among the non-bilaterian phyla remain controversial, but the topology shown reflects the current consensus. Numbers in red are percentages of the total number (2,308) of Pomatoceros ESTs with matches against specific taxonomic groups. Numbers on the boundaries between taxonomic groups are shared exclusively between Pomatoceros and those groups, whereas in the cases of non-bilaterians and bacteria/protists the numbers reflect ESTs shared between Pomatoceros, lophotrochozoans and those taxonomic groups. For example, 7% of Pomatoceros ESTs are shared only with lophotrochozoans and deuterostomes, and less than 1% are shared only with lophotrochozoans and non-bilateral animals or with lophotrochozoans and bacteria or protists. Over half (1,205; 52%) of the total number of Pomatoceros ESTs had no matches in the databases.
One implication of work on the annelid Platynereis dumerilii is that lophotrochozoans may be less derived (are more representative of ancestral character states) than members of Ecdysozoa , but it is not yet clear how representative Platynereis is. Takahashi et al.  have now analyzed a set of ESTs from a second and only distantly related annelid, the serpulid Pomatoceros lamarckii, which differs from Platynereis both morphologically and in lifestyle. Platynereis is a free-living predator, whereas Pomatoceros lives within a tube that it constructs and captures food from the surrounding waters using a crown of feeding tentacles through which water is filtered. Nevertheless, data from the two species lead to the same conclusion, that annelids (and perhaps lophotrochozoans in general) are less derived than the insects and nematodes investigated so far. One focus of the paper  was patterns of gene sharing and gene loss between Pomatoceros and the other major groups of organisms. These figures are summarized in Figure 1: Pomatoceros shares a significant number of genes (158; 7% of the total) only with deuterostomes and other lophotrochozoans, but a much smaller number (23 genes; 1% of the total) only with ecdysozoans and other lophotrochozoans. In addition, 11 genes shared only with non-bilaterians were identified, illustrating the ubiquity of gene loss.
The significance of cnidarians for understanding the urbilaterian gene complement
Although the genome of a choanoflagellate (thought to be the closest living relatives of the animals)  showed us that some 'animal-specific' genes came earlier in evolution, many other genes really are unique to metazoans, and these include components of several signaling pathways (such as the Wnt, transforming growth factor β and nuclear hormone receptor pathways). When the genomes of bilaterians are compared with those (admittedly, as yet few) available for 'lower' (non-bilaterian) animals, one fact that clearly stands out is that a quantitative leap in terms of signaling molecule complexity preceded the emergence of the Cnidaria (the phylum that includes hydras, sea anemones, corals and jellyfish). Whereas most or all of the animal-specific signaling systems seem to be present in the genomes of Porifera (sponges) and Placozoa (placozoans; very simple animals with only three or four distinct cell types), they are much less highly elaborated than in cnidarians or bilaterians. For example, whereas the sea anemone Nematostella has 12 Wnts, most of which are recognizable as homologs of specific Wnt types known from bilaterians , the sponge Amphimedon and the placozoan Trichoplax each have only three Wnts that are not easily assignable. In addition, these lower animals seem to have much less well developed arsenals of signaling molecule antagonists. The situation with respect to transcription factors is a little less clear cut but, for example, the homeobox gene complement of Nematostella is much more bilaterian-like than are those of sponges and placozoans . In summary, cnidarians seem to be particularly important in terms of under standing the urbilaterian gene repertoire.
Have lophotrochozoans lost fewer genes?
There is a widespread perception that ecdysozoans have lost more of the an