PHYLUM PLATYHELMINTHES (FLATWORMS)
• "Platy" for flat; "heminthes" for worm.
• They are free living or parasitic.
• 29,000 species • They are found in marine and fresh water.
• Simplest of the three worm groups.
TYPES
There are three types of flatworms, including Tapeworms/Cestodas (Giardiasis), Flukes/Trematodas (Asian Liver Fluke), and Planarians/Turbellaria (Pseudoceros dimidiatus).
THE TRIO (THREE GROUPS OF FLATWORMS)
FLUKES - These are parasites with complex life cycles and require more than one host. It's estimated that over one million people die annually due of these worms. Many have complex life cycles that include multiple hosts and alternation of generations.
PLANARIANS - These are free living flatworms. Most are not parasitic. They feed on small organisms. They live under rocks, on plant materials, or in freshwater. Lack circulatory and gas-exchange organs. Have simple excretory structures called flame cells that maintain osmotic balance. Is cephalized and has eye-spots which are sensitive to light. Feeds through a ventral feeding tube called a pharynx. Can reproduce sexually, or asexually through regeneration. Have auricles, which are sensitive to chemicals, on the anterior end.
TAPEWORMS - These are also parasites. Can grow to enormous lengths (up to 20m). Parasitize mostly vertebrate hosts. Attach head (named a scolex) to the intestinal walls of host, and absorb the host's nutrients. Following the scolex is a series of repeating structural units called proglottids. Mature proglottids containing thousands of eggs are released with the host's feces to infect other organisms.
SYMMETRY
Bilateral symmetry.
REPRODUCTION
Flatworms reproduce both sexually (internal) and asexually. Flatworms are hermaphroditic (see Sponge description). They reproduce asexually through regeneration, a process in which body parts that have been lost (through predation, for example) can be regrown. A planarian that is cut in half horizontally can grow a new head on the tail end and a new tail on the head end, forming two new genetically identical organisms.
THE ACOELOMATE BODY PLAN
Flatworms are acoelomate (there is no coelomic cavity). A coelom is a fluid filled cavity and serves to protect the organs from crushing. Thus, aceolomates have sold bodies without a fluid-filled body cavity between the gut and body wall. Nutrients and wastes diffuses from one cell to another because there is no circulatory system in these organisms.
WHAT IS CEPHALIZATION?
Cephalization is a concentration of sense organs, nervous control, processors, etc. at the anterior end of the organism, forming a head and a brain. It is considered an evolutionary trend but also deserves in the development of an embryo. This allowed both simple nervous systems and bilateral symmetry to develop in flatworms and all of the later phyla about which you will be reading.
THE GASTROVASCULAR CAVITY IS SPECIAL
The gastrovascular cavity is pretty special in flatworms because it aids both digestion and circulation, which is a new phylogenetic development in animals. The mouth serves as both the entrance and exit from the gut. Thus, there is a one way digestive tract (similar to that of cnidarians).
GAS EXCHANGE & CIRCULATION
Flatworms have no specialized organs for gas exchange and circulation. There is no specialized circulatory or respiratory system in these animals. Since they are so thin, their cells can use the process of diffusion to transport dissolved oxygen and nutrients to all parts of their bodies. Carbon dioxide and other wastes are also removed through diffusion.
MY THOUGHTS
These flatworms are cool little creatures. It is interesting that some flatworms (tapeworms, specifically) can grow to such long lengths (up to 20 meters)! They are colorful, but it's kind of creepy that they live in millions of humans worldwide. They are so insidious.
• They are free living or parasitic.
• 29,000 species • They are found in marine and fresh water.
• Simplest of the three worm groups.
TYPES
There are three types of flatworms, including Tapeworms/Cestodas (Giardiasis), Flukes/Trematodas (Asian Liver Fluke), and Planarians/Turbellaria (Pseudoceros dimidiatus).
THE TRIO (THREE GROUPS OF FLATWORMS)
FLUKES - These are parasites with complex life cycles and require more than one host. It's estimated that over one million people die annually due of these worms. Many have complex life cycles that include multiple hosts and alternation of generations.
PLANARIANS - These are free living flatworms. Most are not parasitic. They feed on small organisms. They live under rocks, on plant materials, or in freshwater. Lack circulatory and gas-exchange organs. Have simple excretory structures called flame cells that maintain osmotic balance. Is cephalized and has eye-spots which are sensitive to light. Feeds through a ventral feeding tube called a pharynx. Can reproduce sexually, or asexually through regeneration. Have auricles, which are sensitive to chemicals, on the anterior end.
TAPEWORMS - These are also parasites. Can grow to enormous lengths (up to 20m). Parasitize mostly vertebrate hosts. Attach head (named a scolex) to the intestinal walls of host, and absorb the host's nutrients. Following the scolex is a series of repeating structural units called proglottids. Mature proglottids containing thousands of eggs are released with the host's feces to infect other organisms.
SYMMETRY
Bilateral symmetry.
REPRODUCTION
Flatworms reproduce both sexually (internal) and asexually. Flatworms are hermaphroditic (see Sponge description). They reproduce asexually through regeneration, a process in which body parts that have been lost (through predation, for example) can be regrown. A planarian that is cut in half horizontally can grow a new head on the tail end and a new tail on the head end, forming two new genetically identical organisms.
THE ACOELOMATE BODY PLAN
Flatworms are acoelomate (there is no coelomic cavity). A coelom is a fluid filled cavity and serves to protect the organs from crushing. Thus, aceolomates have sold bodies without a fluid-filled body cavity between the gut and body wall. Nutrients and wastes diffuses from one cell to another because there is no circulatory system in these organisms.
WHAT IS CEPHALIZATION?
Cephalization is a concentration of sense organs, nervous control, processors, etc. at the anterior end of the organism, forming a head and a brain. It is considered an evolutionary trend but also deserves in the development of an embryo. This allowed both simple nervous systems and bilateral symmetry to develop in flatworms and all of the later phyla about which you will be reading.
THE GASTROVASCULAR CAVITY IS SPECIAL
The gastrovascular cavity is pretty special in flatworms because it aids both digestion and circulation, which is a new phylogenetic development in animals. The mouth serves as both the entrance and exit from the gut. Thus, there is a one way digestive tract (similar to that of cnidarians).
GAS EXCHANGE & CIRCULATION
Flatworms have no specialized organs for gas exchange and circulation. There is no specialized circulatory or respiratory system in these animals. Since they are so thin, their cells can use the process of diffusion to transport dissolved oxygen and nutrients to all parts of their bodies. Carbon dioxide and other wastes are also removed through diffusion.
MY THOUGHTS
These flatworms are cool little creatures. It is interesting that some flatworms (tapeworms, specifically) can grow to such long lengths (up to 20 meters)! They are colorful, but it's kind of creepy that they live in millions of humans worldwide. They are so insidious.