Scyphozoa

Resembling a giant jellyfish (fr: la Méduse, which means the Jellyfish. Scyphozoa is in fact the scientific name for jellyfish, from the Greek words for "cup" and "animal") with long tentacles protruding from its sides, the Scyphozoa is much larger than the rest of Xana's monsters. Its eye is located on a green diamond just in front of its head. The head is bulb-shaped, and sits on its cone-shaped body. Unlike the other monsters, who move through some form of physical transportation (legs or wings), the Scyphozoa is able to float as if it were swimming through water. This enables it to easily reach otherwise unaccessible areas on Lyoko, and also makes it highly maneuverable in close quarters. Unlike Xana's regular monsters (except the Kolossus), the Scyphozoa is the only monster of its kind. The Scyphozoa also makes a unique, creepy (No idea what to compare it to) noise that signifys its appearance and/or approach.

In the second season, the Scyphozoa appears fairly frequently and has only one specialized function: stealing the memory of its target. The Scyphozoa's main target is Aelita, though it targeted Yumi on one occasion. To steal its target's memory, the Scyphozoa wraps its two front tentacles around them. The victim is then encased in a red aura and rendered immobile. The Scyphozoa then places its three tentacles to a victim's head; two at the victim's temples and one at their forehead. It then begins to drain its victim's memory, which is visible as a pinkish fluid that passes through the Scyphozoa's tentacles and into an organ in its bulb-shaped head, supposedly its brain. If it manages to finish its task, it will drops its victim and floats away. If the tentacles are severed before the process is complete, any data it has stolen is returned to the owner. This runs contrary to the episode "Missing Link", where it still managed to steal Yumi's digital genetic code even though its tentacles were severed. It is possible though that since it was only stealing her digital genetic code it had already finished the process and was either about to drop her anyway or it started stealing her memories which restarted the process. The draining process seems to wear out its victim, as its captives usually feel momentary fatigue. It can also drain life points, as evidenced by the Scyphozoa's attack on the Skid in "The Lake".

As with all of Xana's monsters, the Scyphozoa's proposed weakness is the Eye on its head. Only its head and tentacles have ever been hit though. Its head simply absorbs the attacks but the tentacles are severed when hit. This causes it to lose its grasp on its victim. Getting hit in the head can also cause it to lose its grasp on its victim, though the effectiveness of this varies. This monster has also managed to survive every attack inflicted upon it, including Odd's arrows and Manta mines. This suggests that it might be indestructible or, at the very least, extremely durable, since even non-Eye hits have done no visible damage to it. It is only destroyed once, and inadvertently at that, in "The Lake", when Aelita supercharges the Skid's shields using energy from various towers in Lyoko. As it was connected to the shields at the time, the sudden increase in power overloaded and destroyed it. It also seems to be capable of regenerating its limbs over a short period of time. This is seen in "Déjà Vu", when its tentacles were severed by Ulrich yet it came back whole a few minutes later. Regardless of this, the Scyphozoa retreats whenever it faces any significant resistance. Xana is not willing to risk losing Aelita's memories, so the Scyphozoa can also be persuaded to let her go if the group is willing to destroy her rather than let Xana have her memories. This has only happened once, in "Saint Valentine's Day", where Odd threatened to kill Aelita until the Scyphozoa was persuaded to release her.

In "The Key", the Scyphozoa finally managed to steal Aelita's memories, which contained the keys to Lyoko. The theft effectively killed her, since it took every last memory she had. When it succeeded, every tower in Lyoko became controlled by Xana. As Xana escaped the supercomputer, Sector 5 shut down. It was only through the intervention of Franz Hopper that Sector 5 was restarted and all of the towers were purified. His intervention also brought Aelita back to life and restored all of her memories.

Having completed its original purpose, Xana now uses the Scyphozoa to transfer viruses to Aelita in the third season. This enables him to take control of her, afterwards which he can use her to delete sectors in Lyoko by having her enter the code "Xana" into a sector's passage tower. In the last episode of the third season, "Final Round", the Scyphozoa also takes control of William and uses him to destroy Lyoko's core. It continues to use its viruses to possess Aelita in the fourth season and, on occasion, will be deployed to drain energy from the Skid.

The Scyphozoa doesn't utilize any strategy in completing its goals. It merely waits for a decent opening and proceeds to capture its target. Other monsters are usually deployed alongside it to corner a target, making the Scyphozoa's job easier. The Scyphozoa will not attempt to capture a target if there is significant resistance and will retreat if threatened.