Figure 5

Conservation and variation of the segmental nervous system in naidids. A) Structure of a ventral nerve cord ganglion. Image A is an intensity sum projection of a ventral view of a trunk segment from Allonais paraguayensis, with transverse reconstructions to show the structure of the connective (A1) and ventral ganglia at two levels (A2, A3). Specimen was stained for DNA (blue), serotonin (red), acetyl-tubulin (green)), and F-actin (white). Segmental nerves are labeled I-IV; serotonin immunoreactive perikarya are within the parachaetal (p), central (c), axillar (a) or rear (r) group; ventral chaetae (vch) are visible due to birefringence. The paired arrowheads mark the position of the mesodermal septum. The looping, acetyl-tubulin positive structures in the lower and right part of the image correspond to a ciliated nephridium (nf: nephridial funnel; nt: nephrotubule). Scale bar: 25 μm. B) Diagram of non-septate and septate ganglia. Dashed vertical lines represent the mesodermal septa, and horizontal bars indicate the span of a “neural segment” (defined as an entire ganglion and the interganglionic space anterior to it) and an “interseptal segment” (defined as the region between two consecutive septa). C) Generalized pattern of serotonin immunoreactive perikarya in a generic naidid trunk segment. Full circles represent cells that are always or almost always present, while half-circles represent cells whose presence varies among species, individuals and/or segments. D) Nervous system structure of a typical trunk segment for each of the 12 species studied. Diagrams show typical pattern of serotonin immunoreactive perikarya of the ganglion (colored according to putative homology group assignments shown in C), location of peripheral nerve roots and location of septa. The phylogenetic relationships among the species are shown to the right and are based on recent molecular analyses, as described in the Methods section.