Flatworms (Platyhelminthes) are a group of bilateral symmetrical soft-bodied invertebrate animals found in marine, freshwater, and also moist terrestrial environments. Some species of flatworms are free-living, but about 80% of all flatworms are parasitic, that is, they live on or in another organism and receive food from it.
Origin of view and description
The origin of flatworms and the evolution of various classes remain unclear. However, there are two main areas. According to a more generally accepted view, turbellaria represent the ancestors of all other animals with three layers of tissue. However, others agreed that flatworms could be simplified a second time, meaning they could degenerate from more complex animals as a result of evolutionary loss or reduction in complexity.
Interesting fact: The life span of a flatworm is uncertain, but in captivity, representatives of one species lived from 65 to 140 days.
Flatworms fall under the animal kingdom, which is characterized by multicellular eukaryotic organisms. In some classifications, they are also classified as the base group of eumethazoan animals, since they are metazoans that fall under the animal kingdom.
Flatworms also fall under bilaterally-symmetric among eumethazoids. This classification includes animals with bilateral symmetry consisting of the head and tail (as well as the dorsal part and abdomen). As members of the protosome subspecies, flatworms consist of three germ layers. As such, they are also often referred to as protostomes.
In addition to these higher classifications, the type is divided into the following classes:
- ciliary worms;
The class of ciliary worms consists of about 3,000 species of organisms, distributed in at least 10 orders. The class of monogenes, although grouped in another class with trematodes, have many similarities with them.
However, they can be easily distinguished from trematodes and cestodes by the fact that they have a posterior organ known as a haptor. Monogenies vary in size and shape. For example, while larger species may appear flattened and have a leaf shape (leaf shape), smaller species are more cylindrical.
The cestode class consists of more than 4,000 species, commonly known as tapeworms. Compared to other types of flatworms, cestodes are characterized by their long flat bodies, which can grow up to 18 meters in length and consist of many reproductive units (proglottids). All members of the trematode class are parasitic in nature. Currently, about 20,000 species of the trematode class have been identified.
Appearance and features
Photo: What does a flatworm look like?
Symptoms of the representatives of ciliary worms are as follows:
- the case has a conical shape at both ends with a reduced thickness compared to the central part of the case;
- with a compressed dorsal-ventral incision of the body, ciliary worms have a high ratio of surface area to volume;
- movement is achieved using well-coordinated cilia, which repeatedly oscillate in one direction;
- they are not segmented;
- the ciliary worms lack a whole (body cavity located between the body wall and intestinal canal in most animals);
- they have subepidermal rhabdites in the ciliary epidermis, which distinguish this class from other flatworms;
- they lack the anus. As a result, food material is absorbed through the pharynx and ejected through the mouth;
- while most species in this class are predators of small invertebrates, others live as herbivores, scavengers and ectoparasites;
- pigment cells and photoreceptors present at their points of view are used instead of imaging eyes;
- depending on the species, the peripheral nervous system of the ciliary worms ranges from very simple to complex intertwined nerve networks that control muscle movement.
Some of the characteristics of monogenes include:
- all members of the monogenee class are hermaphrodites;
- monogenes have no intermediate hosts in their life cycle;
- although they have certain body shapes depending on the species, it has been shown that they are able to lengthen and shorten their bodies as they move in the environment;
- they do not have an anus and therefore use the protonephridial system to remove waste;
- they do not have the respiratory and circulatory system, but they have a nervous system consisting of a nerve ring and nerves that extend to the back and front of the body;
- like parasites, monogenes often feed on skin cells, mucus, as well as host blood, which causes damage to the mucous membrane and skin protecting the animal (fish).
Characteristics of the cestode class:
- complex life cycle;
- they do not have a digestive system. Instead, the surface of their bodies is covered with small microvilli-like protrusions, similar to those found in the small intestine of many vertebrates;
- through these structures, tapeworms efficiently absorb nutrients through an outer coating (tegment);
- they have well-developed muscles;
- modified cilia on their surface are used as sensory endings;
- The nervous system consists of a pair of lateral nerve ligaments.
