In this lesson, we will explore what a Platyhelminthes is, what its internal body structure is like and how this structure impacts the methods this creature uses for motility.
Intro to Platyhelminthes
When you think of a worm, your mind might automatically shoot right to an image of a small, gray earthworm wriggling in the ground. (Or maybe you think of a gummy worm candy.) Either way, those are worms that are round in shape. But not all worms are round -- they actually come in all shapes and sizes. Some of them are even flat. Let's take a look at one of these flatworms, the interesting, and at times extremely invasive, little group of organisms collectively called Platyhelminthes.
What are Platyhelminthes?
Talk about a mouthful of a name! Well, the term Platyhelminthes comes from the Greek platús, meaning 'flat' and hélmins, meaning 'worm'. Therefore, Platyhelminthes literally means 'flat worm.' And, as you might have suspected, Platyhelminthes are a phylum of worms that are flat in shape.
Just to give you a sense of the phylum itself, the majority of the approximately 25,000 known species of Platyhelminthes are parasitic in nature. Now, if you've ever had of pet that was treated for tapeworms or heard of a lung fluke (which infects the lungs of mammals, including humans), liver fluke (which infects the liver), or planarian (a free-swimming non-parasitic worm), then you know of some of the varieties within this phylum.
So what does it mean to be a 'platy' worm? Well, their shape comes from the fact that they, unlike earthworms, have no coelom (otherwise known as a fluid-filled body cavity). Therefore, Platyhelminthes are acoelomates ('a' meaning 'without'). So why is the presence or absence of a fluid-filled body cavity important? Well, it actually influences everything from the way these creatures interact with their environment to what we will be exploring here -- the way they ambulate, or move.
What are Coelomates?
There are actually three types of body forms: coelomates, acoelomates and pseudocoelomates. A true coelomate (like an earthworm) has a fluid-filled body cavity that is completely encased in a layer of tissue. This tissue not only separates the fluid-filled cavity from the body wall but also from the organs within the body.
Pseudocoelomates (such as intestinal roundworms, a type of nematode), on the other hand, are 'pseudo' (fake) coelomates because their fluid-filled cavity is not fully encased in tissue and makes direct contact with the organs within the organism's body.
Ok, so what role does this fluid-filled cavity play in the movement of an organism anyway? Well, if you think of the last time you squeezed a water balloon in the middle, you'll remember that the circular pressure applied elongated the balloon, right? Well, coelomates and pseudocoelomates use that to their advantage. They use circular muscles (that run around the circumference of their bodies) to squeeze the fluid and elongate their body, and then longitudinal muscles (that run the length of their bodies) pull their backend forward. The pressure of their fluid-filled cavity against their body wall gives coelomates and pseudocoelomates their round shape. Therefore, no fluid-filled cavity means no round shape; thus, the flatworm. Ok, so now that we know that Platyhelminthes are acoelomates, let's take a look at what that means for their body movements.
Species of the phylum Platyhelminthes use one of two methods of movement; either they beat epidermal cilia to move along the surfaces or they use a series of muscles to twist and turn their body for forward movement.
Some species of Platyhelminthes, such as planarian flatworms, have little hair-like projections on their underside, called cilia , that they beat against a fluid. This fluid may come from their environment or be a slime-like fluid that they deposit, and the motion of the cilia against it propels them forward.
Other species rely on contracting and relaxing a series of muscles to create a wriggling-twisting movement. This form of movement is reserved for species living in a fluid environment (either marine, freshwater, or inside a host) and is the method that the majority of the parasitic species use.
By contracting and relaxing their longitudinal muscles (that run the length of their bodies) with their circular muscles (running around the circumference of their bodies), they can twist and turn their bodies, thus propelling them forward in a fluid environment. Their third muscle type, called parenchymal muscle (running through the vertical plane of their bodies, connecting the top tissue to the bottom) can be contracted to create a wave-like motion down the length of their body.
Platyhelminthes are a phylum of worms that are flat in shape, the majority of which are parasitic in nature. Their flat shape comes from the fact that they are acoelomates (having no coelom, or fluid-filled body cavity). This means they must utilize other means than that of a coelomate or a psuedocoelomate for movement. Some species have hair-like appendages called cilia on their underside that they beat against a fluid surface. This fluid may come from their environment (being aquatic or that of their host) or can be a slime-like fluid that they deposit. Other species contract and relax their longitudinal muscles (running the length of their bodies) along with their circular muscles (running the circumference) and parenchymal muscle (running through the vertical plane of their bodies) to create a twisting-turning wave-like motion down their bodies.