Flatworms belong to the phylum Platyhelminthes. They have the simplest body plan of all bilaterally symmetrical animals. They are called flatworms because their bodies are compressed. The mouth is the only opening into the digestive cavity the flatworms have. Food is taken in through this hole and wastes are discharged also through this hole. Flatworms have a well-defined nervous, muscular, excretory, and reproductive system. The flatworm distributes the food it digests through a digestive tube that branches throughout all of its body parts. The fact that the worm's body is flat serves many purposes. It allows the worm to hid in small spaces, to fit into the opening of other animals if the worm is parasitic, and it means that all the cells are close enough to the surface for exchange of oxygen and carbon dioxide with the environment (Meinkoth 399).
There are about thirty thousand species of worms in the phylum Platyhelminthes. Twenty five thousand of these worms are parasitic, which is the vast majority of the phylum. The flatworms only account for four thousand of the worms in the phylum, but they are said to be very beautiful compared to the parasitic worms (Snyderman and Wiseman 83). Flatworms display bilateral symmetry, which means that their bodies have a distinctive head and rear, and their right and left sides are mirror images of each other. The fact that these worms are bilaterally symmetric enables them to move from place to place more efficiently. This trait can help the worms catch their food and get away from their enemies. It can help them find a mate and move from location to location. If they were not made like this, they would have very little control over their bodily movements, and not be able to survive in the ocean (Snyderman and Wiseman 84).
Some other characteristics that make worms of the phylum Platyhelminthes unique are the fact that they are light sensitive. They often try to avoid brightly lit areas. They are so sensitive because they possess' bundles of light-sensitive eyes that are often described as primitive eyes (Snyderman and Wiseman 84). Another interesting characteristic of the flatworms is that they are hermaphroditic. This means that they posses both male and female reproductive organs. When one learns of this they may think that self-fertilization would be common, but it is not. Flatworms also posses amazing regenerative properties. When they lose a body part, a new one will form, and it some cases, the body part often regenerates an entirely new flatworm (Snyderman and Wiseman 84).
One can find Marine flatworms in shallow rubble zones. They like to crawl in and under the rubble. Sometimes, the rubble and debris will rip or tear the flatworm's skin, but because of their amazing regenerative properties, this is not much of a problem. Flatworms are usually spotted on the bottom of the ocean, but some species have been found swimming. They can do this by using an undulating motion of their body. This is due to the fact that they are bilaterally symmetrical (Meinkoth 401).
There are three classes included in the phylum Platyhelminthes. They are the Turbellaria, Trematoda, and the Cestoidea. The turbellaria class includes mostly free living forms. This means that they are not parasitic and do live off another organism. The most well known members of the Turbellaria class are the Tricladida and Polycladida.
These are the members that are most commonly found on the beach (National Audubon Society 399). Triclads have a digestive cavity that contains three major branches. They lay their eggs in a solid bubble that they then attach to an immovable object. Development of the eggs is direct in triclads. The young worms are tiny replicas of adult worms. Unlike the triclads three major branches, polyclads have numerous branches in their digestive cavity. Also dissimilar is the eggs are not enclosed in a shell. The development can be direct in some species and indirect...
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