Characteristics of trematodes:
- they have oral suckers, as well as ventral suckers, which allow organisms to attach to the host. This facilitates the feeding of organisms;
- adults can be found in the liver or circulatory system of the host;
- they have a well-developed digestive tract and excretory system;
- they have a well-developed muscular system.
Where do flatworms live?
Photo: Flatworms in the water
In general, free-living flatworms (turbellaria) can be found wherever there is moisture. With the exception of dark cephalids, flatworms are cosmopolitan in distribution. They are found both in fresh and in salt water, and sometimes in humid terrestrial habitats, especially in tropical and subtropical regions. Darkcephalids, which parasitize on freshwater crustaceans, are found mainly in Central and South America, Madagascar, New Zealand, Australia and the islands of the South Pacific.
While most species of flatworms live in the marine environment, there are many others that can be found in freshwater environments, as well as in tropical terrestrial and humid temperate environments. Thus, they require at least moist conditions in order to survive.
Depending on the species, representatives of the class of ciliary worms exist either as free-living organisms or as parasites. For example, representatives of the order dark ciphalids exist as completely commensals or parasites.
Interesting fact: Some species of flatworms occupy a very wide range of habitats. One of the most cosmopolitan and most tolerant of various environmental conditions is the turbellar Gyratrix hermaphroditus, which is found in fresh water at an altitude from sea level to 2000 meters, as well as in pools with sea water.
Monogenies are one of the largest groups of flatworms, whose members are almost exclusively parasites of aquatic vertebrates (ectoparasites). They use adhesive organs to attach to the host. This design also consists of suction cups. Cestodes, as a rule, are internal worms (endoparasites), which require more than one host for their complex life cycles.
Now you know where flatworms are found. Let’s see what they eat.
What do flatworms eat?
Photo: Flat annelid worm
Free-living flatworms are mainly carnivorous, especially adapted for catching prey. Their encounters with prey seem largely random, with the exception of some species that secrete mucous filaments. Digestion is both extracellular and intracellular. Digestive enzymes (biological catalysts) that mix with food in the intestines reduce the size of food particles. This partially digested material is then absorbed (phagocytosed) by cells or absorbed; digestion then completes in the intestinal cells.
In parasitic groups, both extracellular and intracellular digestion occurs. The degree to which these processes occur depends on the nature of the food. When a parasite perceives fragments of food or host tissue, other than fluids or semi-liquids (such as blood and mucus), as nutrients, digestion is largely extracellular. In those who feed on blood, digestion is mainly intracellular, which often leads to the deposition of hematin, an insoluble pigment formed as a result of the breakdown of hemoglobin.
Although some flatworms are free-living and non-destructive, many other species (in particular, trematodes and tapeworms) parasitize humans, domestic animals, or both. In Europe, Australia, and the Americas, the introduction of tapeworms in humans has been significantly reduced as a result of routine meat examinations. But where sanitary conditions are poor and meat is eaten undercooked, the incidence of tapeworms is high.
Interesting fact: Thirty-six or more species have been reported as parasitic in humans. Endemic (local) foci of infection are found in almost all countries, but widespread infections occur in the Far East, Africa and tropical America.
Features of character and lifestyle
The ability to undergo tissue regeneration, in addition to simple wound healing, is found in two classes of flatworms: turbelaria and cestode. Turbellaria, especially planaria, are widely used in regeneration studies. The greatest regenerative ability exists in species capable of asexual reproduction. For example, pieces of almost any part of a turbulent stenostum can grow into completely new worms. In some cases, the regeneration of very small pieces can lead to the formation of imperfect (eg, headless) organisms.
Regeneration, although rare in parasitic worms in general, occurs in cestodes. Most tapeworms can regenerate from the head (scolex) and neck. This property often makes it difficult to treat people with tapeworm infections. Treatment can eliminate only the body, or the strobila, leaving the scolex still attached to the intestinal wall of the host and thus able to produce a new strobila that restores the invasion.
Several species of cestode larvae can regenerate themselves from excised areas. The branched larval form of Sparganum prolifer, a human parasite, can undergo both asexual reproduction and regeneration.
