There cannot be perpetuum mobile. It must be some source of the energy in the system.
The possible solutions are:
- The worm consumes something that can be used to extract the energy out of it. Even inside a human, there are more microbial cells in the intestines than human cells in all body, and microbial cells feed on cellulose and other food components human cannot digest. Then human digests these micro-organisms, they are important source of some vitamins.
- The worm exists for the limited time. Some insects are known not to eat in the adult stage, they have enough resources from the previous stages of development and do not live for very long.
- New symbionts are constantly coming into the worm from outside, and finally get consumed but may not be immediately and not always. Some highlander species of spiders only eat one another, but the population still holds because it is supplemented by migrant spiders arriving to such an unfavorable ecosystem.
And, obviously, the chain of usual biological parasites must end somewhere. Anything based on regular biochemistry cannot be smaller than an atom of hydrogen, and likely should be much larger. I am not sure about something more exotic, as all elementary particles in the Standard Model are considered point-like and without spatial size, even if their masses vary (with neutrinos being among the lightest).