External digestion is possessed by many representatives of the animal world. It is not a rare occurrence and involves the digestion of food not in the intestine or stomach, but on the outside, that is, when the digestive juices are released into the external environment. Let's take a closer look at this physiological feature.
Who is involved in external digestion
This type of food is common to some invertebrates. Spiders, flatworms, starfish, and even some larvae and other invertebrates use it when the food is too large for them to swallow at a time.
External digestion possess jellyfish. By the way, just touching them can be dangerous for humans. This type of nutrition appeared, most likely due to the fact that invertebrates the digestive tract is not yet as developed as in vertebrates. And it’s more convenient for them to absorb already digested food. In addition, in small animals, prey sizes can exceed many times the size of a predator.
Flatworms
For flatworms intracellular digestion is characteristic. But most of them are capable of extracellular digestion of food. The external digestion process in flatworms can be analyzed using turbellaria, also called ciliary worms, as an example.
They belong to free living, but there are parasites among them. Many types 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. The pharynx is arranged in such a way that at the right time they extend out of the pharyngeal pocket. They simply absorb small prey, and tear off pieces from large prey with the help of strong sucking movements.
Ciliary worms can also attack crustaceans that have a hard shell. But in order to digest them, they secrete and discard digestive enzymes on the victim’s body, which break down tissues. After that, invertebrate swallows already digested food.
We can say that these creatures have 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 “captured weapons”. When she, for example, eats hydra, the stinging cells of the latter, designed to paralyze the enemy, are not destroyed during digestion, but, on the contrary, remain in the worm and protect it already. In addition, ciliary worms themselves are rarely eaten, as they secrete protective mucus.
Spiders
Spiders can also hardly be called vegetarians. They are predators and feed mainly on insects. Although the exception can be called a spider-horse eating green parts of acacia. All other species prefer animal food, and they are characterized by external digestion.
Many of these arthropods weave a web in which various flying insects come across. Entangled in a trap, the victim begins to tremble, which betrays itself.
The spider immediately feels this, thanks to the vibrations of the web, and, as a rule, packs the victim in a cocoon, and then injects digestive juice into it. It softens the victim’s tissues, and ultimately turns them into a liquid that the spider drinks after a while.
We can say that spiders prefer external digestion, as they have no teeth, and their mouth is very small for swallowing, even for those who feed on birds. To inject poison, these predators have special hook-jaws or chelicera. For example, by piercing them into the chitinous shell of a beetle, the spider secretes digestive juice, drinks the digested tissues, then injects the poison again and so on until the whole beetle is digested.
Scorpions
Scorpions feed in much the same way as spiders. And not surprisingly, 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 the scorpion ends with a needle, from which poison is released during muscle contraction. And some individuals are able to "shoot" poison at a distance of one meter.
These creatures differ from spiders in that they digest their prey not in a cocoon from the web, but in their mouth. The scorpion's mouth 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 have no teeth, but they wait, releasing digestive juices in their mouths. When food becomes liquid, it is pumped from the mouth to the intestines.
Larvae
Larvae of the swimming beetle also use the described method of feeding. They are small, their digestive system is poorly developed, and therefore they are characterized by external digestion.
These 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 the prey. Small fish or a tadpole can swim for some time and “digest” on the fly.
The most interesting thing is that even the mouth of the larva is not particularly developed - it is, it is 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.
Marine inhabitants
External digestion is also possessed by the inhabitants of the sea, for example, jellyfish and starfish. Starfish are very beautiful and unusual-looking animals. Belong to the type of echinoderms. There are many different types and forms of stars, and all of them are very elegant and attractive. True, their insidiousness is also unusual, although in appearance they are harmless marine animals leading a sedentary lifestyle and not able to keep up even with the tortoise.
Most often, they have five rays in which the outgrowths of the stomach are enclosed. Having met with a bivalve mollusk, the star envelops it with its body. Sucking to the sink with rays, the echinoderm opens it with the help of muscular efforts. This process can take half an hour. After which the star makes a very tricky maneuver. She turns her stomach inside out, pulls it through her mouth and stuffs it into the sink. The digestion process takes place in the sink, and after four hours the clam is no longer there.