Platyhelminthes and flatworms mean one and same type of worms. As the name reveals, these worms are usually thin, ribbonlike and dorsoventrally flattened. While some species are microscopic in size, others measure several feet in length. Notable characters shared by all flatworms are, they are triploblastic (having three germ layers) and bilaterally symmetrical organisms, i.e., their body can be divided into two equal halves that are mirror images of each other.
About 20,000 species of platyhelminthes are scientifically identified from the freshwater and marine habitats. Of all the types of worms, flatworms are the simplest forms, characterized by presence of a blind gut. These organisms have a single mouth opening, which is responsible for intake of food and excretion of wastes. They neither have a distinctive skeleton nor digestive, nervous, circulatory and respiratory systems. In the upcoming paragraphs, you will get to know about platyhelminthes reproduction in detail.
How do Platyhelminthes Reproduce?
Both nonparasitic and parasitic flatworms are found in nature. The parasitic worms bear hooks, scolex, suckers and other features for attaching to the host organisms. A classic example of platyhelminthes that most of us are aware of is the tapeworm. It is an intestinal parasite that infests humans and vertebrates in general. Speaking more about phylum platyhelminthes, reproduction takes place by means of asexual and sexual methods. Just like other bodily systems, the platyhelminthes reproductive system is very simple to understand. Let's take a look at reproduction processes of platyhelminthes.
The commonly observed platyhelminthes reproduction is asexual type. Several species of the nonparasitic flatworms undergo transverse fission, in which a single organism splits up into smaller fragments through transverse division. Since flatworms have the ability to regenerate themselves, the lost parts develop and each of the fragment grows into a separate individual. Once, they enter the host organism, they travel to the intestinal tract and start feeding on the partially digested foods. Very soon, they break up into smaller fragments, and each single piece then develops into an individual worm later.
According to studies conducted on platyhelminthes breeding, it is found that majority of the species are simultaneous hermaphrodites (or monoecious). In other words, both the male and female reproductive organs are present in a single flatworm. This allows a platyhelminth to undergo sexual reproduction on its own. The actual mode of reproduction may differ slightly from one species to another. For all flatworms, fertilization is of internal type, and the male and female gametes fuse inside the body. Some platyhelminthes release cocoons filled with eggs, which later hatch into young worms that are similar to the matured ones.
In other cases of platyhelminthes (e.g., tapeworm), reproduction is preceded by self-fertilization in a single proglottid (segment containing both male and female reproductive structures) or between two proglottids. On the other hand, there are other flatworms that require cross-fertilization for reproduction. Based on the species, fertilized eggs are released from the host organism through feces, or the proglottid filled with eggs is shed as a whole from the host body. The eggs encased in an outer shell remain as they are, until a prospective intermediate host ingests them. Inside the host body, they hatch to larvae and life cycle continues again.
This was an overview concerning 'how do the platyhelminthes undergo reproduction process'. The parasitic flatworms have a specific pattern of reproduction, wherein the sexual stages are completed in the primary (or definitive) host organisms and the asexual stages are present in the intermediate hosts. The rapid multiplication rate of matured worms is a major concern, while dealing with the parasitic forms of flatworms.