ICR blocked me on Facebook


So, the Institute for Creation Research put up a link on their Facebook page to one of their latest Acts & Facts articles on the whole Ernst Haeckel/vertebrate embryos thing and since the sort of stuff creationists write about this subject is a pet peeve of mine (as readers of this blog will no doubt have gathered) I decided to post a quick comment on the following quote from the article:

Guliuzza: Shouldn’t students be skeptical when they’re told that evolutionists can simply look at folds in embryos and see gill slits? The truth is that these are only folds of tissue in the pharynx region of vertebrates during the pharyngula stage of development. For mammals, birds, and reptiles, they never develop into a structure that is in any way like fish gills.

I wrote that this statement was not true as would be known to anyone who had cracked an embryology textbook and asked if Dr. Guliuzza (the author of the article) was therefore incompetent in this area or if he was being deliberately misleading. Further I provided a link to my blog post on the subject of “gill slits” so that anyone interested could look at the evidence for themselves.

I also corrected one of their other commenters on what Thomas Huxley and Charles Lyell’s professions and religious perspectives were. I also noted to the commenter that all science, not just evolutionary biology, leaves God and other supernatural agents out of its explanations.

I used no harsh language, I did not call anyone any names and I engaged in no mockery (unless you count my pointed question about Guliuzza competency/honesty) and yet the end result was that my link and all my comments have been deleted and I am apparently now blocked from commenting on ICR’s FB page.

I’ll leave the reader to decide what this says about ICR and the robustness of their scholarship.


Responding to a comment on my article: “Gill slits” by any other name…

I have decided to move this up from the comments section (cuz why not?) of my article “Gill slits by any other name…

Andy: Hi Troy, I read your post, and it is very well argued.

Thanks. [He said knowing that a “but” was coming.]

Andy: At the moment, I am a biology student at a local university, and one of my personal headaches is the promotion of either creation-ism or evolution-ism. To be honest with you and with all due respect, I really don’t care for either explanation.

Andy, with all due respect, you should know that for someone like me who has been in the “trenches” debating people pushing pseudoscience for years, the fact that you would refer to evolutionary theory as “evolutionism” is a large red flag that would strongly lead me to suspect that your understanding of evolutionary theory and science itself is likely wanting.

Unfortunately, your next sentence does little to disabuse me of my suspicion. Creationism is set of theological beliefs regarding a creation story written down in the 6th or 5th century B.C.E., accepted on faith and in the face of contravening facts by a tiny minority of people with any scientific background. Its mechanisms (God did it) are untestable and to the extent that it makes claims testable against the empirical world it has been falsified a thousand times over.

On the other hand evolutionary theory is a scientific explanation for a vast number of facts that is testable against further observations of the empirical world and used by the vast majority of the relevant scientific community as a guide to further research…whether you “care” for it or not.

That you would casually place them on the same level is yet another huge red flag.

Andy: As a student, the only thing that I am concerned about is true scientific facts that have been tested at a laboratory to the molecular level.

We are only three sentences in and we have three red flags. In my experience people who use terms such as “true scientific facts” (as opposed to what, “false scientific facts”?) tend to not to be particularly familiar with either the facts or what counts as scientific.

However, that is a nit compared to the philosophical problems with your above statement. Why on Earth would you limit yourself to what is testable in a laboratory at a molecular level? Better yet, why should anyone else take your narrow limits as to what to be concerned with seriously? Using your bizarre limitations, we would be throwing out vast swaths of empirical data, and not just in biology.

Molecules are great but they are not a perfect path to knowledge nor are they the only path.

Furthermore, the point of science is not simply collecting random facts, rather it is about explaining the facts that we observe [insert Darwin quote about collecting pebbles here]. In this case, we have detailed anatomical, physiological and genetic similarities between the pharyngeal structures of terrestrial vertebrate (hereafter “amniotes“) embryos and non-amniote vertebrates (amphibians and “fish”), set against a particular paleontological backdrop that needs a coherent explanation.

You do not like creationism, great. That should be a given and I am with you. However if you do not like evolutionary theory, then what is your better explanation for the gill-like appearance of the pharyngeal structures of amniote embryos (and everything else evolution explains)?

You cannot just sit back and say, “I don’t buy any of it” and expect to be taken seriously, especially in face of the apparent success of evolutionary theory.

