Biology - Body Plan, Shapes and Sizes

Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

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Flatworms come in a variety of sizes and shapes, ranging from tiny microturbellarians that are less than a millimeter in length to tapeworms that can reach many meters.

small flatworm

A microturbellarian


An example of a tapeworm

Polyclad flatworms range from a few millimeters to as much as 30 cm in length. Often their body margins are highly ruffled, although many oval-shaped species with smooth margins exist.

striped flatworm with ruffled margin
An example of a polyclad with ruffled body margin (Pseudobiceros gratus)

A closer look at the "skin" of polyclads reveals that the cells can be filled with pigment granules that impart their color to the worms, giving them beautiful colors and patterns. In addition, many rod-shaped structures called rhabdoids can be found in the epidermal cells. Rhabdoids can be extruded from the cells and upon contact with water, will dissolve to form a mucous sheet enveloping the worms. In some cases, the mucus may be toxic, in others, it may have adhesive properties that help the worms to either attach themseleves to the substrate or helps them entangle prey.

picture of microscope slide showing epidermis with cilia and rhabdites
Photomicrograph of a longitudinal section through the turbellarian epidermis, showing cilia (1) and rhabdoids (2)

In many cases though, the mucus released from the ventral side of the worm acts as a lubricant. Extending from the epidermis are numerous tiny hair-like structures called cilia. These cilia beat back and forth and propel the worm over the substrate that is lubricated with mucus. Most smaller worms will move along in this fashion called ciliary gliding.

Larger worms will use muscles of the body wall to move about. The muscles are located in two distinct layers beneath the epidermis. The outer layer is made up of circularly arranged muscles, the inner layer consists of longitudinal muscles. In some cases, there is a third, diagonal muscle layer interposed between the circular and longitudinal layers. Contraction of these muscle layers results in undulations of the body margin which help to move the animal forward.

schematic cross-section through flatworm

In this schematic diagram of a cross-section through a turbellarian flatworm (specifically Planaria), the circular (1) and longitudinal (2) muscle layers are shown. In addition, dorsoventral muscle bands (3) can be seen spanning across the animal. The large circular structure in the center is the intestine; two gut branches are visible laterally.

The image is from a large image bank at the University of Ottawa. You can connect to it here.

These pages are for educational purposes only. If you use any of the information or the images in your classes, please credit these web pages. © MK Litvaitis, 2005

Photo credits to Leslie Newman, Andrew Flowers, or Anne DuPont, unnless otherwise indicated.


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