Hydra has a mouth without anus, and the residue is still discharged through the mouth. Hydra has no respiratory and excretory organs, and gas exchange and metabolic waste discharge are directly carried out on the cell surface.

The specific mode is:

1. When small animals accidentally meet the tentacles and branches of hydra drifting in the water, they will produce mechanical and chemical stimulation to them. The adherent cells immediately secrete mucus on the tentacles and stick to the food, then the tentacles gradually contract, and at the same time, the hydra rotates and wraps the tentacles around the body. When the food is close to the mouth, the mouth will open and swallow the food.

2. All polyps are digested in the body. It takes about 65,438+0 hours for food to be digested in the mouth, and then further digested in the stomach. Digestion is all digested outside the digestive tube, and the final absorption is absorbed by the cells in the inner wall. After that, the indigestible components are excreted through some structures similar to excretory pores. It is generally believed in academic circles that hydra-like eating and excretion is completed by the structure of the mouth.

Hydra's food is mostly small crustaceans (Daphnia), aquatic oligochaetes and insect larvae.

3. When the hydra senses that the food is close to the tentacle, it will immediately shoot barbed wire to anesthetize or kill it, and then send it into its mouth with the tentacle. Food is swallowed into the digestive circulation cavity, digestive enzymes are secreted by gland cells for extracellular digestion, and digested food particles are extended by endothelial muscle cells for intracellular digestion.

Extended data:

Hydra is an animal with a body length ranging from a few millimeters to 15 millimeters, and the longest is 3 centimeters (1.2 inches), but it can shrink very little.

White, pink, green or brown. The main body is cylindrical; Usually transparent and soft. The body wall consists of two layers of cells separated by a thin layer (mesothelium, composed of unstructured connective tissue). The feet or basal plates at the bottom of the body are attached organs that can secrete mucus. When it encounters external or internal stimuli, it can cause the substrate to slide or somersault.

Some kinds of substrates can secrete bubbles to form air bags, so that hydra can be suspended on the water from the bottom; The other end of the body is the mouth, through which food and sundries can enter and exit. The inside is connected with the intestinal cavity, and the lower end is closed. There are 5 ~ 6 slender tentacles around the mouth.

There are piles of stinging cells on the tentacles. Each tentacle can act independently and has the functions of action, predation and defense against the enemy. The outer body layer can temporarily form two kinds of gonads, generally hermaphroditic.

References:

Baidu encyclopedia-hydra