Platyhelminthes: Body Cavity & Movement

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  • 0:04 Intro to Platyhelminthes
  • 0:33 What Are Platyhelminthes?
  • 1:47 What Are Coelomates?
  • 3:14 Acoelomate Ambulation
  • 4:28 Lesson Summary
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Lesson Transcript
Instructor: Sarah Phenix
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.

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