Fundamental properties of the spiralian developmental program are displayed by the basal nemertean Carinoma tremaphoros (Palaeonemertea, Nemertea).

The first description of the cleavage program of the palaeonemertean Carinoma tremaphoros (a member of a basal clade of the Nemertea) is illustrated by confocal microscopy and microinjection and compared to development of more derived nemerteans and other eutrochozoans (Annelida, Mollusca, Sipunculida and Echiurida). Lineage tracers were injected into individual blastomeres of C. tremaphoros at the 2-, 4-, 8- and 16-cell stage. Subsequent development was followed to the formation of simple (so-called planuliform) planktonic larvae to establish the ultimate fates of the blastomeres. Results of labeling experiments demonstrate that the development of C. tremaphoros bears closer similarity to other Eutrochozoa than development of a previously studied hoplonemertean (Nemertopsis bivittata) and a heteronemertean (Cerebratulus lacteus) in that the first cleavage plane bears an invariant relationship to the plane of bilateral symmetry of the larval body. Additionally, our cell-labeling experiments support the earlier suggestion that the transitory pre-oral belt of cells in the larvae of C. tremaphoros corresponds to the prototroch of other Eutrochozoa. A unique feature of development of C. tremaphoros includes the oblique orientation of the trochal lineages with respect to the anterior-posterior axis of the larva. The significance and application of cleavage characters such as presence of molluscan vs. annelid cross for phylogenetic analyses is reviewed. We argue that molluscan or annelid cross, neither of which are present in nemerteans, are merely two out of much greater variety of patterns created by the differences in the relative size and timing of formation of micromere quartets and none can be considered, by itself, as evidence of close phylogenetic relationship between phyla.