Some of the earliest bilaterians were wormlike, and a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a
coelom or pseudocoelom.[a] Animals with this bilaterally symmetric
body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[4][2]
Having a front end means that this part of the body encounters stimuli, such as food, favouring
cephalisation, the development of a head with
sense organs and a mouth.[5] The body stretches back from the head, and many bilaterians have a combination of circular
muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[2] these enable soft-bodied animals with a
hydrostatic skeleton to move by
peristalsis.[6] Most bilaterians (Nephrozoans) have a gut that extends through the body from mouth to anus, while Xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary
larvae which swim with
cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain
parasitic worms have extremely
plesiomorphic body structures.[4][2]
One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.[3] Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry.[10] However, there is evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).[11]
Fossil record
The first evidence of bilateria in the fossil record comes from trace fossils in
Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[12] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[13][14]Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[15] Burrows believed to have been created by bilaterian life forms have been found in the
Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old.[16] However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran.[17]
The traditional division of Bilateria into
Deuterostomia and
Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa,
Acoela and
Nemertodermatida (together called
Acoelomorpha), and the remaining bilaterians.[18] The latter clade was called
Nephrozoa by Jondelius et al. (2002) and
Eubilateria by Baguña and Riutort (2004).[18] The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian bodyplan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.[18][26] Subsequently the acoelomorphs were placed in phylum
Xenacoelomorpha, together with the
xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses.[26]
A modern consensus
phylogenetic tree for Bilateria is shown below, although the positions of certain
clades are still controversial (dashed lines) and the tree has changed considerably since 2000.[27][25][28][29][30]
A different hypothesis is that the Ambulacraria are sister to Xenacoelomorpha together forming the
Xenambulacraria. The Xenambulacraria may be sister to the Chordata or to the
Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or to Chordata + Protostomia). The phylogenetic tree shown below depicts the latter proposal. Also, the validity of Deuterostomia (without Protostomia emerging from it) is under discussion.[31] The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).[32] While the below tree depicts
Chordata as a
sister group to
Protostomia according to analyses by Philippe et al., the authors nonetheless caution that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".[33]
^Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A.; Turner, F. R.; Chongyu, Y.; Zhou, C.; Yuan, X.; McFeely, M. B.; Stampanoni, M.; Nealson, K. H. (13 October 2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science. 314 (5797): 291–294.
Bibcode:
2006Sci...314..291H.
doi:
10.1126/science.1133129.
PMID17038620.
S2CID25112751.
^Pecoits, E.; Konhauser, K. O.; Aubet, N. R.; Heaman, L. M.; Veroslavsky, G.; Stern, R. A.; Gingras, M. K. (June 29, 2012). "Bilaterian burrows and grazing behavior at >585 million years ago". Science. 336 (6089): 1693–1696.
Bibcode:
2012Sci...336.1693P.
doi:
10.1126/science.1216295.
PMID22745427.
S2CID27970523.