Hydra is the simplest organism from the squamous squad. In almost every body of water this freshwater polyp lives. It is a translucent gelatinous body, similar to a self-moving stomach, where food is digested by the hydra.
How does hydra eat?
The size of this simplest organism rarely exceeds 2 cm. Externally, the hydra resembles a mucous tube of a greenish or brown color. Its color depends on the food eaten. At one end of the body, it attaches to plants, stones or snags in the water, while the other catches prey. Basically, it is small invertebrates - daphnia, cyclops, oligochaeta-naidids. Sometimes small crustaceans as well as fish fry serve as food.
The mouth of the hydra is surrounded by tentacles, which number from six to twenty pieces. They are in constant motion. As soon as the victim touches them, stinging cells located in the tentacles immediately eject a spiky thread containing poison. Piercing an approaching animal, she paralyzes it and, pulling it with tentacles, brings it to her mouth. At the same time, it seems that her body is, as it were, put on a victim, who is thus found in the intestine, where digestion of food begins with hydra. The stinging capsule with poison can be used only once, after which it is replaced by a new one.
Digestive system
The body of the hydra is very similar to a two-layer bag, the outer layer of cells of which is called the ectoderm, and the inner one is the endoderm. Between them is a structureless substance called mesoglay.
The composition of the inner layer, where food is digested in hydra, is mainly made up of glandular and digestive cells. The first secrete digestive juice into the intestinal cavity, under the influence of which the eaten food liquefies and breaks up into small particles. Other cells of the inner layer capture these pieces and draw them inward.
Thus, the digestion process begins in the intestinal cavity, and ends inside the cells of the endoderm. All food residues that could not be digested are thrown out through the mouth.
How does hydra digest food
The digestive cells of the inner layer have from 1 to 3 flagella at the end, with the help of which small particles of food are drawn in and digested. The absence of a transport system in the hydra organism complicates the task of providing the ectoderm cells with nutrients, given that the mesogley is quite dense. This problem is solved due to the 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.
Glandular cells produce digestive enzymes that enter the gastric cavity, where food is digested by the hydra. They break down proteins and prepare food for absorption. This is called abdominal digestion. It is characteristic of higher multicellular animals.
When food is digested inside the digestive vacuoles, it is intracellular digestion characteristic of simple unicellular organisms. The combination of these two types of food digestion, which ensures the distribution of nutrients throughout the body, confirms the relationship of multicellular and unicellular animals.
When food is digested where hydra remains, the waste of cell metabolism remains, it is compressed, resulting in emptying.