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.
However, even closed circulatory systems differ as we move further up the evolutionary tree. One of the simplest types of closed circulatory systems is found in annelids such as the earthworm. Earthworms have two main blood vesselsâa dorsal and a ventral vesselâwhich carry blood towards the head or the tail, respectively. Blood is moved along the dorsal vessel by waves of contraction in the wall of the vessel. These contractible waves are called 'peristalsis.'