That should be the spinal cord which rests inside the spinal vertebra.
The correct answer is that mammals use a closed, double loop , four chambered heart as a circulatory system of the body.
The circulatory system of mammals is closed system that consists of four chambers, two articles and two ventricles. The deoxygenated blood comes into the heart from one loop and the oxygenated blood from other loop from the heart is circulated to the whole body. This is called as double circulation.
Answer:# 1 is salt #2 is fresh #3 fresh #4 salty
Explanation:
