All jellyfish species’ are different in their own way but almost all of them have the exact same rudimentary anatomy.
Jellyfish consist of a bell, concealing the mouth, with many tentacles protruding from the rim of the bell. The bell is made of a soft material called mesoglea which is made of 95% water. Each tentacle has a cnidocyte which has things called nematocysts inside. The nematocysts are what makes a jellyfish sting. Most jellyfish have tendrils or oral arms coated with thousands of microscopic nematocysts (a type of venomous cell). Each nematocyst has a ‘trigger’ paired with a capsule containing a coiled stinging filament armed with exterior barbs. Upon contact, the filament rapidly unwinds, launches into the target and injects toxins. The animal can then pull its prey into its mouth. Most jellyfish use these cells to secure prey or for defense. Others, such as Jellyfish in the order ‘Rhizostomae’ have neither tentacles nor other structures at the bells edges. Instead, they have eight highly-branched oral arms.
Jellyfish lack basic sensory organs and a brain, however, their nervous systems and rhopalia (small sensory structures) allow them to perceive stimuli, such as light and color and enable them to respond quickly.
Jellyfish feed on small fish and zooplankton that become caught in their tentacles. Most jellyfish are passive drifters and slow swimmers, as their shape is not hydrodynamic. Instead, they move so as to create a current forcing the prey within reach of their tentacles. They do this by rhythmically opening and closing their bell-like body.