Flatworm
Platyhelminthes is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called flatworms or flat worms. Being acoelomates, and having no specialised circulatory and respiratory organs, they are restricted to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food can not be processed continuously.
In traditional medicinal texts, Platyhelminthes are divided into Turbellaria, which are mostly non-parasitic animals such as planarians, and three entirely parasitic groups: Cestoda, Trematoda and Monogenea; however, since the turbellarians have since been proven not to be monophyletic, this classification is now deprecated. Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments, such as leaf litter. Cestodes and trematodes have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of hermaphroditic, segment-like proglottids that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host.
Because they do not have internal body cavities, Platyhelminthes were regarded as a primitive stage in the evolution of bilaterians. However, analyses since the mid-1980s have separated out one subgroup, the Acoelomorpha, as basal bilaterians – closer to the original bilaterians than to any other modern groups. The remaining Platyhelminthes form a monophyletic group, one that contains all and only descendants of a common ancestor that is itself a member of the group. The redefined Platyhelminthes is part of the Spiralia, one of the two main groups of Protostomia. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups, Catenulida and Rhabditophora, with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded as paraphyletic, since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians".
A planarian species has been used in the Philippines and the Maldives in an attempt to control populations of the imported giant African snail, which was eating agricultural crops. Success was initially reported for the Maldives but this was only temporary and the role of flatworms has been questioned. These planarians have now spread very widely throughout the tropics and are themselves a serious threat to native snails, and should not be used for biological control. In Northwestern Europe, there are concerns about the spread of the New Zealand planarian Arthurdendyus triangulatus, which preys on earthworms.
Description
Distinguishing features
Platyhelminthes are bilaterally symmetrical animals: their left and right sides are mirror images of each other; this also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians, they have three main cell layers, while the radially symmetrical cnidarians and ctenophores have only two cell layers. Beyond that, they are "defined more by what they do not have than by any particular series of specializations." Unlike most other bilaterians, Platyhelminthes have no internal body cavity, so are described as acoelomates. Although the absence of a coelom also occurs in other bilaterians: gnathostomulids, gastrotrichs, xenacoelomorphs, cycliophorans, entoproctans and the parasitic mesozoans. They also lack specialized circulatory and respiratory organs, both of these facts are defining features when classifying a flatworm's anatomy. Their bodies are soft and unsegmented.Features common to all subgroups
The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable oxygen to reach and carbon dioxide to leave all parts of their bodies by simple diffusion. Hence, many are microscopic, and the large species have flat ribbon-like or leaf-like shapes. Because there is no circulatory system which can transport nutrients around, the guts of large species have many branches, allowing the nutrients to diffuse to all parts of the body. Respiration through the whole surface of the body makes them 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, also known as parenchyma, a connective tissue 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. The genus Paracatenula, whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut. 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 that absorb and digest food. Some species break up and soften food first by secreting enzymes in the gut or pharynx.
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 with 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 concentration of 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 reusable 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 reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called protonephridia.
In all platyhelminths, the nervous system is concentrated at the head end. Other platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies.
Major subgroups
Early classification divided the flatworms in four groups: Turbellaria, Trematoda, Monogenea and Cestoda. This classification had long been recognized to be artificial, and in 1985, Ehlers proposed a phylogenetically more correct classification, where the massively polyphyletic "Turbellaria" was split into a dozen orders, and Trematoda, Monogenea and Cestoda were joined in the new order Neodermata. However, the classification presented here is the early, traditional, classification, as it still is the one used everywhere except in scientific articles.Turbellaria
These have about 4,500 species, are mostly free-living, and range from to 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. The Acoela and Nemertodermatida were traditionally regarded as turbellarians, but are now regarded as members of a separate phylum, the Acoelomorpha, or as two separate phyla. Xenoturbella, a genus of very simple animals, has also been reclassified as a separate phylum.Some 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 their undersides. Most other turbellarians have a pharynx that is eversible, 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 about as large as itself. Predatory species in suborder Kalyptorhynchia often have a muscular pharynx equipped with hooks or teeth used for seizing prey.
Most turbellarians have pigment-cup ocelli ; one pair in most species, but two or even three pairs in others. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly around the edge of the body. The ocelli can only distinguish the direction from which light is coming to enable the animals to avoid it. A few groups have statocysts – fluid-filled chambers containing a small, solid particle or, in a few groups, two. These statocysts are thought to function as balance and acceleration sensors, as they perform the same way in cnidarian medusae and in ctenophores. However, turbellarian statocysts have no sensory cilia, so the way they sense the movements and positions of solid particles is unknown. On the other hand, most 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 most likely smell sensors.
Planarians, a subgroup of seriates, are famous for their ability to regenerate if divided by cuts across their bodies. Experiments show that a new head grows most quickly on those fragments which were originally located closest to the original head. This suggests the growth of a head is controlled by a chemical whose concentration diminishes throughout the organism, from head to tail. Many turbellarians clone themselves by transverse or longitudinal division, whilst others, reproduce by budding.
The vast majority of turbellarians are hermaphrodites which 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. In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae.