Major anatomical steps toward craniotedness: a heterodox view based largely on embryological data

The animals that ultimately attained craniotedness are suggested to have followed an evolutionary pathway including: (1) a pelagic, single-layered and externally ciliated blastiscus stage; (2) a benthonic, double-layered, bilaterally shaped gastriscus stage, from the internal layer of which two bilateral generations of mesodermal outpocketings, branchiothyric and pleurohylomeric, arose in sequence, and from the external layer of which a mid-dorsal tube, the egrediduct, appeared that, along with a pair of posteriorly located flaps, functioned in water-jet locomotion; and (3) a myomerozoon stage, the diagnostic character of which is a paired series of myomeres derived from upper parts of the pleurohylomeres and used for natation. It is further suggested that two evolutionary lineages diverged from the early myomerozoan stock. The holoptychophoric lineage led to the abranchiate myochordates (“Amphioxus”) and the entobranchiate cyclostomes. The hemiptychophoric lineage led to the ecto-branchiate gnathostomes. Hence it follows that the cyclostome and gnathostome crania have evolved independently of one another, and coincidentally that the term craniotes (or craniates) should be recognized as merely a convenient hold-all category for the myomerozoans exhibiting a cephalic skeleton.