From the ancient Greek πλατύς - broad and ἕλμινθος - helminth.

Flatworms (Platyhelminthes) are a type of lower worms. Length from 0.1 to 18 m. The body is bilaterally symmetrical, elongated, flattened in the dorsoventral direction. Primary and secondary cavities are absent. The circulatory and respiratory systems are absent. Mouth on the abdominal part of the body or in front. There is a pharynx. Organs of excretion - protonephridia. The nervous system consists of head ganglia and paired longitudinal trunks connected by ligaments. Most flatworms - hermaphrodites with a complex sexual system, some are separated sexually. Marine and freshwater forms. Some are free-living, others lead a parasitic lifestyle. Over 12500 species. Main classes: ciliated worms, suckers, tapeworms.

When flatworms are viewed from above or below, they appear elongate-oval, except for tapeworms, which are really ribbon (belt) shaped. Most representatives in the cross section flat, which feature gave the name of the type, the so-called microturbellaria may be in this section rounded. In flatworms are distinguished anterior and posterior ends of the body. Most free-living flatworms reach less than 1 mm in length. The smallest individuals belong to the class of trematodes, adults of which can reach 0.2 mm.However, there are known and very large forms, which have a length of more than 1 cm. For example, Rimacephalus arecepta, inhabiting Lake Baikal, reaches 60 cm in length. As a rule, the larger the worm, the more flattened its body (this is due to the peculiarities of internal transport, which is entirely provided by diffusion). Parasites (for example, Fasciola hepatica), reach several centimeters in length. The largest representative of flatworms - Diphyllobothrium latum - has a body length of 25 meters. Some flatworms have a very unusual coloration, because of which they are often confused with cephalopod mollusks and bottom scallops.

The body wall is a musculoskeletal sac, which consists of a combination of coverings and muscular layers. The covers are represented by either ciliary epithelium or immersed epithelium. The ciliary epithelium, in turn, may not be separated from the deeper lying cells (no basal membrane), but is most often represented by a detached epithelium. The epithelial cells are always multiciliated. Sometimes all epithelial cells are fused, forming a single mass - a symplast. Immersed epithelium in free-living species carries cilia, in parasitic species is devoid of them and is called tegument. The coverings also include the basal lamina. Muscle fibers form continuous layers, instead of breaking into separate bundles. The outer layer is formed by ring muscles, underneath it is a layer of longitudinal muscles, deeper down is a layer of diagonal muscles. Almost always there are also muscles connecting the dorsal and abdominal parts of the animal - dorso-abdominal (dorsoventral). Parenchymal musculature is formed by bundles of muscles that provide mobility of individual organs (suckers, trunks, pharynx, etc.). The development of cutaneous musculature is determined by the degree of transition from the ciliary type of movement to the contractile type.

The space between the basal plates of the epidermis and gastrodermis is filled with parenchyma (mesenchyma) - connective tissue reinforced with collagen fibers, which act as skeletal elements and muscle attachment points. In the parenchyma are immersed organs of excretion and osmoregulation, as well as the sexual system. In the parenchyma are located and cover cell bodies. On the mesenchyme by diffusion moves oxygen, nutrients and metabolic products that fall into it from internal organs. Mesenchyme consists of cells of two types: main cells, containing many vacuoles, and stem cells (neoblasts), which can turn into cells of any other types. Unlike many other animal groups, somatic cells of flatworms are incapable of mitosis, so only neoblasts can serve as a source of new cells. Neoblasts play a key role in regeneration and sexless reproduction.

The nervous system of flatworms (with the exception of some Acoelomorpha) consists of the brain, immersed under the musculocutaneous sac, from which a number of longitudinal trunks clothed with sheaths branch off. The latter are often connected by transverse commissures, forming a characteristic rectangular lattice - orthogon. The orthogon is composed of localized thickenings of the nerve plexus located within or under the muscle layers. In addition to the orthogon, there may be epidermal and subepidermal plexus, a reticulate or felt-like interlacing of neuronal outgrowths. These neurons form the peripheral nervous system, which is connected by fibers to the brain and the longitudinal trunks of the orthogon (central nervous system). Most flatworms, including parasitic forms, have richly developed sensory organs and ciliary receptors (collar receptors, sensory cilia) of a wide variety of ultrastructure. Several types of eyes arranged according to the scheme of "pigment glasses" and statocysts are widespread.

The digestive system consists of the mouth and two sections of the intestine: the anterior, represented by the pharynx, and the middle, blindly closed. The intestine is lined with a single layer of entodermal cells that break down and absorb food. In large forms, the midgut forms invaginations to deliver nutrients to organs distant from the intestine. Some parasitic worms (few species of parasitic turbellaria, trematodes and all cestodes) have no digestive system at all, and nutrients are delivered through the covers. There is both extracellular and intracellular digestion, and therefore both phagocytic cells and exocrine glands are present in the intestinal walls. As a rule, flatworms do not have an anal opening, and undigested food remains are excreted through the mouth. However, some species with considerable body length do have an anus, and some representatives have a complex branched intestine, opening outward with several anuses. Some species of free-living turbellarians have unicellular photosynthesizing symbionts.

