Any of a phylum (Platyhelminthes) of soft-bodied, usually much-flattened worms, including both free-living and parasitic species. Flatworms live in a variety of marine, freshwater, and terrestrial habitats worldwide. They range in length from much less than an inch (a fraction of a millimeter) to 50 ft (15 m) and are of three main types: turbellarians (including the planarian), trematodes (see fluke), and cestodes (see tapeworm). Flatworms are bilaterally symmetrical and lack respiratory, skeletal, and circulatory systems and a body cavity. Turbellarians are mostly free-swimming, but trematodes and cestodes are parasites.
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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.
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.
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.
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.
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.
Flatworms were formerly considered 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:
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
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.
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.