(a) Blood is a fluid connective tissue that contains plasma, RBC, WBC and platelets. Lymph is a tissue fluid formed from blood. It contains only lymphocytes.

(b) Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. Closed circulatory system is present in annelids and chordates in which blood pumped by the heart is always circulated through a closed network of blood vessels. Closed system can be better regulated than open system.

(c) Systole is the contraction of heart muscle and diastole is the dilation of the heart muscle. Systole results in increased pressure in heart chambers, whereas diastole results in decreased pressure.

(d) The P – wave represents the electrical excitation (on depolarisation) of the atria, which leads to the contraction of both the arteria. The T – wave represents the return of the ventricles from excited state to normal state (repolarisation). The end of T – wave marks the end of systole.