Hydra - the simplest organism from the orderCoelenterates. Practically in every pond inhabits this freshwater polyp. It is a translucent gelatinous body, similar to a self-moving stomach, where the hydra is digesting food.
The size of this simplest organism is rarelyexceeds 2 cm. Externally, the hydra resembles a mucous tubule of greenish or brown color. Its color depends on the food eaten. At one end of the body, it attaches itself to plants, stones or snags that are in the water, others catch prey. In general, it is small invertebrates - daphnia, cyclops, oligochaetes-naidids. Sometimes the food is small crustaceans, as well as fish fry.
The mouth opening of the hydra is surrounded by tentacles,which number from six to twenty pieces. They are in constant motion. Once the victim touches them, the stinging cells located in the tentacles immediately throw out a pointed thread containing the poison. Getting into the approaching animal, it paralyzes it and, pulling it up with tentacles, brings it to the mouth. At the same time, it seems that her body seems to be put on the victim, which is thus in the intestines, where digestion of food by the hydra begins. A cachet with a poison can only be used once, after which it is replaced with a new one.
The body of the hydra is very similar to a two-layered sac, whose outer layer of cells is called the ectoderm, and the inner layer is called the endoderm. Between them is an unstructured substance called mesogloe.
The composition of the inner layer, where the hydra occursdigestion of food, are mainly glandular and digestive cells. The first release digestive juice into the intestinal cavity, under the influence of which the eaten food is diluted and disintegrates into small particles. Other cells in the inner layer capture these pieces and draw them inward.
Thus, the process of digestion begins in the intestinal cavity, and ends inside the cells of the endoderm. All remnants of food that could not be digested are thrown out through the mouth.
Digestive cells of the inner layer haveend of 1 to 3 flagella, by which small particles of food are drawn inward and digested. The lack of a transport system in the hydra organism complicates the task of providing the cells of the ectoderm with nutrients, given that the mesogloe is sufficiently dense. This problem is solved due to existing outgrowths on the cells of both layers. They cross the intercellular substance, connecting through gap junctions. Organic molecules in the form of amino acids and monosaccharides, passing through them, provide nutrition to the ectoderm.
The glandular cells produce digestiveEnzymes that enter the gastric cavity, where the hydra is digesting food. They break down proteins and provide food preparation for digestion. This is called cavitary digestion. It is characteristic of higher multicellular animals.
When food is digested inside the digestive systemvacuoles are intracellular digestion, characteristic of the simplest unicellular organisms. The combination of these two types of digestion of food, ensuring the distribution of nutrients throughout the body, confirms the relationship of multicellular and unicellular animals.
When there is a waste of cellular metabolism in the place where the hydra is digesting food, it shrinks, resulting in emptying.
