Flatworms: phylum Platyhelminthes

This page is part of a series introducing various animal phyla. For the first day of animal phyla, you should look at these pages:

On other lab days, we'll look at some other animal phyla:

Flatworm features:

Flatworms are anatomically simple compared to nematodes and annelids. Many members of this phylum are parasitic (tapeworms, for example), but the specimens you'll see in lab are Planaria, which is free-living (non-parasitic).

  • Three tissue layers in embryo. Almost all animals share this basic feature; the sponges and cnidarians are exceptions.
  • Acoelomate: Flatworms don't have any kind of coelom or pseudocoelom; their bodies are basically solid. This simple body structure led biologists to conclude that the phylum Platyhelminthes branched off from the rest of the animals before the evolution of the coelom. However, some genetic studies have led some researchers to argue that flatworms descended from an ancestor that had a coelom, and later lost the coelom.
  • Gastrovascular cavity: The digestive tract has only one opening, and branches throughout the body. Flatworms do extracellular digestion, like most animals (but unlike sponges).
  • Pharynx: a muscular tube through which the flatworm can suck food into its gastrovascular cavity. The opening into the pharynx could be considered the mouth, but since this animal has a two-way gut, that opening also must function as the anus.

Planaria whole mount

Planaria whole mount, showing gastrovascular cavity, pharynx, and eyespots

This specimen is stained to show the gastrovascular cavity, showing the small branches called diverticula. Note that there is one main branch of the gastrovascular cavity in the anterior part of the body, but two main branches posterior to the pharynx.

The eyespots are simple and don't form an image; that's why they are called eyespots instead of eyes. However, they are slightly cup-shaped and face toward the sides. With this arrangement, the flatworm can tell light from dark and move toward the dark.

Planaria cross sections

This microscope slide has several different cross sections of a Planaria, showing different regions of the body.

Anterior region

This section anterior to (in front of) the pharynx. The body is more or less solid; the only openings are the diverticula of the gastrovascular cavity.

Planaria, cross section through posterior region

Pharynx region:

This cross section shows the pharynx, which is retracted into the pharyngeal cavity (the empty white space surrounding the pharynx). The pharynx itself is a thick, muscular tube; when Planaria eats, it everts the pharynx, sticking it out of the body to suck up bits of food.

Planaria cross section, pharynx region

Posterior region:

The gastrovascular cavity is divided into two main branches, with smaller diverticula. The vertical pink lines are dorsoventral muscles.

Planaria cross section

References & further reading

Are Platyhelminthes Coelomates without a Coelom? An Argument Based on the Evolution of Hox Genes. Guillame Balavoine, 1998. American Zoologist (1998) 38 (6): 843-858. Flatworms, lacking a coelom, were traditionally thought to represent leftovers from the early days of animal evolution, before any animals had a coelom. The author suggests that modern flatworms descended from a coelomate ancestor, which lost its coelom (and its anus!) through the course of evolution.

Flatworm on Wikipedia.

Molecular phylogeny of the Platyhelminthes. Jaume Baguñà and Marta Riutort, 2004. Canadian Journal of Zoology, 82: 168–193.

Platyhelminthes on Encyclopedia of Life. A brief summary of the phylum, accompanied by a great many images.


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