Today we characterize such a group of animals as intestinal. Representatives, structural features, nutrition, reproduction and movement of these animals - you will learn about all this by reading the article. Flower-like sea anemones, forming huge rocks underwater, corals and transparent umbrella-shaped jellyfish are among the most attractive inhabitants of the ocean. No matter how dissimilar these animals are, they are all intestinal. Representatives of this group are numerous. There are over 9,000 species of aquatic organisms that inhabit mainly shallow water.
What unites the intestinal
The feature that allows you to attribute coral, jellyfish and freshwater hydra to the intestinal cavity 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, rather than independent elements of cell clusters, as is observed in sponges. Intestinal - representatives of the animal world, who were the first to reach a similar level of organization on the evolutionary ladder, and they all have similar features in the structure and location of tissues.
Colonies and solitary organisms
Sea anemones, or sea anemones, are solitary animals, and a plant-like Obelia (pictured above) forms a colony of several hundred polyp individuals. In the case when the polyps differ from each other, they talk about polymorphic colonies. Some marine colonial intestinal cavity are representatives of the type of interest to us, in which there are separate polyps for nutrition, protection and reproduction, and sometimes for resettlement.
So, we briefly described these animals. Now that you have some idea of them, we suggest that you consider the main features of the structure of representatives of the type of Intestinal.
The structure of the intestinal
A mouth surrounded by a corolla of tentacles carrying stinging cells opens directly into the digestive cavity. In the wall of the body, the outer layer, or ectoderm, distant from the inner (endoderm) with a gelatinous layer - mesoglay - is distinguished. Representatives of the intestinal can multiply by budding or sexually. We will also give examples of both methods when we talk in more detail about reproduction. Sperm and eggs are formed in the corresponding male and female genital organs.
Representatives of the class of intestinal have nematocytes. It is a weapon of defense and attack in these animals. Some of them inject paralyzing poison into the sacrifice, others secrete a sticky substance, and others throw out entangling threads. At one end of the cell is a sensitive hair acting as a trigger. If an animal passing by touches him, the nematocyte shoots. The mechanism of the shot is not entirely clear, but is believed to be associated with a sharp increase in fluid pressure in the capsule. Each nematocyte shoots only once, after which it is reset.
Developmental stages
In the development cycle of many intestinal cavities, two distinctly different stages can be seen: the free-floating (medusoid) stage of settlement and the sedentary stage of attachment and growth. This means that some species can inhabit both bottom layers and the oceanic layer at the same time. But either one or another stage prevails in them, which explains the great variety of forms in the group of intestinal.
In Obelia, for example, the medusoid stage does not last long, it is followed by a longer sitting-attached stage, and this development cycle is typical of the intestinal cavity of the Hydrozoa group. Having reached maturity, the colony Obelia forms the special forms of polyps that produce jellyfish. In the Scyphozoa class, the opposite is true: the medusoid stage dominates here. 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 fall directly from the gonads located in certain areas of the endoderm lining the gastric cavity, and then are brought out through the mouth opening.
From fertilized eggs, a larva develops, which settles to the bottom and turns into a new individual. But there are species, especially among Hydrozoa, that make up the 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 anemone in their way of life, except that their sperm 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 strongly reduced or completely absent.
Asexual reproduction
Compared to complex sexual reproduction options , asexual organisms seem like a very simple process. For example, a representative of the intestinal animals, like Hydra, forms new individuals that bud from the parent form. This process is presented in the photo below.
But the sea anemones are simply divided in half. Asexual reproduction can lead to the formation of colonies from individual polyps, united by a common gastric cavity.
The ability of gastrointestinal to reproduce asexually means, in addition, that they regenerate quite easily. Indeed, even a small piece of the animal can develop into a new individual, which is quite capable of sexual reproduction.
Intestinal nutrition
In most intestinal cavities, the tentacles surrounding the oral opening are helpful. Generously dotted with stinging cells (nematocytes), these tentacles infect prey and pull it up. Interacting with each other, they tightly enclose food and push it into the gastric cavity. Then the mouth opening is closed, and the endoderm cells secrete digestive enzymes into the gastric cavity. Enzymes break down prey, turning it either into easily absorbable liquid products, or into a suspension of small particles that can be captured by endoderm cells. Undigested food debris is removed by contractions of the body through the ajar mouth.
Ability to move
All intestinal cavities move, although this process can be limited only by the movement of tentacles and the change in body shape. Intestinal movements are carried out thanks to muscle fibers. They are located both in the ectoderm and in the endoderm. In addition, the base of anemones is abundantly equipped with muscle fibers, allowing these animals to move along the ground. It looks like they are sliding on it. Hydra can also move in this way, but it moves faster due to a kind of “somersaults”. Even the simplest intestinal movements require some coordination. Such coordination is carried out by a diffuse network of nerve cells that penetrate the tissues of the animal and thereby form a primitive nervous system.
So, we briefly characterized the type of entericeloid. Its representatives, as you see, are very diverse in many respects, which makes this group of organisms especially interesting.