Cross references: Deuterostomes Deuterostome Nerves
Genetics in General Genomics in General Genomic Analysis
Ediacaran Genomics Parazoa Genomics Coelenterata Genomics
Hemichordate Genomics Protochordate Genomics
A PCR Survey of Xenoturbella bocki Hox Genes (ref. from above)
14 page PDF available online for free.
"Our data on the Hox genes of Xenoturbella bocki are thus at least consistent with a basal position within the Deuterostomia. ... This suggests that Xenoturbella represents an early stage in the evolution of the deuterostome Hox cluster, in which the middle group genes have not yet been completely expanded to the situation common to Chordata and Ambulacraria."
Fig. 2. Evolution of Hox gene clusters in deuterostomes.
1. See: Genomics in General for a brief discussion of Hox genes.
2. I wasn't able to make this diagram any larger, but if you get up close with a magnifying glass you can almost read the labels. Even without a magnifying glass, you can see that the top line says "Vertebrate Ancestors". That's us.
3. "Ciona", the second label down from the top, is a tunicate.
4. "Branchiostoma", the fourth label down from the top, is another name for the amphioxus. Notice that it has the full compliment of vertebrate Hox genes.
4. "PDA", at the very bottom, stands for 'Protostome-Deuterostome Ancestor.
5. One up from PDA is "Xenoturbella" the most primitive, still-living deuterostome. If I understand correctly, the fact that it doesn't have the full compliment of PDA Hox genes is explained by its passive, almost parasitic, life style.