(a) In open circulatory system, blood flows in spaces called sinuses and cells and tissues are directly bathed in blood. In closed circulatory system, blood flows through a network of vessels. Arthropods and molluscs have open circulatory system, while annelids and vertebrates have closed circulatory system. 
(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.  

(d) When body cavity is absent, the animal is called acoelomate, e.g. platyhelminthes. In some animals; mesoderm is present as scattered pouches between ectoderm and endoderm. Such animals are called pseudocoelomate, e.g. aschelminthes. 
(e) The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. The dorsal nerve cord is a hollow cord dorsal to the notochord. It is formed from a part of the ectoderm that rolls, forming the hollow tube. The notochord is replaced by vertebral column in higher vertebrates. The nerve cord develops into the central nervous system. 
(f) Polyp is a sessile and cylindrical form whereas medusa is umbrella-shaped and free-living. Polyps produces medusae asexually and medusae form the polyps sexually (e.g. Obelia). Examples of polyps are Hydra and Adamasia whereas of medusa is Aurelia.