In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, and man-made vibrations are often generated to investigate shallow, subsurface structures.
Flatworms are considered acoelomate, since they have no body cavity, and roundworms are pseudocoelomate because they have a body cavity, but it is not derived from the splittiâ¦ng of the mesoderm tissue. Also, flatworms only have one opening in their digestive system unlike roundworms with two.
Flatworms also have a feature that the sponges and cnidarians do not, a middle layer of actual organized cells. Flatworms have three cell layers: an ectoderm, an outer covering like the other groups have; an endoderm, a lining of the digestive spaces somewhat like a cnidarians; and a mesoderm between the other two. With actual tissues protected like this, another evolutionary explosion of features happened.