Fig. 12

Ultrastructural details of the outer (A, B) and circumoral nerve rings (C, D) of the anterior nerve center of Owenia borealis. A, C Perikarya, which are surrounded by supportive cells and their projections. B, D The basal portions of neuropil. Varicose of neurite of second type is shown by double arrowheads. Synaptic-like structures, which are characterized by cell membrane density and by synaptic vesicle concentration, are indicated by arrowheads. Abbreviations: dcv – dense-core synaptic vesicle; dsv – dense synaptic vesicles; ecm – extracellular matrix; edg – electron dense granules; if – intermediate filaments; lsv – light synaptic vesicle; mch – mitochondrion; msv – synaptic vesicle with content of middle electron density; nld – neurite of large diameter; nsd – neurite of small diameter; pkn – nucleus of perikaryon; scp – supportive cell; sv – synaptic vesicle