Most arthropods and some mollusks and tunicates have circulatory systems, which means fluid transporting nutrients and oxygen flow directly into internal organs.

a. closed
b. nonexistent
c. open

There are few cells in the body that do not undergo mitosis: most somatic cells divide regularly, some more than others. take for example the cells that line the digestive tract. these cells must be frequently replaced because they are constantly eroded by the movement of food through the tract. what mechanism(s) is/are in place to ensure that these cells are exact copies of the mother cell