Flatworms are bilaterally symmetrical with a defined head and tail region and a centralized nervous system containing a brain and nerve cords. Clusters of light-sensitive cells make up what are called eyespots. The head region of the flatworm also contains other paired sense organs, which are connected to the flatworm's simple brain. They are more evolved than the Cnidaria because they have bilateral symmetry, with a distinct anterior, posterior, dorsal, and ventral end, with a defined head. However, flatworms also lack a complete digestive tract, but instead only have one opening for substances to both enter and leave the body.
Platyhelminths are commonly known as "flatworms," owing to their almost paper-thin structure. Their digestive system (if present) is incomplete, and they possess only a mouth. Flame cells, which encloses a tuft of cilia that beats are often present to move waste into excretory pores, where it leaves the body system. Three classes will be covered. Members of one class (the turbellaria) are free-living, while the other two classes (trematoda and cestoda) are obligate parasites, meaning that must live on or in another organism, the host, in order to survive. The parasite obtains its nutrition and protection from the host, and often harms the host by depriving it of nutrition and interfering internal organ systems. Often the parasite does not kill the host, but keeps it alive so that can keep its supply of nutrition. Scientists believe that parasites began as free-living organisms. Durring evolution, some organs were modified to adapt to parasitism, and some were lost entirely. Because the host often has already digested the food, some parasites lack a mouth or digestive system. Since parasites are often located in the harsh environment of a digestive system, many have a thick outer covering of cells called a tegument and a cuticle, which is a nonliving layer secreted by the epidermis. Some parasites have hooks or suckers to grip their hosts.
FLAME CELL:
TURBELLARIA
Although most live in marine environments, the
most familiar form of turbellaria are the freshwater planarian. The
planarian's body is covered in cilia and moves by swimming with an undulating
motion or by laying down a layer of mucus over which it slides. It
feeds on both dead material and smaller organisms. It's mouth is
located in the middle of its body and feeds with a muscular tube extending
from its mouth called a pharynx. Food is sucked into the pharynx,
passes through the intestine, and is excreted back through the pharynx.
Chemical wastes are eliminated through ducts that run the length of the
body. Each duct contains many flame cells, which help to move the
waste out of the body. Planarians have a cephalized nervous system
(meaning it is centralized in a head-like region). They are able
to sense differences in light due to eye spots and can sense touch, taste
and smell. The information is received in the ganglia, two clusters
of nerve cells that act as a simple brain.
Planarians have a surprising ability to learn. They can learn to do things such as finding a way through a maze. Their memory is stored chemically, which means one planarian can learn by eating another planarian that already posses information.Planarians can reproduce both sexually and asexually. They are hermaphrodites, meaning they have both male and female parts. The most amazing thing about planarians is their ability to regenerate. As shown below, planarians can be cut in half and grow to form two new organisms. It is also possible to cut one partially, resulting in a two headed planarian.
Trematoda
Classes Trematoda the parasitic flukes. These flatworms have similar structures to class Turbellaria but their mouth is at the anterior end. These worms have suckers in the mouth that attach to blood vessels near the intestine. While attached near the intestine, eggs pass out with the primary host's feces (such as a sheep or fish) and eventually contaminate water, where they are ingested by snails (the first intermediate host). The eggs hatch within the snail's gut and eventually burrow out of the snail and become free-swimming larva. If a single egg is ingested by a snail, several hundred free-swimming larva can be produced. The larva then seek out a second intermediate host (a fish or sheep) and burrow through the fish's skin. Once in the fish's muscles they lose their tails and encyst. When raw or poorly cooked fish is eaten the young flukes are released from their cysts by digestive juices in the stomach. Once out of the stomach they make their way up the common bile duct to the liver where they attach and suck the host's blood.
In some areas of China the incidence of parasitism in man is close to 100%. Do you like sushi?
A major world health issue are blood flukes, which
infect 200 to 300 million people, mostly in third world nations.
The diseases, known as schistosomiasis. The life cycle is
similar to other flukes, except the adult form lives in humans. The
spiny eggs lodge in human veins, lungs, intestines, bladder and liver.
They block blood vessels and cause internal bleeding and tissue decay that
can be fatal.
Cestoda
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The cestodes, or tapeworms, differ in a number of ways from other flatworms. Their bodies are long and flat, made up of many segments called proglottids. Each proglottid is a reproductive unit, essentially a factory to produce gametes. Adults lack cilia and their surface is a tegument (as in monogeneans and trematodes), but in cestodes the tegument is covered with tiny projections, microvilli, which increase its surface area and thereby its ability to absorb nutrients from a host. Digestive tracts are absent completely. At the tapeworm's anterior end is a specialized segment called a scolex (pictured above), which is usually covered with hooks or suckers and serves to anchor it to the host.
Largest flatworm ever found, a tape worm, was 90 feet longAll of the 5000 or so known species of tapeworms are endoparasites. Most require at least two hosts, with the host of the adult tapeworm a vertebrate. Intermediate hosts are often invertebrates. A number of tapeworm species inhabit humans. Tapeworm infections pose a serious public health problem in many less developed countries due to poor sanitation conditions. The disease is most common where livestock, such as cattle and pigs, are raised in areas where human feces are not disposed of in a sanitary manner. Another common source of human tapeworms are certain species of freshwater fish. Tapeworm infections tend to occur more frequently in areas of the world where the people regularly eat raw or undercooked beef, pork, or fish.
http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html
http://cas.bellarmine.edu/tietjen/images/platyhelminthes.htm
http://www.kent.k12.wa.us/staff/rlynch/sci_class/chap13/platy.html
http://www.pbs.org/kcet/shapeoflife/animals/platyhelminthes.html