Representative images of rudiments, depicting the most common injection locations.

<p>(<b>A</b>) A schematic diagram of juvenile rudiment tissue layers. Cavities within the rudiment are shown in white, the blastocoel in light grey, and the stomach in dark grey at left. (B–D) Schematic diagrams of RD fluorescence (orange) in (<b>B</b>) the vestibule, (<b>C</b>) the hydrocoel (including primary podia) and (<b>D</b>) the intercoelomic space. (<b>B′–D′</b>) Representative corresponding epi-fluorescent images of RD distribution in these respective regions, one minute after injection. The stomach is in dark grey at left. The area in medium grey in the center of the diagram, to the right of the stomach, connecting the two shown portions of the hydrocoel (H) indicates the out of focus radial canal. All larvae shown and drawn are in anal view sensu <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113866#pone.0113866-Smith3" target="_blank">[30]</a>, posterior down; therefore the "left" side (where the rudiment is found) is seen to the right in these larvae. H = hydrocoel, V = vestibule, IS = intercoelomic space, PP = primary podium, DS = dental sac (derived from left somatocoel). IS is contiguous with the blastocoel, but we give it a distinct term for the reasons described in the text. Scale bars in B′–D′ 35 µm.