Fig. 274: Hypothetical path of evolution (left) of marine invertebrates (echinoderms) to vertebrates.
Hypothetical course of evolution (left) of echinoderms to vertebrates over a free-swimming larva form of tunicates that have supposedly deviated from their normal embryonic development (bottom right) and acquired a chorda (blue) and a neural tube (yellow) though evolution. The similarity of the fish larvae (top right), here of a shark with free-swimming larva forms of tunicates or with amphioxus, is gauged as recapitulation.
After Romer AS & Parsons TS (1991) Vergleichende Anatomie der Wirbeltiere. Hamburg.