ON THE ULTRASTRUCTURE OF THE CEREBRAL GAN GLION OF ENCHYTRAEUS-BUCHHOLZI (OLIGOCHAETA)

The outermost layer of the cerebral ganglion of Enchytraeus buchholzi consists of extensions of glial cells (type I) covered by a discontino us muscle layer and cells of the peritoneal epithelium. The amount and arrangement of glial cells, neurons and neuropile differ in the vario us regions of the ganglion. Most pericarya of neurons are localized in the caudal region surrounded by projections of glial cells. The neuro pile is concentrated mainly in the middle portion of the ganglion. A s ingle large cell (Q-cell) situated on each side of the brain shows a b right fluorescence when treated with dibromobimane indicating the pres ence of sulfur containing neuropeptides (neurosecretion). Q-cells are characterized by numerous membrane-bound ''neurosecretory granules'' a nd a well developed rough endoplasmatic reticulum. However, the cytopl asm of the other neurons contains numerous more or less electron dense granules of similar shape. In the axons of the neuropile two types of synaptic vesicles (electron lucent, dense core vesicles) as well as ' 'neurosecretory'' granules occur. Two types of glial cells can be dist inguished: 1) cells with large pericarya and electron lucent cytoplasm present only in few numbers (type I). Their projections cover the neu rons and extend in the neuropile, where they possess glial filaments; 2) small more electron dense cells (type II) close to the neuropile ex tending projections between the axons. Both contain typical glia grana . In the anterior portion of the brain four to five receptor cells are found having microvilli and numerous cilia, which show a 8 x 2 + 2 mi crotubule-pattern.