Porifera, Radiata and basal Bilateria
Phylogenetic analysis suggests that the Porifera and Ctenophora diverged before a clade that gave rise to the Bilateria, Cnidaria andPlacozoa.[52] Another study based on the presence/absence of introns suggests that Cnidaria, Porifera and Placozoa may be a sister group of Bilateria and Ctenophora.[53]
The sponges (Porifera) were long thought to have diverged from other animals early.[54] They lack the complex organization found in most other phyla.[55] Their cells are differentiated, but in most cases not organized into distinct tissues.[56] Sponges typically feed by drawing in water through pores.[57]Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.[58]However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.[59]
Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, andjellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[60] Both have distinct tissues, but they are not organized into organs.[61] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic.[62] The tiny placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: thedeuterostomes and the protostomes, the latter of which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.
Ecdysozoa
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The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis.[72] The largest animal phylum belongs here, theArthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water.[73] A number are important parasites.[74] Smaller phyla related to them are the Nematomorphaor horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.
The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop with spiral cleavage.
