Blood is called a connective tissue As it helps in the trasport of essential substances to the cells and carries waste substances fro there. It connects the body systems, transports oxygen and nutrients to all the parts of the body, and removes the waste products. Blood also has an extra-cellular matrix called plasma, with red blood cells, white blood cells, and platelets floating in it.

The evolutionary changes in pattern of heart among vertebrates are :
1. Fishe heart is two chambered, one auricle and one ventricle. In there is single circulation of blood. 

2. Amphibian heart : The heart of amphibians have 3 chambers i.e. two auricles and one ventricle. 

3. Reptilian heart : In reptiles heart is three chambered, but ventricle is also incompletely divided by incomplete interventricular septum. 

In crocodile, the heart is four chambered i.e. with wo auricles and two ventricles.

4. Avian and Mammalian heart : In avian and mammals, the heart is four chambered ie with two auricles and two ventricles and there is complete separation of pure and impure blood. 

In the human heart, the contraction is initiated by a special modified heart muscle known as sinoatrial node or the SA-node. It is located in the right atrium. The SA node has the inherent power of generating a wave of contraction and controlling the heart beat. Hence, it is known as the pacemaker. Since the heart beat is initiated by the SA node and the impulse of contraction originates in the heart itself, the human heart is termed myogenic. The hearts of vertebrates and molluscs are also myogenic.