Andy: My teacher presented this same example about the gill slits during a microbial genetics lecture.

I am not sure how the “gill slits” of vertebrate embryos are relevant to the genetics of microbes but OK…

AndyI was just exhausted of listening, so, I raised my hand and said that the information was outdated and more up-to-date data has been collected explaining that they are no longer gill slits, and they are pharyngeal pouches.

Huh, did you read my article to which you are supposedly responding? You are literally arguing semantics here. Regardless of what we call these structures in the embryos of amniotes, they exist, they resemble—in detail—the developing gills of non-terrestrial vertebrates, and we want to understand why this would be so.

AndyHe asked what was my source, and my reply was, with all due respect, professor, but your source is 150 years old and newer information was out that did not concentrate on a belief based teaching. He stopped and said he will do some more research and get back to us. He continued with the lecture as it was intended.

I am sorry but “your source is 150 years old”, even if true, is not an argument; old ≠ incorrect. FA = -FB (for every action there is an equal and opposite reaction) gets robotic probes to Mars despite the fact that Newton came up with the formula over three hundred years ago.

Furthermore, what is this “belief based teaching” to which you refer? Once again, if you read my article you would know that referring to these structures as “gill-slits” is not dependant on a belief in either creation or evolution as is evidenced by the fact that pre-Darwin creationists called them this.

Oh, and what is your source Andy?

AndyI agree; they look like gills until you dissect it, and then they look like pouches. I know in biology we name a lot of things base on morphological characteristics, and that is a great easy way to remember things.

No, no, no, again did you even read my article? Here, look at this diagram again:

Source bionalogy.com, with modifications.

Source bionalogy.com, with modifications.

There are clefts (or grooves) on the outside of the pharynx (red arrows) with corresponding pouches on the inside (blue arrows). These clefts and pouches are each separated by membranes that, in mammals, normally remain unperforated with the first (tympanic) membrane forming the so-called eardrum. Some of the post tympanic membranes do normally perforate and then reclose in some birds and reptiles embryos. They also temporarily perforate in the gill-bearing larvae of some amphibians and, of course, in fish they stay perforated and become gills.

Since I know some people are queasy about diagrams here are a few photographs of coronal sections done on various mammal embryos. First, an electron microscope picture of what I think is a stage 13 human embryo (Etchevers, 2008)(blue arrow points to the pharyngeal pouch and red to the corresponding cleft).

(Etchevers, 2008)

For comparison, here are some stained sections from a mouse (left, Zhang et al. 2005) and a pig (right, Shone & Graham 2014) at a similar point of development; once again blue arrows for pouches and red for clefts. Also, note in these photos that the aortic arches—the blood vessels—are visible within the pharyngeal arches (green arrows).

(Left, Zhang et al. 2005 – right, Shone & Graham 2014)

These are not just “pouches”! There are arteries (the blood vessels shown above), muscles, nerves and cartilaginous structures found on either side of the cleft/pouch pairs in the embryonic gills of “fish” and the corresponding pharyngeal arches of amniote embryos.

How do you get from this to, “they look like gills until you dissect it, and then they look like pouches” without ignoring all of these observable facts? These things exist and you need to explain them Andy.

AndyThe only thing that really gets to me is that either creationist or evolutionist keeps trying to push the subject in school.

The reason a competent science teacher (i.e. one who teaches evolutionary theory) would bring them up is because they are structures that have interested biologists since they were first discovered and whose existence is elegantly explained by descent from a common ancestor (evolution).

AndyHonestly, I paid for one of my books close to $300. I feel that my book should be filled with up-to-date information that has been tested, and that is going to help me in the future and not filled with propaganda of any kind.

The pharyngeal structures of amniote embryos are not propaganda. They exist and are observable by anyone with eyes to see. For example, please note that the photographs of the pharyngeal sections of mouse and pig embryos I reproduced above were both taken from papers published within the twenty-first century. Any modern textbook on vertebrate developmental biology is likely to have similar photos or diagrams of these structures.

AndyI guess it is easier to print the old stuff than to update the books to what they are intended to do, and that is to provide useful information that can help us treat diseases and give our patients a fighting chance.

Sorry Andy regardless of whether it comes from an older source or a current one, pharyngeal clefts, or “gill slits” exist in amniote embryos whether or not you want to accept the facts or the current best explanation for them, so please spare us the overwrought “won’t someone think of the ill people?!” shtick.

