Planarians are part of the phylum platyhelminthes, or the flat worms. They are in the class turbellaria which includes the free-living flat worms, and some parasites which make up the rest of the platyhelminthes classes (Cestoda and Trematoda). Hammerhead planarians are of the family geoplanidae or the land planarians. The hammerhead planarians make up the genus Bipalium within the geoplanidae family.
Hammerhead planarians are predatory organisms which prey on earthworms or terrestrial mollusks, this depends on the species of bipalium. They are also cannibalistic. Land planarians are able to move on land because of a structure called the creeping sole. The creeping sole is a highly ciliated area on the ventral side of the organism, a coat of mucus is secreted from glands onto the substrate which allows the planarian to glide smoothly. Hammerhead planarians find their prey by following the prey’s mucus trails, making the far reaching head very useful. They are able to grow up to 2 feet in length.
When a hammerhead planarian reaches its prey, it attaches itself to it using its muscles. Additionally, an adhesive substance is secreted to prevent escape. They completely entrap their prey then kill it. To eat their prey, they extend the pharynx which is found on the middle section of the ventral part of the organism. Digestive enzymes are excreted, the digested tissues are brought into the body via the same pharynx structure.
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In this video a hammerhead planarian hunts and entraps an earthworm. As soon as the planarian attaches itself to the earthworm it begins to react. The planarian then wraps itself around the worm to entrap it.
For the United States there are four invasive species of hammerhead planarians: B. adventitium, B. kewense, B. pennsylvanicum, and B. vagum. It is believed that these species were brought into the United States via horticultural plants. Hammerhead planarians are very successful as an invasive species since they have no known predators, besides themselves. The surface excretions are distasteful if not toxic to many organisms.
Another reason they are so successful is that they can undergo asexual and sexual reproduction and all species are hermaphrodites. The eggs they lay hatch in about 3 weeks. Having the flexibility of two methods of reproduction and a lack of predators allows the hammerhead planarian to be a hugely successful invasive species.