Chordates    -    Phylum:   Chordata
“Taxonomy is described sometimes as a science and sometimes as an art, but really it’s a battleground.”
Bill Bryson  (b. 1951)  in   'A Short History of Nearly Everything', 2003.
      The Chordates are a phylum of what at first sight appear to be three vastly different groups of animal; Lancelets, Tunicates and Vertebrates. It is however, during their developmental stages that the similarities are obvious as all three subphyla share a number of embryonic features that indicate their common ancestry.  These features include; the notochord  (= spine-like structure),  a dorsal central nervous system, the endostyle  (= a mucous secreting organ),  pharyngeal slits (= gill slits) and a tail that extends beyond the anus.   The above features are not necessarily apparent in the adult stage of the animal; they may either develop into something different or disappear altogether.
      The name Chordata is derived from the Latin 'chorda'  (= cord or string) in reference to the dorsal support structure, the notochord.  With nearly 75,000 described species the phylum: Chordata is by no means the largest but they are by far the most conspicuous and yet a detailed placement and classification of the phylum and its' member species has always proven to be bothersome.  Back in the days of Linaeus when the physical appearance of the organism was the prominent feature upon which classification was based, things were relatively simple although quite a few errors were made. Nowadays though, the more we discover the murkier the waters seem to become.
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      Animals can be divided into two groups,the Deuterostomia and the Protostomia.  The criterium for belonging to either is based on a 'which comes first?' question.  The anus, or the mouth?  If, during very early embryonic stages,  the anus is formed first you are a Deuterostome and if the mouth comes first, a Protostome.  Humans are Vertebrates therefore Chordates and therefore Deuterostomes.   However.  In human embryos the mouth comes first!  And this raises a few problems for taxonomists.
Image 1.  Protostome v. Deuterostome development in early embryonic stages.  (Could it be argued the division is moot? - Ed.)
  Also, Pre-Cambrian fossils of Chordata are relatively rare and few have been found that could confidently be ascribed to them.  Debate continues about the origins of the Chordata with some studies using molecular phylogeny suggesting that the Deuterostomia came to be around 900 Mya.  Other research has suggested our current perception of the various classes and orders within the phylum: Chordata itself is fraught with errors and that, for instance, the Lampreys are not an end-branch but may well be an ancestral one.  And considering we ourselves do not qualify to be where we are, where do we belong?
      The phylum: Chordata is divided into three sub-phylla.  Species within all of which may be encountered in the Northern rivers.  
References  and  links:  

​​^      Image 1.  By YassineMrabet.  CC-BY-SA 3.0  via https://commons.wikimedia.org/wiki/File:Protovsdeuterostomes.svg​

^      Blair, Jaime & Hedges, S.. (2005). Blair JE, Hedges SB.. "Molecular phylogeny and divergence times of deuterostome animals."Mol Biol Evol 22:                                 2275-2284. Molecular biology and evolution. 22. 2275-84. 10.1093/molbev/msi225. 
^      Donoghue, P.C.J. and Keating, J.N. (2014), "Early vertebrate evolution."  Palaeontology, 57: 879-893. https://doi.org/10.1111/pala.12125
^      Donoghue, P.C.J. and Sansom, I.J. (2002), "Origin and early evolution of vertebrate skeletonization."   Microsc. Res. Tech., 59:352-372.                              https://doi.org/10.1002/jemt.10217
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​       https://www.britannica.com/animal/chordate​
      http://www.columbia.edu/itc/hs/medical/humandev/2004/Chapt18-Endoderm.pdf
      https://en.wikipedia.org/wiki/Deuterostome
​       https://en.wikipedia.org/wiki/Embryological_origins_of_the_mouth_and_anus​
       https://en.wikipedia.org/wiki/Endostyle
       https://en.wikipedia.org/wiki/Molecular_phylogenetics
       https://en.wikipedia.org/wiki/Pharyngeal_slit
       https://en.wikipedia.org/wiki/Protostome
     https://www.nature.com/articles/news.2010.45​

Header photo: 
Erik Beringen.
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