The Psychosomatic Phrynosoma is most often found in the abdomen, particularly in the stomach but has been known to roam about the digestive tract and guts according to the stability or lack thereof of its immediate environment.
Eggs are introduced to the host via aural injection, usually riding in upon words that the host does not wish to hear, or that perhaps herald further words that the host does not wish to hear. As these words traverse the earways they trigger various timers and countdowns that will either terminate in their own time or at the behest of external interference.
Once the eggs are deposited in the head they will remain dormant until mindscape surrounding them is at an appropriately fruitful and tense state, at which point they will hatch and begin their journey south in to the literal bowels of their host.
There, depending on the stress under which the host is placed, they will grow into mature adult size which is typically just a little bigger than is comfortable for the host. The host will react by clenching their stomach against this protrusion, which in turn will prompt the Psychosomatic Phrynosoma to extend the many spikes adorning its armour. This conflict will proceed as a struggle of wills, with the Phrynosoma twisting, clawing and scrabbling about with its many spines and claws while the host is most typically sitting calmly in a public place, such as a tram stop or an office desk, gazing distractedly into the distance and working the inside of their lips tensely.
The Psychosomatic Phrynosoma causes no physical damage. The infestation ends once the host's stress drops below viable levels, at which point the Phrynosoma dies, passes through the host's digestive tract, and leaves the body with a moist sigh.
The common name for the Psychosomatic Phrynsoma is "Anxiety".
ETA 28 June 2011: Cockle-warming surprise reading of this post done by the bouncy flouncy Alex Garber.