AndyTo this day, I haven’t seen a mermaid or an angel either in nature or in a lab, so, I will have to dismiss both claims as pending research and proper laboratory testing.


Yeah so at this point, I am starting to have doubts that you are for real Andy and not just a troll looking for attention. What the hell does a mermaid have to do with anything Andy? Please tell me you are not seriously suggesting that evolutionary theory somehow supports the existence of mermaids [Hint: the exact opposite is the case]. Because if you think it does, you may want to rethink your career path—perhaps something in the arts?

AndyPlease don’t take this the wrong way. I am a paying student who is discordant with the never-ending stories.

As I said, after the mermaid comment I am no longer sure how to take you Andy. Perhaps your response or lack thereof will clarify that question.

Regardless I want to thank you Andy. Formulating my response to you has made me consider adding a few things to my “gill-slit” article that I think could use some expansion or clarification. So anyway, Poe, troll or sincere, something positive has come from your comment, thanks.


Etchevers, Heather (2008) Development of the branchial arches (slide presentation downloaded on 10-29-16)

Shone, Victoria & Graham, Anthony (2014) Endodermal/ectodermal interfaces during pharyngeal segmentation in vertebrates, Journal of Anatomy, 225(5):479-91

Zhang, Zhen et al. (2005) Tbx1 expression in pharyngeal epithelia is necessary for pharyngeal arch artery development, Development, 132:5307-5315

Consilience and whale evolution

Way, waaay back in December of 2005 (ye gods has been ten years already?!) I wrote a Feedback response on the Talk Origins Archive to a question about the vestigial pelvic bones found in modern whales. In this case the questioner did not believe them to be truly vestigial, no doubt due to holding erroneous beliefs regarding the subject. In my response I of course took the time to correct their faulty views, however I also used the opportunity to talk about the concept of consilience wherein multiple independent lines of evidence converge on a single explanation, giving us greatly increased confidence that those explanations (hypothesis/theories) are likely to be accurate reflections of reality, i.e. “true”. 

I have now and again thought of going back and using that post as a spring-board for a more detailed examination of this subject and who knows, I may still do so someday. In the meantime however, here is a great video from Stated Clearly that I ran across on Facebook recently that uses the same topic—whales—to essentially do the same thing I did all those years ago; make a point about the consilience of evidence pointing to a pretty definite conclusion with regards to not just the ancestry of cetaceans but the evolution of life in general. Better they include more details than I did and it has animations.

Check it out:

I miss answering the feedback question on Talk Origins…

And now a little poorly concealed bragging



Last weekend while I was awaiting delivery of my copy of Cambridge University historian of science Nick Hopwood‘s new book Haeckel’s Embryos: Images, Evolution, and Fraud (2015) on 19th century biologist Ernst Haeckel and his (in)famous embryo illustrations, (which I have written about a few times here an PCWP and elsewhere), I got a Facebook notification that I had been “tagged” in a post by my friend (in real life, not just Facebook) and colleague Dr. Nick Matzke. The somewhat cryptic post said the following:

Hey look who’s in the acknowledgements – Troy Britain

Attached to this comment was the following picture:


Thereafter the comments conversation between Nick M. and I went like this:

Me: Wait, wait, wait, this isn’t Hopwood’s new book is it (my copy is in route)?!

NickYep it is!!

Me: Holy crap!

Nick: Immortality!

Apparently Professor Hopwood was kind enough to mention me (and Nick Matzke as well) in the acknowledgements section of his new book (page 304).

 The relevant section reads as follows:

For crucial pieces of advice, I thank Thomas Brandstetter, Troy Britain, Solveig Jülich, Ron Ladouceur, Nick Matzke, Signe Nipper Nielsen, Ron Numbers, Jesse Olszynko-Gryn, and Constance Sommerey…

Quite an honor! All the more given the company of people like Nick Matzke, Ron Numbers and the rest.

Thank you Prof. Hopwood, you are too kind! And thank you for writing this book! It needed to be done and I look forward to reading it (or rather the rest of it, I’m up to chap. 3 already)!


