secretes new shell material faster on one side than the other, causing the shell to grow in a spiral. The rates at which shell material is secreted at different points around the opening are presumably determined by details of the anatomy of the animal. And it turns out that—much as we saw in the case of branching structures earlier in this section—even fairly small changes in such rates can have quite dramatic effects on the overall shape of the shell.
The pictures below show three examples of what can happen, while the facing page shows the effects of systematically varying certain growth rates. And what one sees is that even though the same very simple underlying model is used, there are all sorts of visually very different geometrical forms that can nevertheless be produced.