In 1900, communicable diseases, like pneumonia, TB, influenza, chicken pox, and polio, were the most common causes of death. However, with the development of medicine, and vaccines, now In 2006, lifestyle diseases, like cancer, heart disease, and stroke, were the most common causes of death.
Their mode of fertilization is external.
They lay eggs in the water and there is a chance of destruction of these gametes by water current if they are in less quantity.
So they lay more and more eggs in order to rule out the possibility of destruction of all eggs.
Thus at least some eggs are fertilized by sperm and produce zygote.
<span>B. Cilia and white blood cells</span>
