The term "basal metazoans" refers to animals at the base of the metazoans' (multicellular animal) evolutionary tree. The term isn't very well-defined, and may refer to cnidarians (jellyfish and relatives), porifera (sponges), ctenophores (comb jellies), placozoans (the only animal phyla with a single species, Trichoplax adhaerens), and extinct species which may be more primitive than the common ancestor of all living metzoans (also called stem metazoa, mostly Ediacaran fauna).
The most basal of all living metazoans may be Trichoplax adhaerens, a simple, very small (0.5 mm) balloon-like animal named for its propensity to stick to the sides of a glass aquarium. Trichoplax has the smallest genome out of any known animal, with only 50 megabases of DNA, and 6 chromosomes. The sequencing of the Trichoplax genome is currently underway. It is suspected that it may be related to cnidarians and ctenophores. Some recent molecular studies suggested that Trichoplax may have split from the rest of animals after sponges and cnidarians, however.
Although it was long suspected that sponges were the most basal of all major groups of metazoans, a genetic study from 2007 determined that, in fact, cnidarians split off from other metazoans earlier than the sponges. This is quite a surprise, as cnidarians are clearly more complex than sponges. The finding demonstrates that morphology (appearance) cannot be used as a yardstick for how early an organism split off from the others, as some animals become less complex over time, not more. This contradicts many popular assumptions about evolution.
Another interesting group of basal metazoans are the mesozoa, once thought to be intermediary between protozoa (unicellular organisms) and mesozoa (multicellular organisms), now thought to be either a simplified form of metazoan or a true basal metazoan. The two major mesozoan groups are the Rhombozoa and the Orthonectida, simple organisms about which little is known. They are tiny parasites of marine invertebrates, and some only have a few dozen cells. To learn more about these animals, their genomes will need to be sequenced.
A group of extinct metazoans, possibly the most basal of all metazoans, possibly not, are the Ediacaran fauna, a group of quilted mattress and bag-like organisms that lived an extremely long time in the past, during the Ediacaran period, between about 600 and 542 million years ago. These animals have been classified by some workers in their own group, Vendobiota, on the basis of their similar "quilted" appearance, although there is great controversy over this. The Ediacaran fauna has been described as an "early failed experiment in animals." Since this group is long-extinct, we'll never get our hands on its genetic material, and are limited to making guesses based exclusively on crude morphology.