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