Answering feedback on Haeckel’s embryo drawings

A commenter named Brad, who is apparently a creationist, left a comment to one of my recent posts.  In that post, “Creationist foists “fraudulent” embryo picture on his readers“, I talked about how creationist Brian Thomas ironically used a biologically inaccurate picture of a human embryo to illustrate his article in which he denies the evidence for evolution from developmental biology and implies that all such evidence is based on the supposedly fraudulent embryo drawings 19th century biologist Ernst Haeckel.

Not wanting it to be neglected in the comments section, I have moved my response to Brad up to a full post.

Brad: Wow, evolution is like magic. It turns gills into ears, tonsils, and glands (and part of the larynx).

There is no magic involved in evolution, you’re thinking of creationism, the very heart of which is the claim that living things are the miraculous creations of a supernatural being.

Brad: Calculate the odds…

Since by all available evidence it did in fact happen, the odds are 100%.

Brad: Granted, it was a bad choice with the stock photo; however, that’s possibly an oversight vs. the intentional use of known fraudulent drawings for decades in school textbooks.

I would suggest that it was an “oversight” brought on by ignorance of the subject and a lack of curiosity to even check. As for the recycling of Haeckel’s embryo illustrations in textbooks (as illustrations of evidence rather than as a historical reference) this due to the regrettable and all too common laziness on the part of textbook publishers who often recycle not just images but even language as well. See: Stephen Jay Gould’s essay “The Case of the Creeping Fox Terrier Clone” (Gould 1991).

I agree that some* of Haeckel’s drawings exaggerated overall similarity of the embryos they depict and that more accurate illustrations should be used. However, I am unaware of any fundamental anatomical inventions in the illustrations that if corrected would affect the evidence from comparative embryology for evolution.

So, please explain in your own words exactly what anatomical details were altered in Haeckel’s drawings that created evidence for evolution where none exists in actual vertebrate embryos.

Remember, specific anatomical details and in your own words. I don’t want quotes of vague generalities from other people. You’ll note that in my post on pharyngeal clefts at no point did simply I quote some scientists opinion that the pharyngeal apparatus of amniote embryos are homologous to the pharyngeal apparatus of “fish”. Instead I pointed to the anatomical and genetic evidence for this.

If you are going to argue against this, then you are going to have to do likewise.  And if you want to present an alternative scientific explanation you are going to have to present something that is logical, coherent, makes reference to the empirical evidence at hand and does not rely on untestable miracles or divine fiat.

Brad: O’Rahilly and Müller stated that ‘the pharyngeal clefts of vertebrate embryos … are neither gills nor slits’.1

I don’t have this reference handy and I don’t know when I’ll get a chance to get to my local university’s science library (whose website says they have it) I was able to find a later edition on Amazon with a free preview which allowed me to get what seems like the relevant section.  If the older edition has any important differences please let me know (assuming you have the book and didn’t just cut and past the quote from a creationist and/or anti-abortion website).

Here is the the section which seem to contain Brad’s quote with the part Brad quoted in red:

Recapitulation, the So-Called Biogenetic Law.

The theory that successive stages of individual development (ontogeny) correspond with (“recapitulate”) successive adult ancestors in the line of evolutionary descent (phylogeny) became popular in the nineteenth century as the so-called biogenetic law. This theory of recapitulation, however, has had a “regrettable influence on the progress of embryology” (G. de Beer). The work of Carl Ernst von Baer in 1828 was much closer to the mark. According to the “laws” of von Baer, general characters (e.g., brain, notochord) appear in development earlier than special characters (e.g., limbs, hair). Furthermore, during its development an animal departs more and more from the form of other animals. Indeed, the early stages in the development of an animal are not like the adult stages of other forms but resemble only the early stages of those animals. The pharyngeal clefts of vertebrate embryos, for example, are neither gills nor slits. Although a fish elaborates this region into gill slits, in reptiles, birds, and mammals it is converted into such structures as the tonsils and the thymus. According to the hourglass model of evolutionary development, a conserved pattern of developmental gene expression is linked with considerable resemblance among embryos of different species at a constricted phase, whereas divergence is found earlier and later. Morphological evidence for such a phase, however, is unconvincing (Richardson et al., 1997). (O’Rahilly & Müller 2001, p. 16)

Although there are issues I could take with their description of recapitulation as necessarily being about a strict correspondence between embryos and the adult forms of ancestors or with the short shrift they give the concept of the phylotypic stage (at least in this paragraph), however those are really nuanced points that are not necessary to get into here.

One point I will quibble with regards the presence of slits in certain amniote embryos.

