The Cnidarian phylum (cnidarios) is represented by hydras, jellyfish or jellyfish, corals and sea anemones.
The cnidaria are the first animals to have a digestive cavity in the body, a fact that generated the name coenter, highlighting the evolutionary importance of this structure, which was maintained in other animals. The presence of a digestive cavity allowed the animals to ingest larger portions of food, because in it the food can be digested and reduced to smaller pieces, before being absorbed by the cells.
Based on the external aspect of the body, cnidaria present radial symmetry. They are the first animals on the evolutionary scale to have true tissues, although they have not yet formed organs.
In the cnidarian phylum there are basically two morphological types of individuals: the jellyfish, which are native and the polypswhich are sessile. They can form colonies, such as corals (sessile colonies) and caravels (floating colonies).
Polyps and jellyfish, apparently very different forms from each other, have many features in common that define the phylum, as we shall see.
In cnidaria there is a special type of cell called cynidocyte, which despite occurring along the entire surface of the animal, appears in greater quantity in the tentacles. When touched, the cnidocyte launches the nematocyst, a penetrating structure that has a long filament through which the stinging liquid contained within is eliminated. This liquid can cause serious burns to man.
These cells participate in the defense of cnidaria against predators and also in prey capture. Using the substances produced by cnidocytes, they can immediately paralyze the small animals captured by their tentacles.
It was the presence of the cnidocyte that gave the nemo to the phylum Cnidaria (who have cnida = nettle)
Both the polyp and the jellyfish have a mouth that opens in the gastrovascular cavity but have no anus. The food ingested by the mouth, falls into the gastrovascular cavity, where it is partially digested and distributed (hence the name gastro, feeding, and vascular, of circulation).
After the extracellular phase of digestion, the food is absorbed by the cells lining the gastrovascular cavity, completing the digestion.
Digestion is therefore partly extracellular and partly intracellular. Unusable remains are released from the mouth. In the oral region are the tentacles, which participate in the capture of food.
The cell layers that occur in the cnidaria are: epidermis, which covers the body externally, and the gastrodermis, which lines the gastrovascular cavity. Between the epidermis and the gastroderm there is a gelatinous layer called mesogleia. This layer is more abundant in the jellyfish than in the polyps, so the jellyfish look gelatinous, a fact that earned them the popular name of "jellyfish."
The epidermis and gastroderm are two cell layers derived from embryonic tissues generically called germ leaflets. The epidermis derives from the germ leaflet called ectoderm (ect = external, dermis = lining tissue), which lines the embryo's body externally; Gastrodermis is derived from the leaflet called endoderm (endo= internal), which covers the digestive tract of the embryo. Cnidaria are considered animals diblastics.
Porifers have been considered diblastic in several books, some consider them as ablastic (a = negation; blast - woven). However, porifers develop only up to the blastula, so they do not form embryonic leaflets, which explains the non-formation of true tissue in the phylum.
The other animals are triblastics or triploblastsbecause they have three germinal leaflets: the ectoderm, the endoderm and the mesoderm (meso= in the middle), which develops between the ecto and the endoderm.
The Cnidaria are the first animals to have nerve cells (neurons). In these animals, neurons are diffused throughout the body, which is a primitive condition among animals.
Cnidaria present contraction and extension movements of the body, and may present dislocations. They are therefore the first animals to perform these functions.
Porifers are animals that live fixed to the substrate, showing no displacements.
In polyps, the ability to move is reduced and may be of the "palm-sized" or "somersault" type. In the jellyfish, locomotion is more active, being performed by a mechanism called jet propulsion: the body's lips contract, and the water accumulated in the oral phase of the jellyfish is expelled in jet, causing the animal to move in the opposite direction.
The ability to change body shape by determining movements and displacements is due to the presence of special cells with functions of contraction and distention, but which are not true muscle cells, as they arise from the mesoderm, which only occurs in triblastic animals.
Breathing and excretion occur by diffusion across the entire surface of the body. There are no special structures related to these processes, as is the case with sponges.