Figure 1

Overview of musculature of Ophryotrocha diadema. Scale bars = 50 μm, anterior is to the left. A. Dorsal view, phalloidin staining. The most prominent muscles in the prostomium are the ventral diagonal muscles (dVLM). Two transverse muscles are present in the prostomium: PStm2 crosses the dVLM dorsally, whereas PStm1 lies between the dorsal and the ventral longitudinal muscles. The complex musculature of the ventrally situated pharynx (phar) is heavily stained. The inner strands of the dorsal longitudinal muscles (DLM) cross repeatedly at the dorsal midline (circles). Four pairs of parapodia are present, with prominent parapodial muscle complexes (pmc). Transverse (tm) and diagonal muscles (dm) cross the dorsal side irregularly. Several transverse muscles surround the posterior zone of segment proliferation (arrowhead). The last transverse muscle circle marks the pygidium (tagged arrow). B. Ventral view, phalloidin staining (red) and anti-acetylated-tubulin staining (green). The ciliary bands are stained by anti-acetylated tubulin antibody. These are the prototroch (prot) of the prostomium, the metatroch (met) and the ciliary band of the segment like annulus within the peristomium (pcb), and the four ciliary bands of the chaetigers (1–4 BS) anterior to the telotroch (tel) of the pygidium. The boxed region marks the close-up depicted in Figure C (vnc-ventral nerve cord, VLM-ventral longitudinal muscle) C. Close-up from B. Ventral view, phalloidin staining and anti-acetylated tubulin antibody depth coded (in μm). Upper panel shows the ventral nerve cord (vnc), lower panel shows the median ventral longitudinal muscle (mVLM). Depth coding indicates that the mVLM is situated dorsal (deep) to the vnc (circles). D. Histological cross section through chaetiger of an adult. The dorsal longitudinal muscles (DLM) lie just below the epidermis (epi). The uniramous parapodia (ppd) are ventrolaterally situated. The ventral longitudinal muscles (VLM) lie on either side of the ventral nerve cord (vnc). E. Close up from D. Same labeling as in D, additionally labeled are the dorsally situated transverse muscles (tm). F. Close up from E depicting the dorsal longitudinal muscle (DLM) and a transverse muscle (tm) lying above it (supralongitudinally). G. 3D-reconstruction from a series of histological sections in the same body-region as shown in D. Anterior is toward the top of the figure. Depicted are the dorsal longitudinal muscles (DLM) with their dorsomedian crossing strands (arrow), diagonal muscles (dm) and transverse muscles (tm) in a supralongitudinal position. The ventral longitudinal muscles (VLM) and the median ventral longitudinal muscle (mVLM) surround the ventral nerve cord (vnc – only the main tracts of the neuropile shown here). The anterior and posterior dorsal parapodial muscles (adPM, pdPM) stretch diagonally toward posterior and anterior respectively, underneath the dorsal longitudinal muscles.