I know they carry diseases which can harm or kill the animal it is attached to. These diseases can taint the meat of the animal so it wouldn't be wise to eat it which stinks if the animal is bread for eating. They can also irritate the skin and cause rashes, or even eat the flesh of the animal away.
Answer:
When conducting research, scientists use the scientific method to collect measurable, empirical evidence in an experiment related to a hypothesis (often in the form of an if/then statement), the results aiming to support or contradict a theory.
Explanation:
The answer you are looking for is the heart and lungs.the heart pumps blood which we also kinda need in order to breath and the lungs produce oxegen trew the body which allow us to breath.
hope i helped