Jellyfish Have No Brain or Circulatory System
Jellyfish have no brain and no circulatory system. Instead of a centralized brain, they have a simple nerve net that helps them sense stimuli and coordinate movement. They do not need blood or a heart: gases and nutrients move through their tissues, while the gastrovascular cavity helps with digestion and nutrient distribution.
This applies to jellyfish in the usual zoological sense (medusoid cnidarians); the word “jellyfish” is sometimes used loosely for other gelatinous animals.