Digestion is an extremely complex and multi-stage process. In order for nutrients to be absorbed, food must be properly processed: it is exposed to enzymes and only after that it is absorbed.
Digestion Methods
However, in the process of evolution, various species have developed different strategies for digesting prey. In biology, there are five different ways to extract nutrients from food. The intestinal digestion is used by most multicellular, having the intestinal tract:
- protozoa, such as algae, use intracellular digestion;
- predatory plants;
- fungi and most bacteria digest food outside the cell;
- extraintestinal digestion is also common among multicellular ones;
- there is also parietal digestion, which, although it occurs in the intestinal cavities, is fermented in the layer of mucus that covers its walls.
Organisms with external digestion deserve special attention, among which there are many spiders, insect larvae, mushrooms, squid. The structure of these organisms is different, but they all have chosen as a digestive strategy - external fermentation of food. This process is quite complicated and often unsafe for others.
Digestive organisms: pine fungus
Pine fungus is an extremely common fungus in Russia and Europe. It is well known to many, as it has a very specific type of brown growth of the correct form on rotten trees, stumps, and deadwood. This fungus belongs to destructors, that is, organisms that destroy the organic substances that it feeds on, so qualitatively that after its exposure there are no solid residues. The assimilation of nutrients is possible only in a liquid state, so the tinder fungus injects an enzyme into the affected wood, which it decomposes to cellulose. As a result of this process, brown rot appears, which contains the substances necessary for the fungus to grow and actively bear fruit.
Like other organisms with external digestion related to fungi, the tinder fungus is propagated by spores. In ecological systems, the mushroom occupies an important place, as it contributes to the disposal of dead wood and processing into easily digestible substances that can be used by other members of the ecological community.
Digestive organisms: ant lion larva
While researchers know very little about the method of feeding adult individuals of an ant lion, the feeding 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 watch its prey by setting up a trap in the sand in the form of a funnel into which insects fall. Spiders, beetles and ants, being trapped and not having the strength to get out, fall into the crescent jaws, with which the larva stings its victim, injecting digestive juice, consisting of enzymes and bacteria, into it. After all insect entrails are softened with juice and turn into a soft slurry, the larva absorbs it and discards the empty chitinous skeleton with 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 the Belarium Planaria, which many unjustly consider a parasite. This flat worm lives in fresh waters and is dangerous only for the inhabitants of home aquariums.
Like many other organisms with external digestion, the white planaria leads a predatory lifestyle. For hunting, the planarium has a special organ, which, responding to an approaching victim, advances and captures the prey.
In most cases, the production is swallowed, but if this is not possible, then pieces come off from it, each of which enters the cavity of the planarium in turn.
However, it also happens that the extraction has a hard coating (for example, crustacean) and is too large to prevent swallowing. In this case, the planaria sticks the retractable jaw into the victim’s body and infects it with digestive juice full of enzymes and poisons. At the same time, the victim is not released, and when the gastric juice completes its work, the planaria absorbs the soft contents, and discards the shell.
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 generating energy and nutrients. Over time, the digestive system became more complex and reached its peak in a person who has an extremely complex digestive system, which includes many stages and involves a variety of mechanisms in the body.