7. Which of the following systems is NOT found in a flatworm: a. Digestive system
b. Respiratory system
c. Nervous system
d. All of the above

Veins have a much lower blood pressure than arteries. which of these prevents backflow of blood in veins? a. pressure applied by the heart b. one–way valves in veins c. thin muscular walls of veins

According to theories of how life began, how did early organic molecules begin to separate from the outside world? a: specialized enzymes were required b: chains of amino acids created a barrier c: formation of microspheres or vesicles d: rna catalyzed the formation of membranes

Describe the relationship between isotonic solutions, equilibrium, and water movement into and out of a cell.