whatA shell does not know its own pattern. One excitable line of pigment cells rides the growing edge, and every tick of growth lays that line down as one more row of shell -- the pattern you see is the diary of a one-dimensional argument. Meinhardt read shells this way; this piece grows them.
whyThe claim: cone-snail tents, zigzag lightning, and banded rhythms emerge row by row from one mechanism -- pigment cells burn slow-regrowing fuel and ignite their neighbours. A lit region widens at wave speed while its interior exhausts itself, then collapses: a filled triangle, the tent of Oliva porphyria. When recovery is fast the whole edge flashes together, again and again: bands. Both are measured as they happen -- triangles counted as segments that widen then die (count and mean base printed), the band rhythm's period read from the autocorrelation of the row fill. Two sliders morph species; click the shell to drop a spark on the edge.
howA 260-cell excitable line: igniting needs well-rested fuel (0.8) beside a lit neighbour, but once lit a cell persists down to 0.35 -- that hysteresis is what makes tents rare, big, and deep; fuel burns at 0.05 a row and regrows at the recovery rate; rare seeded sparks nucleate new fires. The shell keeps its last 420 rows; the print takes the whole visible shell. Everything seeded and deterministic.