Temporal range: 40–0 Ma
|A marine species Pseudobiceros bedfordi (Bedford's Flatworm), a member of the Polycladida|
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.
The Acoelomorpha and the genus Xenoturbella were formerly included in the Turbellaria, but are no longer regarded as Platyhelminthes. All the exclusively parasitic Platyhelminthes form a monophyletic group Neodermata, and it is agreed that these are descended from one small sub-group within the free-living Platyhelminthes. Hence the "Turbellaria" as traditionally defined are paraphyletic.
Traditional classifications divide the Platyhelminthes into four groups: Turbellaria and the wholly parasitic Trematoda, Monogenea and Cestoda. In this classification the Turbellaria include the Acoelomorpha (Acoela and Nemertodermatida). The name "Turbellaria" refers to the "whirlpools" of microscopic particles created close to the skins of aquatic species by the movement of their cilia.
Features common to all Platyhelminthes
Platyhelminthes are bilaterally symmetrical animals, in other words their left and right sides are mirror images of each other; this also implies that they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians they are triploblastic, having three main cell layers.The radially symmetrical cnidarians and ctenophore are diploblastic having only two cell layers. Unlike most other bilaterians, platyhelminthes have no internal body cavity and are therefore described as acoelomates. They also lack specialized circulatory and respiratory organs 
The lack of circulatory and respiratory organs limits platyhelminthes to sizes and shapes that enable gas exchange (of oxygen and carbon dioxide), to take place all over their bodies by simple diffusion. Hence many are microscopic and the large species have flat ribbon-like or leaf-like shapes. The guts of large species have many branches, so that nutrients can diffuse to all parts of the body. Respiration through the whole surface of the body makes platyhelminthes vulnerable to fluid loss, and restricts them to environments where dehydration is unlikely: sea and freshwater; moist terrestrial environments such as leaf litter or between grains of soil; and as parasites within other animals.
The space between the skin and gut is filled with mesenchyme, a connective tissue that is made of cells and reinforced by collagen fibers that act as a type of skeleton, providing attachment points for muscles. The mesenchyme contains all the internal organs and allows the passage of oxygen, nutrients and waste products. It consists of two main types of cell: fixed cells, some of which have fluid-filled vacuoles; and stem cells, which can transform into any other type of cell, and are used in regenerating tissues after injury or asexual reproduction.
Most platyhelminths have no anus and regurgitate undigested material through the mouth. However some long species have an anus and some with complex branched guts have more than one anus, since excretion only through the mouth would be difficult for them. The gut is lined with a single layer of endodermal cells which absorb and digest food. Some species break up and soften food first by secreting enzymes in the gut or the pharynx (throat).
All animals need to keep the concentration of dissolved substances in their body fluids at a fairly constant level. Internal parasites and free-living marine animals live in environments that have high concentrations of dissolved material, and generally let their tissues have the same level of concentration as the environment, while freshwater animals need to prevent their body fluids from becoming too dilute. Despite this difference in environments, most platyhelminths use the same system to control the level of concentration in their body fluids. Flame cells, so called because the beating of their flagella looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some re-usable material, and drive it into networks of tube cells which are lined with flagella and microvilli. The tube cells' flagella drive the water towards exits called nephridiopores, while their microvilli re-absorb re-usable materials and as much water as is needed to keep the body fluids at the right level of concentration. These combinations of flame cells and tubule cells are called protonephredia.
Planarians are famous for their ability to regenerate if divided by cuts across their bodies. Experiments show that, in fragments that do not already have a head, a new head grows most quickly on those that were closest to the original head. This suggests that the growth of a head is controlled by a chemical whose concentration diminishes from head to tail.
Features specific to Turbellaria
These have about 4,500 species, are mostly free-living, and range from 1 mm (0.039 in) to 600 mm (24 in) in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded humid locations such as leaf litter or rotting wood. However some are symbiotes of other animals such as crustaceans, and some are parasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.
Turbellarians have no cuticle (external layer of organic but non-cellular material). In a few species the skin is a syncitium, a collection of cells with multiple nuclei and a single shared external membrane. However the skins of most species consist of a single layer of cells, each of which generally has multiple cilia (small mobile "hairs"), although in some large species the upper surface has no cilia. These skins are also covered with microvilli between the cilia. They have many glands, usually submerged in the muscle layers below the skin and connect to the surface by pores through which they secrete mucus, adhesives and other substances.
