Fig. 5

Overall organization of the terminal arm segment of A. kochii. (a) Low magnification view of a cross section. Toluidine blue. (b–d) Transmission electron microscopy. (b) Parasagittal section. Two opposing arrows indicate the level where the fusion between the ectoneural neuroepithelium and the epidermis takes place. (c) Cross section through the sphincter in the water-vascular canal at a level indicated by a dotted line in (b). (d) Detailed view of a parasagittal section showing the fusion between the epineural cuticle (cn) and the cuticle of the surface epidermis (ce). It approximately corresponds to the boxed area in (A). ce—cuticle of the epidermis; cn—epineural cuticle; e—surface epidermis; ec—epineural canal; ee—epineural epithelium; en—ectoneural neuroepithelium of the radial nerve cord; pbe—epidermis of the pit bottom; s—sphincter in the water-vascular canal