Chordates

Cross references:  Deuterostomes    Hemichordates    Hemichordate Hormones   
Protochordates      Early Behavior    

Chordate (Wiki)  
http://en.wikipedia.org/wiki/Chordate 

    "Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail."   
    "All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna
"   
    "...
the current consensus is that chordates are monophyletic, meaning that Chordata contains all and only the descendants of a single common ancestor which is itself a chordate
..."   
    "
The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, and chordates are deuterostomes.

    "
The phylum Chordata consists of three subphyla: Urochordata, represented by tunicates; Cephalochordata, represented by lancelets; and Craniata, which includes Vertebrata. The Hemichordata have been presented as a fourth chordate subphylum, but they are now usually treated as a separate phylum.



1939   
Chordate anatomy p. 346-358 (BHL) -  in Amphioxus - NRC
Chordate anatomy / by Herbert V. Neal ... and Herbert W. Rand ... with 378 illustrations. - Biodiversity Heritage Library   
http://www.biodiversitylibrary.org/bibliography/6383#/summary   
    My comment
This is a scanned copy of a book published in 1939.  It starts on page 348.  It's mostly of historical interest.   
   
   

2000   
Evolution of the chordate body plan...
Abstract:
http://www.ncbi.nlm.nih.gov/pubmed/10781046
HTML:  
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC18258/?tool=pubmed
from the abstract   
    "
The deuterostome phyla include Echinodermata, Hemichordata, and Chordata. Chordata is composed of three subphyla, Vertebrata, Cephalochordata (Branchiostoma), and Urochordata (Tunicata).  
    Careful analysis of a new 18S rDNA data set indicates that deuterostomes are composed of two major clades: chordates and echinoderms + hemichordates. This analysis strongly supports the monophyly of each of the four major deuterostome taxa: Vertebrata + Cephalochordata, Urochordata, Hemichordata, and Echinodermata.
    Hemichordates include two distinct classes, the enteropneust worms and the colonial pterobranchs. Most previous hypotheses of deuterostome origins have assumed that the morphology of extant colonial pterobranchs resembles the ancestral deuterostome. We present a molecular phylogenetic analysis of hemichordates that challenges this long-held view.  
    We used 18S rRNA to infer evolutionary relationships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranchs may be derived within enteropneust worms rather than being a sister clade to the enteropneusts. The nesting of the pterobranchs within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more closely resembled a mobile worm-like enteropneust than a sessile colonial pterobranch.
"  


2003   
Chordate evolution in a new light.
HTML:   http://www.sciencedirect.com/science/article/pii/S0092867403004720   
from the HTML   
    "... the nerve chord is dorsal in chordates and ventral in arthropods. Thus, if there was only a single  origin of the central nervous system, one has to propose  an axis inversion during evolution.  The alternative, namely at least two independent events leading to the evolution of a central nervous system from an ancestor with a diffuse system,"   
My comment
    I'm deeply prejudiced against "parallel evolution".  I think an axis inversion is much, much more likely.     


2005   
Key characters uniting hemichordates and chordates: homologies or homoplasies?
    Full-length PDF no longer available online for free. 
    Abstract: 
    "
Four chordate characters  —    
        dorsal hollow nerve cord,  
        notochord,  
        gill slits, and  
        endostyle — are compared morphologically, molecularly, and functionally with similar structures in hemichordates to assess their putative homologies.
    The dorsal hollow nerve cord and enteropneust neurocord are probably homoplasies. The neurocord (= collar cord) may be an autapomorphy of Enteropneusta that innervates a unique pair of muscles, the perihemal coelomic muscles. Despite the apparent lack of organ-level homology, chordates and enteropneusts share a common pattern of neurulation that preserves a "contact innervation" between neuro- and myo-epithelia, which may be the primitive deuterostome pattern of neuromuscular innervation.  
    The chordate notochord and hemichordate stomochord are probably homoplasies. Other potential notochord antecedents in hemichordates are examined, but no clear homolog is identified. The comparative morphology of notochords suggests that the "stack-of-coins" developmental stage, retained into adulthood only by cephalochordates, is the plesiomorphic notochord form.  
    Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton, as occur in adults of Cephalodiscus spp., developmentally in some enteropneusts, and in many urochordates.  
    Functional morphology, I125-binding experiments, and genetic data suggest that endostylar function may reside in the entire pharyngeal lining of Enteropneusta and is not restricted to a specialized midline structure as in chordates.  
    A cladistic analysis of Deuterostomia, based partly on homologs discussed in this paper, indicates a sister-taxon relationship between Urochordata and Vertebrata, with Cephalochordata as the plesiomorphic clade."      
    My comment:   
The closest this reference gets to hormones is a very indirect reference to "Iodine125-binding experiments". 











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