Everything I have seen on the subject indicates that the pharyngeal clefts of some “reptiles” and birds do normally perforate during development and are therefore technically “slits” for a short time before they re-close. This also sometimes occurs in mammals, including humans, though it is not a normal condition. (see my earlier article for links). Also they’ve left out any reference to the larvae of amphibians some of which do develop functional gill slits.

Beyond that, they are absolutely correct, the pharyngeal apparatus of amniotes never function as gills and as I said in one of my earlier posts, I don’t know of any biologist who has ever claimed that they do, save one, the creationist Louis Agassiz.

The Great Satan, Ernst Haeckel, stated in no uncertain terms in his writings that these structures never function as respiratory organs in amniote embryos.

It is not required that the pharyngeal apparatus of amniote embryo function as gills for them to be evidence—not absolute, stand alone proof, but evidence—of their ancestors aquatic nature any more than it is necessary for the hind limb buds of whale embryos to function as working legs for them to be evidence that their ancestors once walked on land.

Brad: Blechschmidt is more forceful, concluding that ‘the so-called basic law of biogenetics is wrong.

The late Erich Blechschmidt (1904-1992) was a scientific outlier (as are some of his followers, i.e. Brian Freeman). That is his views were not representative of developmental biologists as a whole. Creationists love to quote Blechschmidt and other outliers such as Alan Feduccia (who denies that birds are descended from dinosaurs) and Charles Oxnard (who denies that australopithecines are human ancestors) in an attempt to make it seem as if they are tapping into some real controversy amongst scientists. In reality they are just quoting people who are only slightly less on the fringe of science than themselves.

You can find someone with an advanced degree that will say just about anything. For example there is PhD astronomer (and creationist) Gerardus Bouw who argues that the Earth is the center of the solar system.

In this case Blechschmidt (and Freeman) seem to be the source for creationists absurd argument that pharyngeal clefts are merely “flexion folds” caused by the bending of the embryonic neck (Freeman 2001).

As for the biogenetic law, no one is arguing for that. If you read the article you were commenting under you would have seen that I quoted Michael Richardson (a critic of Haeckel) as saying that there are studies which support the recapitulation of individual character transformations, just not whole ancestral stages (Richardson & Keuck 2002, p. 522)

Brad: ” ‘No buts or ifs can mitigate this fact.’ He adds that the gill stage myth is ‘not even a tiny bit correct or correct in a different form…It is totally wrong’. 2 This view is shared by many mainstream embryologists.

No, it is not.

Although among the strongest opponents to Haeckel in recent years, and favourably differing from him by a paramount respect for detail, Blechschmidt resembled Haeckel in several respects: they both were intrepid and very persuasive defenders of their views, they were both artists at heart – witness Haeckel’s illustrations and Blechschmidt’s giantsized models of human embryos – and, most importantly in the present context, they both stuck to some outdated ideas not shared by the majority of contemporaneous biologists: Haeckel remained Lamarckist, defending the inheritance of acquired characters throughout his life, and Blechschmidt strove to maintain the special status of Man by condemning any embryological interpretation or terminology suggesting his descent from other vertebrates. (Sander 2002, p. 531, emphasis mine)

So Blechschmidt apparently had issues with accepting the evolutionary development of humans from other animals. As I said an outlier with an extreme minority view.

Brad: in the end, this is probably an argument evolutionists should give up on.

You’ll understand, I hope, that this evolutionist will not be taking advice on what is or is not valid evidence from a creationist.

Brad: But keep right on with it if you insist, it just makes dogmatic evolutionists sound nuts.

Only to those too ignorant to know better and/or too intellectually lazy to look up the evidence for themselves.

*Note: I am thinking primarily of the first stage shown in the comparative embryo illustration in the 3rd edition of his Anthropogenie (1874)


Freeman, Brian (2001) “The Myth of the “Biogenetic Law””, The American Biology Teacher 63(2):84

Gould, Stephen Jay (1991) “The Case of the Creeping Fox Terrier Clone”, in: Bully for Brontosaurus: Reflections in Natural History,  W.W. Norton & Co., pp. 155-167

O’Rahilly, Ronan & Müller, Fabiola (2001) Human Embryology & Teratology, 3rd Edition, Wiley-Liss

Richardson, Michael & Keuck, Gerhard (2002) “Haeckel’s ABC of evolution and development“, Biological Reviews, 77: 495-528

Sander, Klaus (2002) “Ernst Haeckel’s ontogenetic recapitulation: irritation and incentive from 1866 to our time”, Annals of Anatomy 184:523-533

Creationist foists “fraudulent” embryo picture on his readers

I decided I am not going to bury the lead on this one. Brian Thomas of the Institute for Creation Research (ICR) just posted another in a long line of creationist screeds attacking the evidence for evolution from comparative embryology, which as usual claims that the evidence is based on fraud and pins much of the blame for it on 19th century biologist Ernst Haeckel.

