The evidence of metamery in adult brachiopods and phoronids
There are both metameric and nonmetameric animal body plans in each of the three main branches of the bilaterian tree - the Ecdysozoa, the Deuterostomia and the Lophotrochozoa. Has metamery originated independently in these groups or is it a synapomorphy of all Bilateria? If the latter is correct, we might expect to find remnants of metamery in nonmetameric forms. The Lophophorata seems to be the only group of main bilaterian groups that lacks metamery. Here, we infer that the lateral mesenteries of brachiopods and phoronids are metameric in nature and originated from dissepiments between segments of trunk coelomic sacks of an oligomerous ancestor. In addition to preoral and lophophore coeloms, brachiopods and phoronids demonstrate a metameric subdivision of the body coelom. The trunk coelom of recent brachiopods and phoronids is a product of partial fusion of three or two segments, respectively. The lateral mesenteries in phoronids and brachiopods bear funnels of excretory organs like the dissepiments of true metameric animals (for example, annelids). In both groups, the lateral mesenteries are situated at an angle to the main axis of the body and always at a right angle to the axis of metamery. We conclude that metamery was present in ancestral Lophrophotrochozoans and in the common ancestor of all Bilateria but has since been reduced in some groups. The reduction of metamery in phoronids and brachiopods is correlated with strong changes in their body plan. We suggest that lophophorates are primitive lophotrochozoans because they retained some plesiomorphic features.