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.
example of a tapeworm
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.
An example of a polyclad
with ruffled body margin (Pseudobiceros gratus)
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.
Photomicrograph of a longitudinal
section through the turbellarian epidermis, showing
cilia (1) and rhabdoids (2)
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 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.
is from a large image bank at the University of Ottawa. You
can connect to it here.