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

 

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.

 

CLASS TREMATODA

 

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.

 

CLASS MONOGENEA

 

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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CLASS CESTODA

 

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:


 

 

OVERVIEW OF PHYLUM PLATYHELMINTHES