Hate the implicit teleology…

…but love the unforeseen consequences.

Guts really do make the bilaterian. Everything follows from making a tube-shaped body plan — all the increased potential for signaling between different tissues multiplies the number of possible developmental histories, and leads to all kinds of novelties. Like a central nervous system. You wouldn’t be reading this if there’d been no gastrulation. Brains, a coelom, a circulatory system, etc. were all side-effects of a successful feeding strategy and triploblasty. You should be grateful to your guts for making you.

Although, if they hadn’t, I suppose the life of a jellyfish isn’t all that bad.


  1. says

    Indeed. My marine biology instructor in grad school actually said, “For those contemplating reincarnation, a good reason not to choose to be a cnidarian is the lack of an anus.”

    The bad news for us is that we are actually built backwards in this regard. Perhaps PZ will comment on that, or perhaps it’s best left unremarked.

  2. says

    Well, the majority are chordates, which as I say somehow swapped their mouths and anuses. The vast majority of phyla, and accordingly metazoan species, are not represented.

  3. nomdeplume says

    @4 yes – 2 elasmobranchs but one insect, 2 plesiosaurs and a turtle, a mouse, a lion, and a woman in 19th century dress for mammals. It looks like a random collection of images that happened to be available, rather than an attempt to illustrate bilaterians. Also seems to me, and I may well be wrong PZ, that “rolling” a body to form a gut tube needn’t necesarily have resulted in bilateralism, adult Ascidians come to mind, that is it isn’t so much having a gut as having a gut formed in a certain way. But maybe I’m not thinking it through properly.

  4. wzrd1 says

    Well, another characteristic of bilateria is an enteric nervous system and indeed, it’s postulated to have actually been the first brain, before that entire central nervous system thing even began evolving.
    Which places that octopus in quite an interesting place, having a primary brain, but overall having a more distributed nervous system.

  5. says

    Well of course the echinoderms, who are the other deuterostomes and presumably the closest relatives of chordates, have very diffuse nervous systems and bizarre hexagonal body plans. Nevertheless sea stars are capable of quite complex predatory behavior, albeit they move slowly. One might think the arthropods are most closely related to chordates, since they have superficially similar body plans — head, thorax and abdomen compartments, with legs, and a brainlike ganglion — but that is evidently coincidence, or convergent evolution. The echinoderms, despite their very idiosyncratic body plans, are our cousins in eating with their assholes and shitting out their mouths. The average person probably doesn’t need to know this.

  6. nomdeplume says

    Ah, forsooth indeed, so it is! In my (weak) defence the image was too small on my ipad to really see the face.

  7. ginckgo says

    cervantes @ 7: echinoderms are still primarily bilaterians, as evidenced by their embryonic physiology; they become pentaradial only later.
    The closest relatives of chordates are probably not echinderms, but rather the hemichordates – including the extinct graptolites – though I often see them placed on the same branch as the echinoderms.