The next animal to consider is the flatworm. This animal belongs to Class Turbellaria of the Phylum Platyhelminthes. This phylum also includes the tapeworms, Class Cestoda, and the flukes, Class Trematoda. Flatworms are motile animals and have bilateral symmetry instead of radial symmetry as seen in the sea anemone. There is also the beginning of the development of the anterior region into a centralized location for stimulus reception (eyespots and chemo-sensory auricles) and integration (the anterior ganglia or "brain"), a process known as cephalization.

(A) Muscles and Acoelomate Body cavity

Flatworms have a much more highly organized body wall than the cnidarians. Distinct muscle cells are organized into recognizable muscle layers. Obtain a slide showing a cross section of the flatworm Planaria and you'll see a ciliated epithelium followed by a layer of circular muscles and finally a longitudinal muscle layer. Compare this to the figure in Rust. Also visible will be one or more branches of the gut. Also notice that between the gut and the muscular body wall there is no open space or cavity, a condition called acoelomate (without a coelom). Instead there is a loose packing of mesodermally-derived cells known as parenchyma or mesenchyme.

Place a live planarian in a watch glass with pond water and observe its movement. You'll notice it glides along over even surfaces on its ventral ciliated surface. The planarian accomplishes this by secreting a mucous layer against which the cilia can push. It is also capable of crawling over or around obstacles using muscular undulations and exhibits fairly refined movements in response to mechanical stimulus. How does this animal move? Structurally the longitudinal and circular muscles layers serve as potential antagonists but where is the prerequisite non-compressible fluid or pressurized cavity?

(B) Where is the prerequisite non-compressible fluid or pressurized cavity?

The answer lies in the mesenchymal cells which act as a non-flowing but non-compressible medium against which the antagonistic circular and longitudinal muscle layers can act. You'll notice that the planarian is capable of lifting the front of its body and turning it sideways, a refined bit of movement made possible by yet a third set of muscles running dorsi-ventrally in the parenchyma. Thus the combination of a well-organized, muscular body wall and a cellular "hydrostatic fluid" allows the flatworm to achieve a fairly sophisticated range of movement. However the ciliated gliding movement is only effective for a relatively small animal and the broad, flat body plan makes the flatworm very susceptible to drying out and a very ineffective burrower. Thus the flatworm is somewhat limited in both size and habitat.