The flatworms 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
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
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