Organisms with external digestion: from worms to mushrooms

Digestion is an extremely complex and multistage process. In order for the nutrients to be absorbed, the food must be appropriately processed: exposed to enzymes and only then assimilated.

Ways of digestion

However, in the course of evolution, various biological species developed different strategies for digesting prey. In total, in biology, there are five different ways to extract useful substances from food. Intestinal digestion is used by most multicellular cells that have an intestinal tract:

• The simplest organisms, such as algae, use intracellular digestion;
• Predatory plants;
• Fungi and most bacteria digest food outside the cell;
• Extraintestinal digestion is also common among multicellular;
• There is also parietal digestion, which, although occurs in the intestinal cavity, is fermented in the mucus layer covering its walls.

Special attention should be paid to organisms with external digestion, including many spiders, insect larvae, fungi, squid. The structure of these organisms is different, but all of them have chosen as a digestive strategy - external fermentation of food. This process is rather complicated and often unsafe for others.

Organisms with external digestion: pine trunk

The pine trumpet is an extremely common fungus in Russia and Europe. To many it is well known, since it has a very specific kind of brown built-up of the correct form on rotten trees, stumps, and windmills. This fungus belongs to destructors, that is, to organisms that destroy organic substances, which they feed, so qualitatively that after its action there are no solid residues left. Assimilation of nutrients is possible only in the liquid state, so the tinder sprays an enzyme into the affected wood, which decomposes it to cellulose. As a result of this process, brown rot appears, which contains substances necessary fungus for growth and active fruiting.

Like other organisms with external digestion, related to fungi, the tinder reproduces by spores. In ecological systems, the fungus occupies an important place, since it facilitates the utilization of dead wood and processing into easily digestible substances that can be used by other members of the ecological community.

Organisms with external digestion: larva of ant lion

While little is known about the feeding habits of adult ant lion , the food behavior of its aggressive larvae has been repeatedly observed and described in detail. Like many other organisms with external digestion, the larva leads a sedentary lifestyle, preferring to ambush its prey by arranging a trap in the sand in the form of a funnel into which insects enter. Spiders, beetles and ants, trapped and unable to climb, fall into the sickle-shaped jaws, with which the larva stings its prey by injecting into it a digestive juice consisting of enzymes and bacteria. After all insides of the insect are softened with juice and turn into soft gruel, the larva sucks it, and the empty chitinous skeleton is thrown off by an active movement of the head.

The method used by worms

Organisms with external digestion are extremely common in nature. In the marine environment, they are represented by a white planar, which many unjustly consider a parasite. It is a flat worm, living in fresh waters and dangerous only for the inhabitants of domestic aquariums.

Like many other organisms with external digestion, the white planaria leads a predatory lifestyle. For hunting, the planaria has a special organ that reacts to the approaching victim, advances and captures prey.
In most cases, prey is swallowed, but if this is not possible, then pieces come off of it, each of which falls into the cavity of the planaria in turn.

It happens, however, and such that the mining has a hard coating (for example, a crustacean) and too large a size that prevents swallowing. In this case, the planar plunges the retractable jaw into the victim's body and strikes her with digestive juice full of enzymes and poisons. In this case, the victim is not released, and when the gastric juice completes its work, the planaria absorbs soft contents, and the shell discards.

Value for evolution

The presence of a large number of examples allows us to conclude that organisms with external digestion are very common in nature. It turns out that at a certain stage of evolution this method was optimal for obtaining energy and nutrients. Over time, digestive systems have become more complex and the peak of their development has been achieved in a person who has an extremely complex digestive system that includes many stages and involves a variety of mechanisms in the body.