Type of the intestines: representatives. The main features of the structure of coelenterates

Today we characterize such a group of animals as coelenterates. Representatives, features of the structure, nutrition, reproduction and movement of these animals - you will learn all this after reading the article. Similar to flowers, sea anemones, forming huge rocks under water, corals and transparent umbelliform jellyfish are among the most attractive inhabitants of the ocean. No matter how similar these animals are, they are all coelenterates. Representatives of this group are numerous. There are over 9,000 species of aquatic organisms inhabiting mostly shallow waters.

What unites coelenterates

A feature that allows to attribute corals, jellyfish and freshwater hydras to coelenterates is the presence of a wide digestive (gastral) cavity in the center of the body. The body of these animals is formed by concentric layers of cellular groups composing primitive tissues in which cells work interconnected, as parts of a single whole, rather than independent elements of cellular aggregations, as observed in sponges. The intestinal - the representatives of the animal world, who first reached this level of organization on the evolutionary ladder, and all of them have similar features in the structure and arrangement of tissues.

Colonies and solitary organisms

Marine anemones, or anemones, are solitary animals, and a plant similar to Obelia (pictured above) forms a colony of several hundred polyp specimens. In the case when polyps differ among themselves, they speak of polymorphic colonies. Some marine colonial coelenterates are representatives of the type we are interested in, in which there are separate polyps for feeding, protecting and propagating, and sometimes for settling.

So, we briefly described these animals. Now that you have some idea of them, we suggest examining the main features of the structure of representatives of the type of the intestinal.

Structure of coelenterates

The mouth, surrounded by a halo of tentacles bearing stinging cells, opens directly into the digestive cavity. In the body wall, an outer layer is distinguished, or an ectoderm, remote from the internal (endoderm) gelatinous layer - mesogloea. Representatives of coelenterates can reproduce by budding or sexually. Examples of this and the other way, we still give, when we talk in more detail about reproduction. Spermatozoa and ovules are formed in the corresponding male and female genital organs.

Representatives of the class of intestinal cysts have nematocytes. These are weapons of defense and attack in these animals. Some of them sacrifice paralyzing poison, others secrete sticky substance, others throw out entangling threads. At one end of the cell is a sensitive hair acting as a trigger. If an animal passing by will hit him, the nematocite shoots. The mechanism of the shot is not entirely clear, but it is believed to be associated with a sharp increase in the pressure of the liquid in the capsule. Each nematocyst shoots only once, after which it is discarded.

Stages of development

In the cycle of development of many coelenterates, two distinctly different stages can be seen: a free-floating (medusoid) stage of settling and a sedentary stage of attachment and growth. This means that some species can inhabit both the bottom layers and the oceanic stratum simultaneously. But they are dominated by either one or another stage, which explains the wide variety of forms in the coelenterate group.

In Obelia, for example, the medusoid stage lasts for a relatively short time, followed by a more prolonged sitting-attached period, and this cycle of development is typical for coelenterates from the Hydrozoa group. Having reached maturity, the Obelia colony forms special forms of polyps that produce jellyfish. In the class of Scyphozoa the situation is reversed: here the medusoid stage dominates. In the third class of coelenterates - Anthozoa, which includes corals and anemones (pictured above), the attached stage completely displaces the medusoid. In all these groups, the oocytes and spermatozoa fall directly from the gonads located in certain areas of the endoderm lining the gastric cavity, and then through the mouth opening outward.

A larva develops from the fertilized eggs, which settles to the bottom and turns into a new individual. But there are species, especially among Hydrozoa, constituting an exception. So, for example, representatives of the Hydra genus (one of them you can see in the photo above) do not have a medusoid stage at all and resemble actinium in their way of life, except that their spermatozoa and ovules develop from the outside, not inside the polyp. And there are, on the contrary, such species, which are dominated by the medusoid stage, and the stage of the polyp either is strongly reduced, or completely absent.

Asexual reproduction

Compared to complex variants of sexual reproduction, asexual in these organisms seems very simple process. For example, such a representative of coelenterate animals, like Hydra, forms new individuals that bud off from the parent form. This process is presented in the photo below.

But the actinia just split in half. Asexual reproduction can lead to the formation of colonies from individual polyps, united by a common gastral cavity.

The ability of coelenterates to reproduce asexually means, in addition, that they are easily regenerated. Indeed, even a small piece of the animal can develop into a new individual, which is fully capable of sexual reproduction.

Feeding of coelenterates

Most coelenterates feed on the tentacles surrounding the mouth opening. Generously strewn 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. Then the oral opening closes, and the endoderm cells secrete digestive enzymes into the gastric cavity. Enzymes break down the prey, turning it either into easily digestible liquid products, or into a slurry of fine particles that can be captured by the cells of the endoderm. Undigested food remains are removed by contractions of the body through the slightly opened mouth opening.

Ability to move

All coelenterates move, although this process can be limited only by moving the tentacles and changing the shape of the body. Movements of coelenterates are carried out with the help of muscle fibers. They are found both in the ectoderm and in the endoderm. In addition, the base of the actinium is abundantly provided with muscle fibers, allowing these animals to move along the soil. It looks as if they are sliding on it. Hydra can also move this way, but it moves faster due to a kind of "somersaults". Even the simplest movements of coelenterates require certain coordination. Such coordination is carried out by a diffuse network of nerve cells piercing the tissues of the animal and thus forming a primitive nervous system.

So, we briefly described the type of the intestinal. Representatives of it, as you can see, are very diverse in many respects, which makes this group of organisms particularly interesting.