Jellyfish does not come to you and bite you.
Instead they release thousands of venom filled capsules in to the water. Millions of those capsules are developed in their tentacles. (not testicles
) Jellyfish released thousands of capsules when your presence disturbed it or touch it.
Those capsules has a mechanism to identify and get attracted to you skin when they were released to the water. As soon as they touch your skin they get the stimulation signal to inject the venom to your skin with a huge force .
So even you kill it or hit it with something before touching your body 1000s of its capsules will come towards you and get the revenge.
Nidariya has developed a water proof skin lotion to distract those capsules from your skin.
A: Jellyfish and jellyfish tentacles
B: Tentacles and stinging clusters
C: Clusters with hundreds of stinging mechanisms
D: Cluster with stinging capsules and folded needles
E: Resting and discharge stinging mechanism
The following image described how the venom capsule come towards you and inject its liquid into you.
The oval shape body is the copusule. 1000s of such are released to the water.
A. Stimulants from the skin initiate discharge process.
B. High internal pressure of 200 atmospheres is built in the capsule.
C. With 40,000xg of acceleration the needle drills a hole into the skin
D. A tubule follows the shaft and injects poison
into the body.
more at http://www.nidaria.com/Nidaria/Templates/showpage.asp?DBID=1&LNGID=1&TMID=84&FID=836