Phylum Cnidaria

Phylum Cnidarians are predominantly saltwater aquatic species, sessile or free-swimming. They are diploblastic, which exhibit tissue level of organisation and show radial symmetry. They have stinging capsules or nematocysts present on their tentacles and bodies (cnidoblasts and cnidocytes). Cnidoblasts help in catching the prey, anchorage, and anti-predator defence. Nematocysts present on cnidoblasts releases hypno toxins or neurotoxins, which paralyses the prey or may result in death. Most Cnidarians use their nematocysts to sting and capture food.

Their work is a little harder since they aren’t a strong predatory species, so they can’t easily hunt down prey; therefore, they depend on the incoming food. 

The name Cnidaria comes from the stinging capsules or nematocysts present on their tentacles and bodies (cnidoblasts and cnidocytes). They have a single opening, mouth on hypostome, in a central gastro-vascular cavity. Digestion can be extracellular or intracellular. Cnidarians, such as corals, have skeletons made of calcium carbonate. The body forms they exhibit are polyps and medusas. Polyps are sessile and cylindrical, like a hydra, whereas medusae are umbrella-shaped and free-swimming, like Aurelia or jellyfish. Those Cnidarians that exist in both forms undergo alternation of generation (metagenesis), that is, polyps form medusae asexually, and medusae form polyps sexually (e.g., Obelia).

How Does Phylum Cnidaria Get Their Food?

All the species in the phylum Cnidaria may look very different, but they share one common feature—they have stinging cells called nematocysts. The stinging cells may be useful for anti-predator defence or for catching prey (food).

Cnidarians Eat, But How?

We know pretty well how animals eat – the food enters the body through the mouth, travels through the digestive system, and exits through the anus. Do you wonder how jellyfish consume food?

As jellyfish have no mouth or anus, and their transparent, small bodies are difficult to imagine as having a digestive system, it is challenging to comprehend how they eat or digest food.

There are no obvious means of eating and digesting food in most of the Cnidarians. They can’t make their food through photosynthesis as they are animals, not plants. So, most of the Cnidarians use their nematocysts to sting and capture food.

When nematocysts present on the animal’s body come in contact with a prey (potential meal), they release stinging barbs into the prey, which stuns or even kills them. The severity of the nematocyst’s sting depends on the species—jellyfish being the most lethal. It releases hypno toxins or neurotoxins (a combination of proteins and phenols), which paralyses the prey or may result in death.

The size of their mouth varies by species, but even the tiniest coral polyps have a mouth area. It does not look or function like a human mouth, but it serves the same purpose.

What Do Cnidarians Eat?

Cnidarians have to work a little harder as they aren’t strong predatory species; they can’t easily hunt down their prey, so they must rely on the incoming food. This comes from floating in the water column or from coming in contact with the Cnidarian by accident.

All Cnidarians are carnivores (meat-eating animals) whose energy requirements are derived solely from animal tissues. They use their cnidae and toxins to capture food (prey). Meat consumption is influenced by the size of the species eating it, which ranges from microscopic zooplankton (animal larvae) to small fish.

Conclusion

Cnidarians are diploblastic and show radial symmetry. They are predominantly saltwater aquatic species – sessile or free-swimming. They possess stinging capsules or nematocysts present on their tentacles and bodies (the cnidoblasts and cnidocytes). Catching prey, anchorage, and anti-predator defence are some of their functions. They have a tissue-level organisation. Their mouth is positioned on a hypostoma, and their gastro-vascular cavity is central. 

Cnidarians, such as corals, have calcium carbonate skeletons. They have polyps and medusas as their basic forms of body. The polyp is stipulate and cylindrical like a hydra, while the medusa is umbrella-shaped and free-swimming, like an aurelia or jellyfish. Cnidarians that display both forms of life exhibit alternation of generation (metagenesis), meaning medusae generate polyps asexually, while polyps generate medusae sexually (i.e., Obelia).