Jellyfish are part of philum cnidaria. At the most primitive cnidaria (hidrozoars), the nervous system is called a diffuse nervous system;it has multipolar neurons on the surface of its body.
The nervous system in jellyfish (scifozoars) is more complex because it is concentrated more on the region of the umbrella. They don't have a brain, but they have brain cells that are organized well enough to fulfill the needs of the jellyfish.
About the blood and heart- jellyfish are diploblasts. They only have 2 layers of cells. The digestion takes place in the gastral cavity and the food gets absorbed through there. The nutrients get to the exterior layer through diffusion, so blood and a heart are unnecessary.
Jellyfish also don't have eyes- they navigate with the photosensitive pigments called ocelli, located around their swimming bell.
Source: {http://www.biology-online.org/biology-forum/about1027.html}
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