The Phylum Platyhelminthes contains over 20,000 free-living and parasitic
species of acoelomate animals called flatworms. In flatworms, the body that is
flattened dorsoventrally, with the mouth and genital pore usually located in a
ventral position. The space between the gut and outside is filled with
mesodermal muscle fibers and undifferentiated parenchyma. Although fluid-filled
spaces in the parenchyma serve as a hydrostatic skeleton for support and to aid
in internal transport, the animals lack a body cavity, which is why they are
called acoelomate. Most free-living flatworms have a gastrovascular-type
digestive system (a mouth is present but no anus), while parasitic forms
generally have no digestive system.
Flatworms have a centralized nervous system consisting of pair of cerebral
ganglia and longitudinal nerve cords connected to transverse nerves. The
excretory system (absent in some forms) consists of two lateral canals with
protonephridia bearing flame cells. Although many flatworms are free-living, the
phylum includes some very important parasitic species as well.
In terms of reproduction, flatworms can reproduce sexually or asexually. Most
species are monoecious but practice cross fertilization. Many freshwater
turbellarians can reproduce asexually by fission in which the animal simply
divides into two halves, each of which regenerates the other half. In some
turbellarians (as it is in most other animals), the yolk that provides nutrition
for the developing embryo is containing within the egg cell itself, a condition
described as endolecithal. In the monogeneans, trematodes and cestodes (as well
as in a few turbellarians), yolk is contributed by cells released from organs
called yolk glands, and the eggs are therefore described as ectolecithal.
Development may be direct or indirect.