Many representatives of the animal world have external digestion. It is not a rare occurrence and involves the digestion of food not in the intestines or stomach, but outside, that is, when digestive juices are released into the external environment. Let's take a closer look at this physiological feature.
Who tends to have external digestion
This type of food is characteristic of some invertebrates. Spiders, flatworms, starfish, and even some larvae and other invertebrates use it when the food is too large for them to swallow in one go.
Jellyfish have external digestion. By the way, one touch to them can be dangerous for a person. This type of nutrition appeared, most likely, due to the fact that in invertebrates the digestive tract is not yet as developed as in vertebrates. And it is more convenient for them to absorb already digested food. In addition, in small animals, the size of the prey can be many times greater than the size of the predator.
Flatworms
Intracellular digestion is characteristic of flatworms. Butmost of them are capable of extracellular digestion of food. The external process of digestion in flatworms can be analyzed using the example of turbellarians, which are also called ciliary worms.
They are free-living, but there are also parasites among them. Many species of these worms are characterized by extraintestinal digestion. And the pharyngeal glands and the retractable pharynx itself play an important role in the digestive tract.
Having found its future food, the worm covers it and then swallows it. Their pharynx is arranged in such a way as to protrude from the pharyngeal pocket at the right time. They simply absorb small prey, and tear off pieces from large prey with the help of strong sucking movements.
Ciliary worms can also attack hard-shelled crustaceans. But in order to digest them, they secrete and release digestive enzymes onto the victim's body that break down tissues. After that, the invertebrate swallows already digested food.
It can be said that these creatures have a mixed digestion - it can be both internal and external. In addition, Turbellaria is not a simple worm, it has another interesting feature - the use of "trophy weapons". When she, for example, eats a hydra, then the stinging cells of the latter, designed to paralyze the enemy, are not destroyed during digestion, but, on the contrary, remain in the integument of the worm and already protect it. In addition, the eyelash worms themselves are rarely eaten, as they secrete a protective mucus.
Spiders
Spiders can hardly be called vegetarians either. They are predators andfeed mainly on insects. Although an exception can be called a jumping spider that eats the green parts of acacias. All other species prefer animal food and have external digestion.
Many of these arthropods weave webs that catch various flying insects. Entangled in a trap, the victim begins to flutter, which betrays himself.
The spider immediately senses this, thanks to the vibrations of the web, and usually encases the prey in a cocoon and then injects the digestive juice inside. It softens the tissues of the victim, and eventually turns them into a liquid, which the spider drinks after time.
Spiders can be said to prefer external digestion, as they have no teeth and their mouths are too small to swallow, even those that feed on birds. To inject poison, these predators have special hook-jaws or chelicerae. For example, piercing them into the beetle's chitinous shell, the spider secretes digestive juice, drinks the digested tissues, then injects poison again, and so on until the entire beetle is digested.
Scorpions
Scorpions eat in much the same way as spiders. And it is not surprising, because they are relatives of spiders, they also belong to the order of arthropods and the class of arachnids, and they also have external digestion. Scorpions live exclusively in hot countries and 50 of their species are dangerous to humans.
The tail of a scorpion ends with a needle, from which poison is released when the muscles contract. And some individuals are capable“shoot” poison at a distance of up to one meter.
These creatures differ from spiders in that they digest their prey not in a cocoon of cobwebs, but in their mouths. The mouth of a scorpion is large and roomy, unlike that of a spider. They stuff there more pieces torn from the victim. But they don’t chew, because they don’t have teeth, but they wait, releasing digestive juices into their mouths. When the food becomes liquid, it is pumped from the mouth to the intestines.
Maggots
The larvae of the swimming beetle also use the described way of feeding. They are small, they have a poorly developed digestive system, and therefore they tend to digest externally.
The named larvae live in ponds, where they can even attack tadpoles or small fish. To do this, they have sharp jaws, with which they cling to prey. Small fish or a tadpole can swim for a while and "digest" on the go.
The most interesting thing is that even the mouth of the larva is not particularly developed - it is there, tightly closed, but it is impossible to open it. But the appetite of these creatures is absolutely incommensurable with the size. They suck out the tissues of the defeated victim, and through special tubules the digested liquid enters the body.
Sea inhabitants
Sea dwellers, such as jellyfish and starfish, also have external digestion. Sea stars are very beautiful and unusual looking animals. They belong to the phylum Echinodermata. There are many different types and shapes of stars, and they are all very graceful and attractive. True, their deceit is also unusual, although in appearance they are harmless marine animals leading a sedentary lifestyle and unable to keep up even with a turtle.
Most often they have five rays, which contain outgrowths of the stomach. Having met with a bivalve mollusk, the star envelops it with its body. Sticking to the shell with rays, the echinoderm opens it with the help of muscle efforts. This process may take half an hour. After that, the star makes a very cunning maneuver. She turns her stomach inside out, pulls it out through her mouth and puts it in the sink. The process of digestion takes place in the shell, and after four hours the mollusk is no longer there.
