Deathworm Morphant

Deathworms (also known as morphants) were symbiotic creatures used by the Morgs to enable them to survive beyond death. When a deathworm was consumed, it would incubate in its host's body until the moment of death, then leave with the host's consciousness preserved inside it and seek out a new host.

It could survive most assaults, including disintegration. The Master consumed one to survive his execution on Skaro. (EDA: The Eight Doctors) As told in The Eight Doctors The Master (Gorden Tipple) goes to the Morg to get a Deathworm, perhaps knowing of his upcoming execution in The Enemy Within. After extermination by The Daleks the Deathworm hides in the ashes of The Master which is then carried to safety by The Doctor, and after it escapes the Doctor, it then possesses an ambulance driver called Bruce. However the Master's borrowed body began to decay almost as soon as he took possession of it.

Although the procedure appeared to be both permanent and safe when carried out by the Morgs, the Master's borrowed body began to decay almost as soon as he took possession of it. (DW: Doctor Who (1996))