Phylum Platyhelminthes
Flatworms
Flatworms are the simplest organisms that have tissues organized into organs and organ systems. They have a rudimentary nervous system where information is stored and processed in a simple brain. This "brain" is just a collection of nerve cells in the anterior portion of the worm. There are also several nerve cords that run from the "brain" throughout the length of the worm.
Flatworms are similar to cnidarian in that they only have one opening for digestion.
They have well developed muscle systems.
Have a mesoderm (middle tissue layer).
No body cavity.
The most commonly seen flatworm is the turbellarians. These are mostly free-living carnivores. Some live inside or on the surface of oysters and crabs. See the video below of a beautiful Turbellarian "flying" through the water. If the video does not come up, you can click here.
Flatworms are similar to cnidarian in that they only have one opening for digestion.
They have well developed muscle systems.
Have a mesoderm (middle tissue layer).
No body cavity.
The most commonly seen flatworm is the turbellarians. These are mostly free-living carnivores. Some live inside or on the surface of oysters and crabs. See the video below of a beautiful Turbellarian "flying" through the water. If the video does not come up, you can click here.
The turbellarians in this video below are performing a mating ritual. The worms are hermaphroditic and each tries to penetrate the skin of the other which its needle-like reproductive organ to inject its sperm. The first to succeed acts as the male and avoids the energy costs of healing the wound and developing the eggs. If the video does not show, you can click here.
The largest group of flatworms are the flukes, or trematodes. All flukes are parasitic. Adult flukes live in a vertebrate organisms but the larva inhabit snails, clams or fish. The larve is eaten when another vertebrate easts one of these and the adult fluke lives in this vertebrate and reproduces. Flukes are common in fish, seabirds and whales. One fluke in particular, a schistosome, infects snails. The If you enter contaminated water, you may be infected and get what is called "swimmer's itch". A marine variety of this occurs often in Australia and is called "pelican itch". The fluke spends part of its life cycle in snails (marine or freshwater). Larvae of the fluke then leave the snail and float freely in the water until they encounter a bird. Then they infect the bird where they will complete the next cycle of their reproduction. In the case of swimmer's itch, because the fluke normally hosts in birds, they are not too troublesome for humans. They can enter the skin of a human, but the promptly die and typically cause an itchy rash for a few days. Other schitosomes DO use humans as one of their life stages and can cause great illness for infected people.
Below, you can see a typical life cycle for a schistosome (fluke) that is harmful to humans.
Below, you can see a typical life cycle for a schistosome (fluke) that is harmful to humans.
Tapeworms, or cestodes, are also parasitic flatworms. They typically have a long body of segmented parts. They attach in the intestines of most species of vertebrates. Tapeworms lack a gut or a mouth. Rather, they just absorb nutrients across the body wall. The host's intestines have done all of the digestion for them! Tapeworms can get very large. The largest one on record was found in a whale and measured 130 ft long!
Tapeworms attach using an interesting collecting of hooks and suckers (as seen inthe photos below). their eggs are made and stored in the proglottids (or segments). As the worm grows, new proglottids are added behind the neck. An organism will defecate and a segment of eggs will go out with the feces. Then, if another organism touches the feces and ingests some of the eggs (like not washing your hands after going to the bathroom and then touching your mouth at lunch time), the eggs will travel into the intestines and attach.
Tapeworms attach using an interesting collecting of hooks and suckers (as seen inthe photos below). their eggs are made and stored in the proglottids (or segments). As the worm grows, new proglottids are added behind the neck. An organism will defecate and a segment of eggs will go out with the feces. Then, if another organism touches the feces and ingests some of the eggs (like not washing your hands after going to the bathroom and then touching your mouth at lunch time), the eggs will travel into the intestines and attach.