Expression patterns of five Wnt genes in 48 hours post-fertilization trochophore larvae of the annelid Platynereis. The expression of these Wnt genes is observed either in reiterated ectodermal segmental stripes and/or in the pygidial/proctodeal presumptive territory. A schematic description of the trochophore larva is given in (k). pt: the prototroch, a ciliated belt used for swimming, also highlighted by a black line on larvae photographs, divides the larva into an apical episphere and a vegetal hyposphere; pr: proctodeum; s: stomodeum; ps: peristomium, a band of embryonic tissues around the forming mouth; 0: anterior-most segmental unit; 1-3: presumptive areas of the larval appendage-bearing segments. h: the future head of the worm formed by the episphere plus peristomium plus segment 0; t: the future trunk of the worm formed by larval segments 1-3 plus the pygidium. (l) shows the two approximate focal planes that are used for larvae photographs. The first and third panel columns (a), (c), (e), (g), (i) show ventral views of trochophore larvae, focusing mainly on tissues of the ventral neuroectoderm that will form the ventral nerve cord. The second and fourth panel columns (b), (d), (f), (h), (j) are frontal optical section focusing on the lateral parapodia-forming fields. The Wnt stripes corresponding to each presumptive larval segment are numbered 1, 2, 3. In addition, a more anterior metameric unit located just below the prototroch is numbered 0. This unit does not produce a larval segment but fuses with the head early in development. Black asterisks show expressions in the pygidial/proctodeal area. Green asterisks show expressions in the stomodeal bulb that will give rise to the mouth. The midline expression of Pd-Wnt4 is indicated by a black arrow. Internal Pd-Wnt5 and Pd-Wnt11 expression potentially located either in the segmental mesoderm or in ectodermal cells of the chaetal sacs are shown by red arrowheads. Additional expression of Wnt genes in the nascent brain are described elsewhere .