Why doesn't a hydra have a circulatory system?

When animals are very, very small, such as a hydra or a very early human (up to Carnegie stages 9 or 10), diffusion of nutrients and gasses is sufficient to keep the cells of the organism alive. read more

By then, a vascular system, blood cells, and a pumping heart have developed (Carnegie stage 10 ~28 days) that allow gas diffusion as well as nutrient/waste transport to occur to/from developing cells throughout the body. That's why we get big and the hydra stays small. read more

This system may beseen strictly as a blood distribution network, but some considerthe circulatory system as composed of the cardiovascularsystem , which distributes blood, and the lymphaticsystem , which distributes lymph. The circulatory system comprises of the heart, the lungs and theblood vessels. read more