Deuterostome Nerves

Cross references:    Deuterostomes    Deuterostome Neurotransmitters     Deuterostome Genomics     Nerves in General    Hemichordate Nerves

The deuterostome ancestor (PubMed)  - 2006   
Only abstract available online.   
    "We find that an anteroposterior map of gene expression domains, representing 42 genes of neural patterning, is closely similar in hemichordates and chordates, though it is restricted to the neural ectoderm in chordates whereas in hemichordates, which have a diffuse nervous system, it encircles the whole body." 
    "The dorsoventral dimension has undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, perhaps from the gut midline, and relocation of the mouth.

Evolutionary convergence of higher brain centers (PubMed) - 2008
Only abstract available online.     
Currently available evidence supports a single origin for the centralized nervous system of bilaterally symmetrical animals.  
    Beneath the staggering diversity of protostome and deuterostome nervous systems lies a fundamental groundplan consisting of a tripartite brain and a nerve cord divided into distinct antero-posterior and medio-lateral zones.    
    As divergent lineages have taken independent paths towards increased encephalization, complex brain centers have arisen that serve multiple levels of sensory processing and advanced behavioral coordination and execution.  
    Many questions arise as one surveys the distribution of these brain centers across the bilaterian phylogenetic trees.  
    What environments did these lineages encounter that promoted the acquisition of energetically expensive brain centers composed of thousands, millions or even trillions of neurons? What novel behavioral capabilities did these brain centers in turn give rise to?

Centralization of the Deuterostome Nervous System Predates Chordates (PubMed)
Abstract:    - 2009   
from the abstract       
The origin of the chordate central nervous system (CNS) is unknown.  
    One theory is that a CNS was present in the first bilaterian and that it gave rise to both the ventral cord of protostomes and the dorsal cord of deuterostomes.    
    Another theory proposes that the chordate CNS arose by a dramatic process of dorsalization and internalization from a diffuse nerve net coextensive with the skin of the animal, such as enteropneust worms (Hemichordata, Ambulacraria) are supposed to have.    
    We show here that juvenile and adult enteropneust worms in fact have a bona fide CNS, i.e., dense agglomerations of neurons associated with a neuropil, forming two cords, ventral and dorsal. The latter is internalized in the collar as a chordate-like neural tube.  
    Contrary to previous assumptions, the greater part of the adult enteropneust skin is nonneural, although elements of the peripheral nervous system (PNS) are found there. We use molecular markers to show that several neuronal types are anatomically segregated in the CNS and PNS. These neuroanatomical features, whatever their homologies with the chordate CNS, imply that nervous system centralization predates the evolutionary separation of chordate and hemichordate lineages.
from the HTML  
    NOTE:  The HTML contains many pictures and 46 references with active links to abstracts.         

    "... the nervous system of enteropneusts consists of cord-like masses of neurons overlying a neuropil. Moreover, the dorsal cord is internalized at the level of the collar in a manner indistinguishable from the chordate neural tube. A CNS has also been described in pterobranches in the form of a ganglion situated
at a position similar to that of the enteropneusts’ collar cord [39–41], suggesting that whether pterobranches are the sister group of enteropneusts [5] or are nested in their midst [42], the possession of a CNS is primitive for hemichordates.