Interesting features are characteristic of a group of multicellular animals that belong to the type Cnidaria, or Cnidaria. Cnidarians have a simple structure, but have real tissues, an intestinal cavity. One of the unofficial names of the group is coelenterates. An important role in the body is played by stinging cells (cnidocytes, nematocytes). They serve to attack prey and defend against enemies.
Which organisms have cnidocytes?
Cnidaria are marine and freshwater animals that live in almost all latitudes. The radially symmetrical body of cnidarians has one of two body types - polypoid or jellyfish. Representatives of the first type differ significantly in appearance, some are more like plants. In jellyfish, the mouth and tentacles are directed downwards. As a rule, these coelenterates are free-swimming, and the two body forms alternate in differentgenerations. Almost all cnidarians have stinging cells; they are located on the tentacles. There are fewer freshwater coelenterates than marine ones. There are single and colonial organisms among them.
Cnidaria type combines the following classes of animals:
- hydroids (Hydrozoa);
- Scyphozoa;
- coral polyps (Anthozoa);
- cubozoa;
- polypodia (Polypodiozoa).
How are stinging cells arranged?
In translation from Greek, the word "cnidos" means "nettle", which is associated with the presence of capsules filled with a poisonous secret in the outer cover of animals. As a rule, stinging cells are concentrated in the tentacles of cnidarians and are equipped with a sensitive cilium. Inside the cnidocyte there is a small pouch and a folded miniature tube - a stinging thread. It looks like a compressed spring with a harpoon. An important role in the activation of burning cells belongs to calcium ions, a change in the concentration and pressure of the solution inside the capsule. It should be noted that cnidarians do not respond to all external stimuli, so as not to waste stinging cells. An animal's body has nerve endings, or receptors, that help detect changes in the environment.
What is the function of stinging cells?
Small contact with prey or enemy, change in water pressure from a moving object can stimulate the sensitive hair. Cnidocytes are also able to respond to protein substances. Here's what happens wheneffect on the stinging cell:
- Opens the lid on the top facing the environment.
- The stinging thread straightens and, together with sharp spikes at the base, pierces the victim's body.
- Cnidocyte coils around or sticks to prey.
- Emitted poison causes paralysis or burns.
- Having completed their function, cnidocytes die, and new ones develop in their place after 48 hours.
Due to the high concentration and coordinated activity of cnidocytes on the tentacles, coelenterates strike a predator or potential prey. Neurotoxins inside the capsules of stinging cells paralyze small prey and cause burns in large organisms.
Who do the cnidating animals prey on?
In the course of experiments, it was found that the cnidocyte throws out a "harpoon" and poison within 3 milliseconds after contact with another animal. The lightning-fast cellular reaction has practically no analogues in wildlife. Its speed and force with which the stinging thread is released is enough to penetrate the hard shells of some crustaceans! Large representatives of the coelenterates attack fish and hermit crabs. But for most cnidarians, small organisms like plankton and benthos serve as a source of food. It should be noted that even stinging cells do not save many coelenterates from predators. Possessing such a formidable weapon in their tentacles, they still become the object of hunting for other animals.
How do the "flowers" of the animal world eat?
Coral polyps form colonies in the seas and oceans. Anemones or sea anemones live alone, attaching their soles to stones, shells, rocks and reefs. The tentacles and mouth of polyps, which belong to the class Anthozoa, are usually located at the top, the lower part is attached to the substrate. The mouth of an anemone is surrounded by tentacles, on which cnidocytes are located. The function of sea anemone stinging cells is to attack prey and defend against enemies. Anemones are able to paralyze and entangle small animals with burning threads. Some cnidarians extend their tentacles, which is necessary for a stationary lifestyle.
The problem of foraging is also solved by the very rapid action of stinging cell neurotoxins. Upon contact, they can immobilize prey and repel predators.
Where do hydroid animals live?
Representatives of the Hydrozoa class are found in fresh water, Antarctic waters, deep ocean trenches. Hydras, limnomedusas, siphonophores, and other subclasses and orders belong to this group. Most of them are predators that hunt with the help of cnidocytes. Stinging cells of coelenterates, related to hydroids, have significant differences in size and strength of the poison. There is a division of functions between groups of organisms in polyp colonies: some feed, others protect, others serve for reproduction. Some jellyfish get their food by drifting in the water with motionlesstentacles into which plankton enters, while others actively swim in search of food. There are coelenterates that are able to purposefully hunt for prey, the approach of which is signaled by receptors on the surface of the body.
Are scypho- and box jellyfish cnidocytes dangerous?
The size of animals belonging to the class Scyphozoa range from 12 mm to 2.4 m in diameter. Even large forms do not have a skeleton, head, or respiratory organs. A typical representative of this group - translucent eared Aurelia - is less poisonous than other jellyfish. Adults feed on plankton that stick to their tentacles. Scyphomedusa have many cnidocytes and receptors surrounding the mouth and tentacles. Their main purpose is to recognize and paralyze prey.
Deadly for small animals are the stinging cells of the giant cyanide (Cyanea arctica). And upon contact with a person, cnidocytes cause burns of varying severity. More often there is a rash and redness from exposure to toxins that enter the skin. Box jellyfish - inhabitants of the warm waters of the seas and oceans - are able to move quickly. Some of them are dangerous to humans: burns resulting from such "communication" can be fatal.
Coelenterates and humans
Problems of relationships between humans and animals that belong to the Cnidaria type are very diverse. Many divers and fans of beach holidays on the ocean are familiar with the stinging propertiescoelenterates. Stinging cells are characteristic of jellyfish floating in the water column. Even light contact with many of them can lead to painful conditions, burns, and skin irritation. To enjoy diving or swimming, you just need to follow the rule, which is as follows: "Look, but do not touch." The best remedy for jellyfish tentacle burns is hot water, then a cold compress and taking antihistamines. One of the complex problems of interaction between the population and coelenterates is the extraction of corals for the manufacture of jewelry and souvenirs. In recent years, scientists have been alarmed by the death of polyps - the builders of rich and complex underwater structures. They create a habitat not only for themselves, but also for other invertebrates, as well as fish. Coral reefs in warm oceans and seas around the world are greatly affected by changes in climate, salinity and other water properties.
Polyp colonies grow very slowly, increasing by only a few millimeters per year. Without coral buildings, it is difficult to imagine the underwater world, which so attracts with its unique beauty and special charm.
