A combination of factors is likely causing the decline of blue spruce trees throughout Michigan. Posted on March 7, 2011 by Dennis Fulbright, Michigan State University Extension, Department of Plant Pathology, and Jill OâDonnell, Michigan State University Extension.
Flatworms lack a respiratory or circulatory system; these functions take place by absorption through the body wall. Nonparasitic forms have a simple, incomplete gut; even this is lacking in many parasitic species. Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle.
They also lack a circulatory system, and their nervous system consists only of two cords of nerve cells that send messages through the worm's body. Considering the simplicity of their anatomy, we should not be surprised that roundworms lack a respiratory system and rely on a more simplistic gas exchange process to take in oxygen.