Cross references:  Bilateria    Protostome       Chordates         Hemichordates   
Deuterostome Nerves     Deuterostome Neurotransmitters     
Deuterostome Genomics
      Sea Urchin   

Deuterostome - Wikipedia   
    "Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "second mouth") are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes. Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates. Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo are not determined by the identity of the parent cell. Thus, if the first four cells are separated, each cell is capable of forming a complete small larva; and if a cell is removed from the blastula, the other cells will compensate."  


1 Formation of mouth and anus
2 Origins
3 Classification
4 References
5 External links

Here are the following phyla\subgroups of the deuterostomes.

Chordate (Wiki) 
The chordates and three sister phyla, the Hemichordata, the Echinodermata and the Xenoturbellida, make up the deuterostomes, one of the two superphyla that encompass all fairly complex animals.

"Most researchers agree that,  ... Xenoturbellida, appears to be basal within the deuterostomes, in other words closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.

Xenoturbella (Wiki)   
Xenoturbella is a genus of bilaterian animals; it contains two marine worm-like species.

Xenoturbella has a very simple body plan: it has no brain, no through gut, no excretory system, no organized gonads (but does have gametes; eggs and embryos occur in follicles ...), or any other defined organs except for a "statocyst" containing flagellated cells; it has cilia and a diffuse nervous system.
1 Description
2 Species
3 Phylogeny
4 References
5 Additional material
6 External links

Problems with Characterizing the Protostome-Deuterostome Ancestor (Goog)       

    "Since Darwin’s time, the origins and relationships of the bilaterian animals have remained unsolved problems in historical biology (Conway Morris 2000). One of the central difficulties is characterizing the common ancestor of the protostomes and deuterostomes." 

Genomic View of the Sea Urchin Nervous System (PubMed) 
  See:   Sea Urchin 

The deuterostome ancestor (PubMed)     
Only abstract available online.  I got the PDF from the library. 
    "Hemichordates, the phylum of bilateral animals closest to chordates, can illuminate the evolutionary origins of various chordate traits to determine whether these were already present in a shared ancestor (the deuterostome ancestor) or were evolved within the chordate line.  
    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. This map allows an accurate alignment of the anterioposterior axes of members of the two groups. 
    We propose that this map dates back at least to the deuterostome ancestor." 

Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives (PubMed)     
Full length HTML and PDF available online for free. 
Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates.

Traditionally, the group was recognized on the basis of a shared embryonic development pattern: gastrulation occurs at the vegetal pole and the blastopore becomes the anus, while the mouth forms secondarily (Chea et al. 2005).

Five major clades make up the Deuterostomia: craniates, cephalochordates and tunicates, which together form the Chordata, along with echinoderms and hemichordates (figure 1).

In summary, the members of all five major deuterostome groups can be distinguished on adult synapomorphies by the end of the Lower Cambrian
all five major clades of deuterostomes were already present in the Lower Cambrian, ca 520 Myr ago (figure 2).

    "Nodal is so far one of the few described deuterostome-specific genes, evolving from the duplication of a BMP-like ancestor (Duboc et al. 2004; Chea et al. 2005). In deuterostomes, there is early nodal embryonic expression (not shown), but later nodal signalling results in left–right asymmetry of certain organs in bilateral embryos (figure 3). In all chordates, later nodal expression is on the left side during development, specifying asymmetry (Chea et al. 2005).

Molecular genetic insights into deuterostome evolution from the direct-developing hemichordate Saccoglossus kowalevskii (PubMed)    
Full length HTML and PDF available online for free. 
This is a broad, up-to-date review that doesn't reach any definite conclusions. 
The molecular genetic body patterning data presented in this review reveal some critical insights into the body plan of the deuterostome ancestor, and a unique way to compare the adult hemichordate body plan to that of chordates.  
    The detailed similarities in the transcriptional and signalling networks are not likely to be a result of recruitment of individual genes into convergently similar expression topologies. These exquisite similarities are almost certainly a result of homology.  
    However, what we can most confidently reconstruct is an ancestral gene network rather than ancestral morphologies. Most of the gene networks discussed have been used comparatively to investigate the nature of ancestral nervous systems, and yet hemichordates are a good example of how homologous gene regulatory networks can be deployed to regulate the development of nervous systems with fundamental differences in their organizational base.  
    While gene networks are conserved over large evolutionary time scales, the broad range of morphologies that they regulate have not been constrained by the higher-level regulatory control. ... Broader sampling and incorporation of fossil datasets will all be required for a more rigorous assessment of ancestral features of early deuterostomes.

The mitochondrial genome structure of Xenoturbella... (PubMed)    
Full length article available online for free.   

from the abstract   
    "The mitochondrial genome of Xenoturbella bocki has a very conserved gene arrangement in the deuterostome group, strikingly similar to that of the hemichordates and the chordates, and thus to the ancestral deuterostome gene order.  
    Similarity to the hemichordates in particular is suggested by inversion and breakpoint analysis.  
    Finally, while phylogenetic analyses of the mitochondrial sequences support a basal deuterostome placement, support for this decreases with the use of more sophisticated models of sequence evolution.

Crumpled Sock? Churro? No, That's Just Xenoturbella - Scientific American   
    My comment
Long, very interesting article in more-or-less plain English.