Small aquatic species use the cilia for locomotion, while larger ones use muscular movements of the whole body or of a specialized sole to creep or swim. Some are capable of burrowing, anchoring their rear ends at the bottom of the burrow, then stretching the head up to feed and then pulling it back down for safety. Some terrestrial species throw a thread of mucus which they use as a rope to climb from one leaf to another.
Some Turbelleria have spicular skeletons, giving the appearance of annulations.
Diet and digestion
The acoel Convoluta roscoffensis swallows cells of the green alga Tetraselmis and does not feed as an adult, presumably relying on the alge to provide nourishment as endosymbionts. In other acoels the gut is lined by a syncitium. These and some other turbellarians have a simple pharynx lined with cilia and generally feed by using cilia to sweep food particles and small prey into their mouths, which are usually in the middle of the underside.
Most other turbellarians are carnivorous, either preying on small invertebrates or protozoans, or scavenging on dead animals. A few feed on larger animals, including oysters and barnacles, while some, such as Bdelloura, are commensal on the gills of horseshoe crabs. These turbellarians usually have an eversible pharynx, in other words, one that can be extended by being turned inside-out, and the mouths of different species can be anywhere along the underside. The freshwater species Microstomum caudatum can open its mouth almost as wide as its body is long, to swallow prey as large as itself.
The intestine is lined by phagocytic cells which capture food particles that have already been partially digested by enzymes in the gut. Digestion is then completed within the phagocytic cells and the nutrients diffuse through the body.
Concentration of nervous tissue in the head region is least marked in the acoels, which have nerve nets rather like those of cnidarians and ctenophores, but densest around the head. In other turbellarians, a distinct brain is present, albeit relatively simple in structure. From the brain one to four pairs of nerve cords run along the length the body, with numerous smaller nerves branching off. The ventral pair of nerve cords are typically the largest, and, in many species, are the only ones present. Unlike more complex animals, such as annelids, there are no ganglia on the nerve cords, other than those forming the brain.
Most turbellarians have pigment-cup ocelli ("little eyes"), one pair in most species, but two or even three pairs in some. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly round the edge of the body. The ocelli can only distinguish the direction from which light is coming and enable the animals to avoid it.
A few groups – mainly catenulids, acoelomorphs and seriates – have statocysts, fluid-filled chambers containing a small solid particle or, in a few groups, two. These statocysts are thought to be balance and acceleration sensors, as that is the function they perform in cnidarian medusae and in ctenophores. However turbellarian statocysts have no sensory cilia, and it is unknown how they sense the movements and positions of the solid particles.
Most species have ciliated touch-sensor cells scattered over their bodies, especially on tentacles and around the edges. Specialized cells in pits or grooves on the head are probably smell-sensors.
Many turbellarians clone themselves by transverse or longitudinal division, and others, especially acoels, reproduce by budding. The planarian Dugesia is a well-known representative of class Turbellaria.
All turbellarians are simultaneous hermaphrodites, having both female and male reproductive cells, and fertilize eggs internally by copulation. Some of the larger aquatic species mate by penis fencing, a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs.
Although the acoels have no distinct gonads at all, in other turbellarians there are one or more pairs of both testes and ovaries. Sperm ducts run from the testes, through bulb-like seminal vesicles, to the muscular penis. In many species, this basic plan is considerably complicated by the addition of accessory glands or other structures. The penis lies inside a cavity, and can be everted through an opening on the posterior underside of the animal. It often, although not always, possesses a sharp stylet. Unusually among animals, in most species, the sperm cells have two tails, rather than one.
Classification and evolutionary relationships
Detailed morphological analyses of anatomical features in the mid-1980s and molecular phylogenetics analyses since 2000 using different sections of DNA agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians") and Nemertodermatida (another small group previously classified as "turbellarians") are the sister group to all other bilaterians, including the rest of the "Platyhelminthes".
The "Platyhelminthes" excluding "Acoelomorpha" contain two main groups, Catenulida and Rhabditophora, and it is generally agreed that both are monophyletic, in other words each contains all and only the descendants of an ancestor which is a member of the same group. Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group, but a study in 2008 concluded that they could, and therefore that "Platyhelminthes" could be redefined as Catenulida plus Rhabditophora, excluding the "Acoelomorpha".
It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups (Cestoda, Monogenea and Trematoda) is monophyletic, and that together these form a larger monophyletic grouping, the Neodermata, in which the adults of all members have syncitial skins. It is also generally agreed that the Neodermata are a relatively small sub-group a few levels down in the "family tree" of the Rhabditophora. Hence the traditional sub-phylum "Turbellaria" is paraphyletic, since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".
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