Hari haran Rajasekar

In different groups of animals , from unicellular organisms like Amoeba to human beings , there is a progressive increase in the complexity of body organisation . A series of organs and organ systems have developed . These different organs and organ systems serve the various needs of the organism . For example , the digestive system performs the functions of nutrition , respiratory system helps in breathing and gaseous exchange , and circulatory system is for the internal transport of materials . To support and protect the organs of the different systems and to keep the body upright , a rigid framework called the skeleton or skeletal system is present . This system also helps in movement shown by organisms .