Homology and Evolution of the Chaetae in Echiura (Annelida)
Fig 2
Ventral chaetae of Echiurus echiurus (A, B) and Thalassema thalassemum (C-F).
A. Extended focus light micrograph of a critical point dried part of a specimen showing the apical hooks of the ventral chaetae. B. Extended focus light micrograph, frontal view of a pair of dissected ventral chaetae (Ch) showing the chaetal radial muscles (rm) and interconnecting muscle (icm). Chaetal sacs are delicate and transparent, arrow marks a developing chaeta. C. Maximum projection of a phalloidin (cyan) stained and chitin autofluorescent (yellow to light blue) dissected pair of ventral chaetae, frontal view, CLSM. Apical microvilli of the chaetoblast marked by arrow heads. Large arrows mark developing chaetae, small arrows mark the collar of the ventral chaetae. Inset: early chaetogenesis at higher magnification. D.-F. Light micrographs of semi-thin (0.5 μm) toluidine blue stained parasagittal sections. D. Apical part of ventral chaeta (Ch), chaetal sac with follicles (bold arrows, cf) and developing chaeta (dCh). Large arrows mark the uppermost follicle cells. Note muscles running between follicles. E. Basal part of a completed ventral chaeta, large arrows mark follicle cells with their strong bundles of intermediate filaments, small arrows mark microvilli brush border of chaetoblast. F. Developing chaeta (dCh) with chaetoblast (cb) plus microvilli brush border (small arrows) and follicle cells (fc). Note that intermediate filaments are absent at this stage. Coelothel (ct) surrounds the follicle except for the apical section. bwm body wall muscles, coel coelom, Cu cuticle, ep epidermis, ecm extracellular matrix, en enamel, icm interconnecting muscle, lm longitudinal muscle, rm radial muscle.