To: All Msg #69, Mar3093 11:05AM Subject: Re: Fishy Fetuses In article 93Mar26.032230.2742

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From: Chris Colby To: All Msg #69, Mar-30-93 11:05AM Subject: Re: Fishy Fetuses Organization: animal -- coelomate -- deuterostome From: (Chris Colby) Message-ID: <> Newsgroups: In article <> (Joel Bradford Klammer) writes: [about gills in vertebrates] >Now, my source for this claim was the latest edition of the >Encyclopedia Britannica, which I am inclined to trust. My >question is: who's right, that creationist speaker or Britannica? >Please answer in detail. In all vertebrates, paired pharyngeal pouches form on the inside of the pharynx in early development. Different hagfish species have between six and fourteen. Lampreys have seven. Fish of the classes Chrondrichythes (sharks) and Osteichthyes (bony fish) typically have six. Amphibians usually have five and amniotes (reptiles, birds and mammals) have four -- the fourth pouch may be a composite of two or three pouches. In fish and amphibians, some or all of the pouches perforate, opening to the outside. They are called pharygeal clefts or gill slits when this occurs. In amniotes, the pouches may fail to open or do so only briefly (I think they open in humans, but I'm not sure). In fish, the first remains closed, but the rest remain open into adulthood and develop into gills. In amphibians the first and last never open, the remaining three close in most amphibians. They remain open in some urodeles (salamanders). In tetrapods (amphibians, reptiles, birds and mammals) the pharyngeal pouches do not form complete gills (except in some salamanders). The pouches extend and become the tympanic cavity (in the middle ear), the eustachian tube (between pharnx and ear) and the thymus gland and parathyroid gland. So, it appears that in the lineage leading to mammals (including humans) our ancestors stopped running the full developmental program for gills and depending more on lungs (like modern lungfish and some amphibians, our ancestors way back had both). The remnant structures from gill formation were then modified for new purposes. A partial phylogeny of the vertebrates (or craniates): hagfish lampreys sharks bony-fish amphibians mammals reptiles birds + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Reference for most of the information here: Torrey and Feduccia, 1979, Morphogenesis of the Vertebrates: Fourth edition, Wiley, New York Chris Colby --- email: --- "'My boy,' he said, 'you are descended from a long line of determined, resourceful, microscopic tadpoles--champions every one.'" --Kurt Vonnegut from "Galapagos"


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