I began writing a rebuttal straight away but then I happened to take a second look at the bright pink image of an embryo atop the article and it brought me to a sudden halt. So, having backed up, let me start again.

Thomas: German zoologist Ernst Haeckel is perhaps most famous for defending evolution with the argument that creatures replay their evolutionary past when developing in the womb. …In his zeal to promote evolution, Haeckel foisted faulty embryo sketches onto his readers, and the zeal of his followers has perpetuated those falsehoods for over a century. (Thomas 2012, emphasis mine)

Yeah, about that…


That’s right, yet another irony meter has been reduced to subatomic particles by a creationist.

Read on»

“Gill slits” by any other name…

Charles Darwin once said that he thought the evidence from the comparative anatomy of embryos was “by far the strongest single class of facts” in favor of common descent (Darwin, 1860) and while it has since been eclipsed by genetics, it remains one of most compelling subsets of evidence for evolution. And perhaps the single most striking detail in the comparative embryology of vertebrates, are the structures colloquially known as “gill slits”.  

Embryonic “gill slits” or “branchial clefts” (branchia is Greek for gill) or more properly pharyngeal clefts (grooves, folds, etc.) are part of what is called the “pharyngeal apparatus” found in front (ventral) and sides (lateral) of the head/neck region of all vertebrates in the “pharyngula stage” of development. In “fish”, and the larva of amphibians, these develop into respiratory organs used to extract oxygen from water while in amniotes (“reptiles”, birds and mammals) they are modified into other structures.

Before I go on, a brief digression about “fish”. Throughout this article I will often use “fish” in the generic sense; but it should be noted that the term as it is commonly used—to refer to any vertebrate that swims in the water, has fins and gills—is not a valid scientific classification. This is because the three main types of animals commonly called “fish” —the Chondrichthyes (sharks, rays, skates and chimaeras), the Actinopterygii (ray fined fish, which constitutes the majority of living fishes), and the Sarcopterygii (lobe fined fish, the group from which four legged land animals, i.e. tetrapods, evolved)—are not a monophyletic group. That is they are not very closely related to each other despite some of their outward similarities (like gills). For example the living Sarcopterygii, lung fish and coelacanths share a more recent common ancestor with us (and all tetrapods) than with the other “fishes”.

OK, so the “pharyngeal apparatus” consists of a series of paired pharyngeal arches and fissures which develop on the exterior with a corresponding set of pharyngeal pouches on the inside of the throat, separated from the external fissures by a thin membrane (more on the details in a moment). And in fact the possession of these structures at some point in development, along with a hollow dorsal nerve cord, a notochord and a post anal tail, are the defining characteristics of the phylum chordata to which we and all other vertebrates belong.

Copyright © 1999 McGraw-Hill Companies, Inc.

Please note that the above illustration is diagrammatic and not intended to be photographically accurate (I have to say that lest I be accused by creationists of conveying a fraud). Below are actual photographs of both a skate embryo and a human embryo for comparison. Also note: the gill structures in the embryos of Elasmobranch fishes—the subdivision of Chondrichthyes which contains sharks, rays and skates—are much less derived than in other “fishes” and therefore generally more similar to those of amniote embryos than the corresponding structures in the bony “fishes” (which are significantly modified).

(Gillis et al 2009, p.5721)

The first of the arches, the mandibular arch, forms the jaw in all jawed vertebrates (Gnathostomes). Most vertebrates develop a total of six arches but the full complement is usually only retained into adulthood by hexanchiform sharks. Hexanchiformes are very plesiomorphic which means that they are more like earlier types of sharks.  Some species of hexanchiformes even develop a seventh arch. Likewise the extant jaw-less vertebrate, the lamprey, also have seven gill openings.

Read on»