Flatworm
The flatworms (Phylum Platyhelminthes from the Greek platy, meaning "flat" and helminth, meaning worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which rapidly colonized large areas of Ireland and Scotland since its unintentional introduction in the 1960s and has since destroyed most of the indigenous earthworms     . Most flatworms are free-living, but many are parasitic. There are four classes: Trematoda (flukes), Cestoda (tapeworms), Monogenea, and Turbellaria.

# Description
The flatworms cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated  epidermis. There is also no true body cavity (coelom) except the gut; hence, flatworms are classified as acoelomates.  The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue.
Flatworms have a hydrostatic skeleton, which consists of a water filled body cavity that is controlled by muscles.
Flatworms exhibit an undulating form of locomotion.
Depending on species and age, individuals can range in size from almost microscopic to over 20 m long. The longest ever recorded flatworm was a tapeworm over 90 ft (27 m) long.

## Haptor
The haptor is a part of Platyhelminthes used to attach to its host. These differ between the prohaptor which is on the anterior end of the body and the opisthaptor, which is part of the posterior end of the body.
The prohaptor has various adhesive and feeding structures. In some species, the prohaptor may have a number of cephalic or head glands that secrete a sticky adhesive substance, and shallow muscular suckers, all used for attachment. In other species there is an oral sucker, with various degrees of muscularisation that surrounds the mouth.
The opisthaptor is primarily responsible for the attachment of the monogeneans to the host. The morphology of the opisthaptor is highly variable. It may have suckers in various degrees of development, large hooks called anchors (or hamuli), small hooks that are remnants from the larval stage, or complex clamps that may be either muscular or sclerotised.

# Circulation and nervous system
There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products.
Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange.
However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.

# Feeding
Usually the digestive tract has one opening, so the animal can feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts are able to do. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below).They eat rarely, maybe a few times per year(once or twice). Their diet includes small bugs found in soils and other types of worms, smaller then itself.
Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level of organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs.
Muscular contraction in the gut causes a strong sucking force which allows flatworms to ingest food.

# Reproduction
Flatworm reproduction is hermaphroditic, meaning  each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. Some flatworms, such as Pseudobiceros hancockanus engage in penis fencing, in which two individuals fight, trying to pierce the skin of the other with their penises; the first to succeed inseminates the other, which must then carry and nourish the fertilized eggs. Flatworms usually do not fertilize their own eggs.
Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these planarian flatworms is cut in half, each half will regenerate, forming two separate, fully-functioning flatworms.

# Classes
Flatworms were formerly consider to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. These findings, however, are still not accepted by all biologists. The systematic position of Catenulida seems uncertain, although Donoghue and Cracraft would place it as a sister group to all other non-Acoelomorpha flatworms. Xenoturbella was at first believed to be a flatworm as well, but it is now obvious that it belongs in its own phyla. The remaining and true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:
- Trematoda - flukes, probably paraphyletic to Cestoda.
- Cestoda - tapeworms
- Monogenea - ectoparasitic flukes with simpler life cycles than Trematode flukes. They live an exclusively parasitic existence.
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
- Catenulida
- Macrostomida
- Lecithoepitheliata
- Rhabdocoela
- Prolecithophora
- Proseriata
- Tricladida
- Polycladida - Marine flatworms
Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause massive damage to humans and other animals.

# Biochemical memory experiments
In 1955, Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1962, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms.  Incredibly these flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.
This experiment intended to show that memory could perhaps be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results, . The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed its behaviour. McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias. No double-blind experiment has ever reproduced his results.