Annelids are considered evolutionary precursors of molluscs as they do not have metameric segmentation.However, Neopilina a living fossil shows metameric segmentation.Besides,its trochopore larva, it has structural similarities to annelid trochopore larva.These two evidences suggest that molluscs have descended from annelids. 

The molluscs have advancement over annelids, as they have distinct head, better sense organs and gills or lungs for respiration.

In leeches, there is no true blood vascular system, the coelomic space and fluid have been modified to form the circulatory system.Hence, it is called haemocoelomic system and the coelomic fluid is called haemocoelomic fluid.Blood is red due to the presence of respiratory pigment haemoglobin or erythrocyanin found dissolved in the plasma.Free amoeboid blood corpuscles are present, but there are no red blood corpuscles

The animals having bilateral symmetry move with a anterior end forward this has led to cephalisation i.e differentiate of a define head at the anterior end. cephalization involves concentration on nervous, tissue(brain) and sense organs in the head.
Thess arrangement is an asset in an adjustment in enviornment, has the anterior end of moving animal is first to encounter changes in the area it is entering.

Viviparous animals: Viviparous animals are those animals in which both the fertilization (i.e. the fusion of male and female gametes) of egg that results in zygote and development of embryo take place inside the body of the female in the mother's womb.
These animals give birth to young ones directly.
Examples – mammals such as cows, goats, dogs, humans, etc.
Oviparous animals: Oviparous animals are those animals in which both the process of development of embryos take place outside the body of the female.
a) In oviparous animals fertilization can occur inside the mother’s body or outside the mother’s body.
b) Oviparous animals lay eggs outside their body. Example – birds, hen, frogs, etc.
Number of eggs or young ones produced by the oviparous and viviparous can be equal or not:
a) No, the number of eggs or young ones produced by an oviparous and viviparous mother cannot be equal.
b) The number of eggs laid by the oviparous animals are always more in number than the young ones produced by viviparous animals.
c) This is because the events of fertilization or development of embryos takes place outside the mother’s body so there are more chances that the eggs will get damaged prior to fertilization or development. So, to ensure the complete development of at least one embryo, oviparous animals lay more numbers of eggs than viviparous animals.
d) On the other hand in viviparous animals fertilization and development of embryos both occur inside the body of the female so there are 100% of its development.