a. In open circulatory system, blood flows in spaces called sinuses and cells and tissues are directly bathed in blood. In a closed circulatory system, blood flows through a network of vessels. Arthropods and molluscs have open circulatory system, while annelids and vertebrates have closed circulatory systems.
b. In oviparity; animals lay eggs and young ones hatch from them after some time, e.g. reptiles, birds and some mammals. In viviparity; animals give birth to young ones, e.g. scoliodon and most of the mammals.
c. When the young ones are different from adult animals; this case is called indirect development, e.g. frog, cockroach, silkworm, etc. When the young ones resemble the adult animal; this case is called direct development e.g. birds, mammals, reptiles, etc.