Like other multicellular animals, flatworms have to maintain a water-ionic balance with their environment. Parasitic species and inhabitants of marine waters have to fight dehydration, while freshwater forms, on the contrary, have to prevent excessive accumulation of water in tissues, as water tends to penetrate inside their bodies by virtue of osmosis. For these purposes, as well as to get rid of the end products of metabolism, flatworms have special excretory organs - protonephridia. The protonephridium has the appearance of a tubule of ectodermal origin, which opens outward at one end and is blindly closed at the other end by a specialized flagellar/ciliary cell called a terminal cell, or cyrtocyte. The cyrtocyte is shaped like a vesicle with a chamber (cavity) inside. The walls of this cavity are bounded by the cytoplasmic outgrowths of the cyrtocyte. Such a system of outgrowths of the cyrtocyte is figuratively called "verso". In some cases, part of the cytoplasmic outgrowths belong to the initial cells of the tubule, which communicates with the chamber of the cyrtocyte. Between these outgrowths remain narrow gaps, covered with a thin filter film. Due to the aggregate of outgrowths and the basal lamina, the inner cavity of the cyrtocyte is shielded from the surrounding cells and extracellular matrix. 

The cavity of the cyrtocyte passes into the cavity of the tubule, the walls of which are built of cells of the usual epithelial type, in some cases these cells carry cilia. From the base of the cyrtocyte there are flagella/cilia. In flatworms, the cyrtocytes contain many cilia and their coordinated biology resembles the flame of a flickering candle, so the term "ciliated (flickering) flame cells" is often applied in such a case. The fridoid may include one or more cyrtocytes. Excreta are transported diffusely and enter the tissue fluid (extracellular matrix). Flow of fluid into the chamber of the cyrtocyte is ensured by filtration through the "verruca". The pressure difference required for filtration is due to the work of cilia. In the course of filtration, water with dissolved salts, organic substances, including metabolites, enters the cyrtocyte chamber in this way. As the filtrate (primary urine) moves along the tubule, reabsorption and secretion occurs. Due to the fact that transport in flatworms is made by diffusion, which is effective only at a short distance, cyrtocytes in these animals are many, and they are more or less evenly scattered throughout the parenchyma. The ducts branching from them unite, so that eventually one or two main excretory trunks are formed. The protonephridia open outward by one or more pores. Sometimes there is an extension at the end of the protonephridial canal, the bladder. The nature of branching of the protonephridia and the number of excretory pores are different in different groups. Gutless turbellarians are characterized by the absence of protonephridia. The excretory function can be performed by special parenchyma cells - atrocytes and intestinal epithelial cells, which are usually organs of accumulative excretion, i.e. isolate and accumulate harmful metabolites without their excretion.

Almost all representatives of flatworms are hermaphrodites. The exceptions are extremely rare, for example, separated sex is characterized by representatives of the family of blood bivalves. Fertilization in all cases is internal. The structure of the sexual system is diverse and usually complex, it includes sex glands, gonoducts, a variety of devices that provide internal fertilization, primarily the copulatory organ, organs that provide reception and storage of foreign sperm. In a part of flatworms ovules are endolecital, i.e. yolk reserves are concentrated in the ovule itself, this type of yolk arrangement can be considered typical for animals. In another part of flatworms, ovaries form ovules, and yolk cells (which by origin are modified ovaries or a modified part of the ovary) produce yolk cells carrying the yolk reserve. In a special section of the sexual system the formation of a complex egg takes place - a multicellular shell-covered formation containing a zygote and some number of yolk cells. Eggs of this structure are called exolecital and outside of flatworms are practically not found. Among the flatworms there are groups whose spermatozoa have one flagellum, two flagella, and flagellum-less spermatozoa. In addition, flatworm spermatozoa are characterized by the absence of acronemes. Free-living turbellarians can move in a variety of ways. Some glide along the surface of the bottom (or other substrate) or swim with the help of cilia located on the surface of the body. Others may use muscles for the same purpose. 

A wide range of movements can be observed in different turbellarians. Their bodies can contract or extend, they can bend their bodies at high angles, turn in all directions, and perform undulatory movements. Peristaltic waves run along the body of some of them. The mechanism of locomotion mainly correlates with body mass - smaller tubellarians more often use only the movement of cilia (peristalsis, if any, is intended for the digestive process), and larger ones use the muscles of the body. In some groups, the ventral surface of the body forms a specialized organ of locomotion, which is carried out by its contraction. Some large Polycladidae perform translational movement by dorsoventral undulation of the lateral edges of the body. In contrast to free-living turbellarians, Neodermata have limited locomotion abilities, manifested mainly in those phases of the life cycle in which the parasite needs to leave the previous host and find and then infect a new host. Trematodes, for example, have a cilia-covered epidermis during the miracidium stage that helps the larva swim freely in the water until it meets its host, a gastropod mollusk. The later developmental stage of trematodes, the cercaria, has a muscular tail that allows them to swim in search of their next host (which is a vertebrate animal). 

Aspidogaster have one larval stage with cilia. Monogenes have one intermediate developmental stage, the free-swimming larva of Oncomiracidium. Its epidermis carries several bands of ciliated cells, with the help of which the larva can swim. Monogenes are mostly ectoparasites and, like trematodes, retain limited mobility during the adult stage. They have an attachment disk at the posterior end of their body by which the worm can attach itself to the host's body surface. Many species of endoparasitic tapeworms do without locomotion mechanisms at all. This occurs when the life cycle of the worm does not involve an aquatic phase, as occurs in representatives of the family Taeniidae. Other species of cestodes have a stage of ciliated free-swimming larvae like other Neodermata. In tapeworms, for example, in representatives of the genus Diphyllobothrium, such a larva is called coracidium.