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The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of relatively simple soft-bodied invertebrate animals. Flatworms are found in marine, freshwater, and even damp terrestrial environments. Most are free-living forms, but many are parasitic on other animals. There are four classes: Trematoda (Flukes), Cestoda (Tapeworms), Monogenea, and Turbellaria.


 


The flatworm’s cephalized soft body is ribbon-shaped, flattened 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.

There is also no true body cavity except the gut. The interior of the body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue. There is no true circulatory or respiratory system, but as a requirement of all organisms, flatworms do take in oxygen. Extracellular body fluids percolate between cells to help distribute nutrients, gases, and waste products.

Depending on species and age, individuals can range in size from almost microscopic to over 20 m long (some tapeworms can attain this length).


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.

Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. 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 or 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 upper end of the gut causes a strong sucking force allowing flatworms to ingest their food and tear it into small bits. The gut is branched and extends throughout the body, functioning in both digestion and transport of food. Flatworms exhibit an undulating form of locomotion.


Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. They 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 flatworms is cut in half, each half will regenerate into two separate fully-functioning flatworms.

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 Kingdom: Animalia

SubKingdom:  Eumetazoa

Superphylum:  Platyzoa

 Subphylum: Platyhelminthes