Today we will characterize such a group of animals as coelenterates. Representatives, structural features, nutrition, reproduction and movement of these animals - you will learn about all this by reading the article. Flower-like sea anemones, corals that form huge rocks underwater, and transparent umbellate jellyfish are among the most attractive inhabitants of the ocean. No matter how different these animals are from each other, they are all coelenterates. Representatives of this group are numerous. There are over 9,000 species of aquatic organisms, mostly found in shallow waters.
What unites coelenterates
A feature that allows corals, jellyfish and freshwater hydras to be classified as coelenterates is the presence of a wide digestive (gastric) cavity in the center of the body. The body of these animals is formed by concentric layers of cell groups that make up primitive tissues in which cells work interconnectedly, as parts of a single whole, and not independent elements of cell clusters, as isobserved in sponges. Coelenterates are representatives of the animal world who were the first to reach this level of organization on the evolutionary ladder, and they all have similar features in the structure and arrangement of tissues.
Colonies and solitary organisms
Sea anemones, or sea anemones, are solitary animals, while the plant-like Obelia (pictured above) forms a colony of several hundred polyps. In the case when the polyps differ from each other, they speak of polymorphic colonies. Some marine colonial coelenterates are representatives of the type of interest to us, in which there are separate polyps for nutrition, protection and reproduction, and sometimes for settlement.
So, we have briefly described these animals. Now that you have some idea of them, we propose to consider the main structural features of representatives of the Coelenterates type.
Structure of coelenterates
The mouth, surrounded by a rim of tentacles bearing stinging cells, opens directly into the digestive cavity. In the wall of the body, the outer layer, or ectoderm, is distinguished, distant from the inner (endoderm) by a gelatinous layer - mesoglea. Coelenterates can reproduce by budding or sexually. We will give examples of both methods when we talk more about reproduction. Sperm and eggs are produced in the respective male and female reproductive organs.
Representatives of the class Coelenterates have nematocytes. This is a weapon of defense and attack in these animals. Some of them inject a paralyzing poison into the victim, others secrete a sticky substance, and still others throw out entangling threads. At one end of the cell is a sensitive hair that acts as a trigger. If a passing animal touches it, the nematocyte fires. The mechanism of the shot is not entirely clear, but is believed to be due to a sharp increase in fluid pressure in the capsule. Each nematocyte fires only once and then resets.
Development stages
In the development cycle of many coelenterates, two distinct stages can be seen: a free-floating (medusoid) stage of settling and a sessile stage of attachment and growth. This means that some species can inhabit both the bottom layers and the oceanic layer at the same time. But they are dominated by either one or the other stage, which explains the wide variety of forms in the coelenterates.
In Obelia, for example, the medusoid stage lasts a relatively short time, followed by a longer sessile-attached stage, and this developmental cycle is typical of coelenterates from the Hydrozoa group. Once mature, the Obelia colony develops special forms of polyps that produce jellyfish. In the class Scyphozoa, the situation is reversed: here the medusoid stage dominates. In the third class of coelenterates - Anthozoa, which includes corals and sea anemones (pictured above), the attached stage completely displaces the medusoid. In all these groups, the eggs and sperm drop out directly from the gonads,located in certain areas of the endoderm lining the gastric cavity, and then through the oral opening are brought out.
The larva develops from fertilized eggs, which settles to the bottom and turns into a new individual. But there are species, especially among the Hydrozoa, that are an exception. So, for example, representatives of the genus Hydra (you can see one of them in the photo above) do not have a medusoid stage at all and resemble sea anemones in their lifestyle, except that their spermatozoa and eggs develop outside, and not inside the polyp. And there are, on the contrary, those species in which the medusoid stage dominates, and the polyp stage is either greatly reduced or completely absent.
Asexual reproduction
Compared to the complex forms of sexual reproduction, asexual reproduction in these organisms seems to be a very simple process. For example, such a representative of intestinal animals as Hydra forms new individuals that bud from the parent form. This process is shown in the photo below.
But anemones just split in half. Asexual reproduction can also lead to the formation of colonies from individual polyps, united by a common gastric cavity.
The ability of coelenterates to reproduce asexually means, in addition, that they regenerate quite easily. Indeed, even a small piece of an animal can develop into a new individual that is fully capable of sexual reproduction.
Coelenterates nutrition
In most coelenterates, the tentacles surrounding the mouth opening help feed. Lavishly studded with stinging cells (nematocytes), these tentacles strike prey and pull it up. Interacting with each other, they tightly cover the food and push it into the gastric cavity. The mouth opening then closes, and endoderm cells secrete digestive enzymes into the gastric cavity. Enzymes break down the prey, converting it either into easily digestible liquid products or into a suspension of small particles that can be captured by endoderm cells. Undigested food residues are removed by contractions of the body through the ajar mouth opening.
Mobility
All coelenterates move, although this process may be limited to moving the tentacles and changing the shape of the body. The movements of the intestinal cavity are carried out thanks to muscle fibers. They are found in both ectoderm and endoderm. In addition, the base of the sea anemones is abundantly supplied with muscle fibers, which allow these animals to move along the ground. It looks like they are sliding on it. The Hydra can also move in a similar way, but it moves faster due to a kind of "tumbling". Even the simplest movements of the coelenterates require a certain amount of coordination. This coordination is carried out by a diffuse network of nerve cells penetrating the tissues of the animal and thereby forming a primitive nervous system.
So here we are in a nutshellcharacterized the type of intestinal. Its representatives, as you can see, are very diverse in many ways, which makes this group of organisms especially interesting.