Social structure and reproduction
Photo: Green flatworm
With very few exceptions, hermaphrodites and their reproductive systems are usually complex. Numerous testicles are usually present in these flatworms, but only one or two ovaries. The female system is unusual in that it is divided into two structures: ovaries and vitellaria, often known as vitelline glands. Vitellaria cells form the components of the yolk and eggshell.
In tapeworms, the tape-like body is usually divided into a series of segments or proglottids, each of which develops a complete set of male and female genitalia. A rather complex copulative apparatus consists of the eternal (able to turn outward) penis in a man and the canal or vagina in a woman. Near its opening, the female canal can differentiate into various tubular organs.
Reproduction of ciliary worms is achieved using a number of methods, which include sexual reproduction (simultaneous hermaphrodite) and asexual reproduction (transverse division). During sexual reproduction, eggs are produced and bind into cocoons, from which young individuals hatch and develop. With asexual reproduction, some species are divided into two halves, which are restored, forming the missing half, thus turning into a whole organism.
The body of true tapeworms - the cestode - consists of many segments, known as proglottids. Each of proglottids contains both male and female reproductive structures (like hermaphrodites), which are capable of reproducing independently. Given that one tapeworm can produce up to a thousand proglottids, this allows tapeworms to continue to thrive. For example, one proglottid is capable of producing thousands of eggs; their life cycle can continue with another host when the eggs are swallowed.
The host who swallows the eggs is known as an intermediate host, given that it is in this particular host that the eggs are hatched to produce larvae (coracidia). Larvae, however, continue to develop in the second host (the final host) and mature in the adult stage.
Natural enemies of flatworms
Photo: What does a flatworm look like?
Predators have access to freely moving flatworms from the class of turbelaria - in the end, they are in no way limited to the bodies of animals. These flatworms live in a wide variety of conditions, including streams, streams, lakes and ponds.
Extremely humid environments are an absolute must for them. They tend to hang out under rocks or in heaps of foliage. Water bugs are one example of the diverse predators of these flatworms - especially bugs diving into the water, and young dragonflies. Crustacean, tiny fish and tadpoles also usually dine with these species of flatworms.
If you have a reef aquarium and you notice the sudden presence of annoying flatworms, they can invade your sea corals. Some aquarium owners prefer to use certain types of fish for biological control of flatworms. Examples of specific fish that often feed on flatworms with enthusiasm are six-stringed rodents (Pseudocheilinus hexataenia), yellow rodents (Halichoeres chrysus), and spotted tangerines (Synchiropus picturatus).
Many flatworms are parasites of involuntary hosts, but some of them are also real predators. Sea flatworms are mostly predators. Tiny invertebrates are a particularly favorite food for them, including worms, crustaceans and rotifers.
Population and species status
Currently, more than 20,000 species have been identified, the type of flatworms is one of the largest types after chordates, mollusks and arthropods. About 25-30% of people are currently infected with at least one parasitic worm species. The diseases they cause can be devastating.Helminth infections can lead to various and chronic diseases, such as eye scarring and blindness, swelling of the extremities and immobility, blocking digestion and malnutrition, anemia and fatigue.
Not so long ago, it was thought that human diseases caused by parasitic flatworms were limited by poor resources throughout Africa, Asia, and South America. But in this age of global travel and climate change, parasitic worms are slowly but surely moving to parts of Europe and North America.
The long-term effects of an increase in the spread of parasitic worms are difficult to predict, but the harm caused by the infection emphasizes the need to develop control strategies that can mitigate this threat to public health in the 21st century. Invasive flatworms can also cause serious disturbances in ecosystems. Researchers at the University of New Hampshire have discovered that flatworms at the mouths can indicate ecosystem health by destroying it.
Flatworms - bilaterally symmetric organisms with multicellular bodies that demonstrate the organization of organs. Flatworms, as a rule, are hermaphroditic - functional reproductive organs of both sexes are found in one individual. Some current evidence suggests that at least some species of flatworms can be secondarily simplified from more complex ancestors.