By: Javier Ovalles
Introduction to Platyhelminthes
Platyhelminthes, commonly known as Flatworms, have no body cavity and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. Flatworms lack an anus because the same pharyngeal opening both takes in food and expels waste. Hence of the lack of any other body cavity, the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat, requiring respiring by diffusion, and having a flattened shape.
Tapeworm (Cestoda)
True tapeworms are exclusively hermaphrodites; they have both male and female reproductive systems in their bodies. The reproductive system includes one or many testes, cirrus, vas deferens and seminal vesicle as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs. There is a common external opening for both male and female reproductive systems, known as a genital pore, which is situated at the surface opening of the cup-shaped atrium. Even though they are hermaphrodites, self-fertilization is a rare event. In order to permit hybridization, cross-fertilization between two individuals is often practiced for reproduction. During copulation, the cirrus of one individual connects with that of the other through the genital pore, and then exchange their spermatozoa.
Bedford's flatworm (Pseudobiceros Bedfordi)
Like all flatworms in the genus Pseudobiceros, they are hermaphroditic and have two penises. During mating, they fence with one another using their penises attempting to stab and inject sperm in their opponent, while avoiding being fertilized themselves. They are able to inseminate their opponent by injecting their sperm into any region of the other's body they are able to penetrate. After successfully injecting the other, the sperm streams through their partner's body on their way to ovaries, where they will fertilize the eggs. The stream is visible through the body tissue of the worm, appearing as pale streaks like lightning jags.
Planarian (Planariidae)
There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its excretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration, allowing adult stem cells to divide and differentiate, thus resulting in two worms. Some species of planaria are exclusively asexual, whereas some can reproduce both sexually and asexually.