinging cells that both ward off predators and help capture a tasty meal. Some Cnidarians are quite deadly because they have very toxic stinging cells, while others will only leave you with a lasting memory of pain. While this may be the case, if you’re like me, you’d rather enjoy a Cnidarian from afar, where you don’t have the chance to find out which kind it is!
Animals come in all forms, shapes, sizes, and colors.
One of the best ways for us to classify animals is through their body symmetry, and the animals of Radiata are no exception. The animals in this super-phylum all have radial body symmetry, which is where the name ‘Radiata’ comes from.The two phyla that make up Radiata are Ctenophora, which are the comb jellies, and Cnidaria, which are the other jellies, corals, and sea anemones.Besides being radially symmetrical, all Radiatans also have a body made up of two layers of specialized tissue separated by a gelatinous middle layer. They also have one single opening that serves as a route for both food intake and waste output.Comb jellies get their name from rows of fused cilia that form combs.
These combs act like little oars, propelling the comb jellies through the water. Cnidarians are a bit different in that they have stinging cells and may either be medusae or polyps. Some even go so far as to form hard skeletons, like corals that anchor themselves along reefs.
At the end of this lesson you should be able to:
- State the main characteristics of animals in the Radiata super-phylum
- Compare Cnidaria and Ctenophora and give examples of each
- Explain the polyp and medusa forms of cnidarians