This example presents an interactive, collapsible tree diagram illustrating a hierarchical classification of animals, from a root "Animals" node down through dietary categories (Herbivores, Omnivores, Carnivores) to specific species. It uses D3.js version 3.5 with the `d3.layout.tree` and `d3.svg.diagonal` APIs to compute the layout and draw curved links, while SVG circles and text nodes render the data. Clicking a node triggers a transition to expand or collapse its children, with colors for strokes and fills defined by `type` and `level` properties in the hardcoded JSON data.