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8.3. Coelenterates

Phylum Cnidaria or Coelenterates

Cnidarians or coelenterates are animals that, like sponges, have sac-shaped bodies but their cells have already specialized somewhat and have muscle and nerve cells. Even so, they lack true tissues and organs and are, along with the porifera, the simplest organizational animals.

Cnidarians have radial symmetry, that is, their body can be divided by different imaginary planes, in equal parts.

Cnidarians are invertebrate animals that live in the aquatic environment, either  marine (jellyfish, anemones, corals) or  freshwater (hydra).

Its body is shaped like a sac, with a gastric cavity called the coelenteron, which communicates with the outside through an opening that functions as a mouth and anus, which is surrounded by tentacles with stinging cells, called cnidoblasts, which secrete a liquid with which paralyze their predators and their prey.

Cnidarians are carnivorous, although they do not need respiratory or excretory system, since each cell is individually responsible for exchanging gases and expelling waste to the outside.

Cnidarians can present two forms:

  • Polyp, sessile, sac-shaped, whose mouth or opening is oriented towards the upper part of the animal. Its reproduction is asexual, by fragmentation  or budding

Tubastraea. un tipo de pólipo

By Will Thomas (Sun Coral) [CC BY 2.0 ], via Wikimedia Commons

  • Medusa, free-living organism, with the appearance of a gelatinous umbrella and whose reproduction is sexualIn jellyfish, the mouth is located in the lower part of the body.

Medusas en un acuario

Many species have alternating reproduction, in which the two generations or vital forms alternate. The egg cell or zygote, formed by the union of the gametes (egg  and sperm) of the jellyfish, gives rise to a polyp that, when fragmented, will give rise to a jellyfish.

In other cases, the polyps reproduce by budding, and the new individuals remain with the parent, forming large colonies, and as they develop a calcareous exoskeleton, they form coral reefs.

Curiosity: Anemones and clownfish

Even if you haven't seen the movie Finding Nemo, you probably know Nemo the clownfish. Clownfish and anemones live in a mutualistic relationship in which both species benefit.

The clown fish is protected by the tentacles of the anemones, since they have a mucous coating that immunizes them against the deadly stings of the anemone, with which they kill other fish. In addition to being protected from predators, clownfish feed on leftover food.
The clown fish cleans the anemone of parasites and, in addition, the currents it generates around the tentacles of the anemones cause them to oxygenate.
The name of this fish, Nemo, seems to come from Captain Nemo, the character in Jules Verne's novels who, in turn, was called Nemo (nobody in Latin) because nobody knew his name if he had ever had it. But others say that it was called that because of the clownfish's relationship with the anemones .

Pez payaso en una anémona

Imagen de R/DV/RS con licencia CC.