As my dissection course draws to a close, it naturally begins moving toward the clade of animals most familiar to my students: the chordates. On its way there, it visits the chordates’ closet living relatives, the echinoderms. Students usually need some convincing that echinoderms and chordates and echinoderms have anything in common, because the adult forms do indeed have no real similarities. The evidence of the kinship between these groups, and between the vertebrates and the other chordates, is mostly genetic and embryonic. These highly divergent animals have a number of highly improbable similarities in the way their embryos form and the anatomy of their larvae, revealing that their adult forms are highly specialized rather than ancestrally distinct.
I usually start with the primitive chordates, because they’re somewhat less spectacular in shape than the echinoderms. The students get to see whole lancelets as well as numerous sections through them, so this sister group to the vertebrates is well known to them. The other non-vertebrate chordate group is the highly underrated Urochordata. These animals have all of the classic chordate characters as larvae but lose almost all of them as adults. All urochordates are filter feeders, but they have highly dissimilar means of realizing that lifestyle.