PHYLUM PLATYHELMINTHES
The phylum Platyhelminthes represents all flatworms in the kingdom Animalia.
Flatworms are relatively primitive animals that
represent an early lineage in the kingdom. Unlike the even more primitive phylum
cnidaria, all flatworms of the phylum
Platyhelminthes are bilaterally symmetrical, having an anterior, posterior,
dorsal, and ventral end and two seemingly identical lateral
surfaces. However, flatworms are the only type of bilaterally symmetrical animal
that lacks any type of body cavity, a fluid filled
space between the digestive tract and the body wall. In the flatworms, this area
is filled with tissue. Flatworms have primitive brains,
comprised of clusters of nervous tissue. Two long nerve cords run the length of
the body, and they branch to form small nerves
running throughout the body. Another hallmark of flatworms is a lack of a
complete digestive tract. Flatworms take in food and
excrete waste from one single hole. The opening in the gastrula never fully
evolves in flatworms to make a tract that travels from one
hole in the anterior end to another hole in the posterior end, which would make
the tract complete. There are four classes that make
up the phylum Platyhelminthes, each with unique structures and living styles.
There are four major classes of flatworms:
Class Turbellaria are the nonparasitic, free-living flatworms. These worms have
large eyespots and flaps on each side of the head to
detect chemicals. These worms live in water, usually on the underside of rocks.
They have a highly branched digestive tract, but the
one opening, or mouth, is on the ventral surface, rather than in the anterior
end. The tube extending from the mouth is used to suck
in materials as the worm uses cilia to crawl on the floor of the water.
This class belongs in the phylum Platyhelminthes, consisting of all flatworms.
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 lack a
body cavity, a fluid filled region between the epidermis and the digestive
tract. Flatworms also lack a complete digestive tract, but
instead only have one opening for substances to both enter and leave the body.
This is because the gastrula opening in the
development of a flatworm never fully evolves into a hole in the anterior and
posterior end. The class Turbellaria is one of four classes
in this phylum.
Classes Trematoda and Monogenea represent 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. The
smaller female fluke fits into a groove on the male, and they are often found
attached, a position in which they can copulate freely.
The eggs pass out in the host's feces and the larva can infect another organism
and assexually divide to reproduce. Often, a fluke
life cycle will involve living in more than one host. Many flukes can infect
humans, and cause diseases such as schistosomiasis,
infecting millions in developing countries.
These classes belong in the phylum Platyhelminthes, consisting of all flatworms.
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 lack
a body cavity, a fluid filled region between the epidermis and the digestive
tract. Flatworms also lack a complete digestive tract, but
instead only have one opening for substances to both enter and leave the body.
This is because the gastrula opening in the
development of a flatworm never fully evolves into a hole in the anterior and
posterior end. The classes Trematoda and Monogenea
are two of four classes in this phylum.
Classes Trematoda and Monogenea represent 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. The
smaller female fluke fits into a groove on the male, and they are often found
attached, a position in which they can copulate freely.
The eggs pass out in the host's feces and the larva can infect another organism
and assexually divide to reproduce. Often, a fluke
life cycle will involve living in more than one host. Many flukes can infect
humans, and cause diseases such as schistosomiasis,
infecting millions in developing countries.
These classes belong in the phylum Platyhelminthes, consisting of all flatworms.
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 lack
a body cavity, a fluid filled region between the epidermis and the digestive
tract. Flatworms also lack a complete digestive tract, but
instead only have one opening for substances to both enter and leave the body.
This is because the gastrula opening in the
development of a flatworm never fully evolves into a hole in the anterior and
posterior end. The classes Trematoda and Monogenea
are two of four classes in this phylum.
The forth class of flatworms, class Cestoda, include all tapeworms. These
flatworms have suckers and teeth that grasp the host in
the small head. Behind a short neck are repeated parts of the worm, each
containing reproductive structures that contain both eggs
and sperm, which can pass out through the host's feces, like fluke eggs. These
repeated parts are in order, with the oldest parts
farthest away from the head of the tapeworm. The pieces give the worm a
ribbonlike structure, beneficial for absorbing nutrients from
the intestine, where these parasitic tapeworms live.
This class belongs in the phylum Platyhelminthes, consisting of all flatworms.
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 lack a
body cavity, a fluid filled region between the epidermis and the digestive
tract. Flatworms also lack a complete digestive tract, but
instead only have one opening for substances to both enter and leave the body.
This is because the gastrula opening in the
development of a flatworm never fully evolves into a hole in the anterior and
posterior end. The class Cestoidea is one of four classes
in this phylum.
TISSUE LAYERS OF A FLATWORM:
LIFE CYCLE OF PORK TAPEWORM:
