One of these differences was unexpected and especially striking: in Branchiostoma, the two most anterior body cavities (the preoral pit and the rostral coelom) originate by enterocoely, whereas in Asymmetron, their likely homologs arise by schizocoely.
A parallel situation exists for the phylum Hemichordata, wherein enteropneusts form their coeloms largely by enterocoely, but pterobranchs do so by schizocoely (Lester, 1988); however, an evolutionary explanation of the difference is hampered by current uncertainty as to whether pterobranchs are basal or derived within the phylum (Sato et al., 2008).