. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 182 L. P. NEZLIN ET AL. Figure 8. FMRFa-ir (A) and LEnk-ir (B) cells in the gill epithelium ofAnucliiiiUi lygncii, and LEnk-ir (C) cell in the mantle epithelium of Bitccinwn undaliim. FITC-labeling. Arrowheads label the borders of the epithelium. Scale bars = 20 jim. be a conservative feature of homologous nerve cells (Sak- harov, 1970). In the simple os| f iitonnu. all primary sen- sory neurons labeled \v; a to the two neuropep- tides are neuroepithelial ^- is: that is, they are situated within the epithelium of the organ.
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 182 L. P. NEZLIN ET AL. Figure 8. FMRFa-ir (A) and LEnk-ir (B) cells in the gill epithelium ofAnucliiiiUi lygncii, and LEnk-ir (C) cell in the mantle epithelium of Bitccinwn undaliim. FITC-labeling. Arrowheads label the borders of the epithelium. Scale bars = 20 jim. be a conservative feature of homologous nerve cells (Sak- harov, 1970). In the simple os| f iitonnu. all primary sen- sory neurons labeled \v; a to the two neuropep- tides are neuroepithelial ^- is: that is, they are situated within the epithelium of the organ. This situation refers to the FMRFa-ir sensory cells in the osphradium ofAm- piillariiis and Bncciniiiu, and to some degree to the LEnk- ir sensory cells. In pulmonates, which are more recent gastropod molluscs, homologous primary sensory cells are true ganglionic neurons incorporated into the osphradial ganglion (Nezlin el al. 1994). Similarly, two ultrastruc- turally different types of primary sensory neurons were demonstrated underneath the osphradial epithelium in the opisthobranch Aplysia californica (Theler et al., 1987). The concentration of specific nerve elements in specific sites and the migration of neuroepithelial cells into central nervous structures are both widely recognized general trends of neural evolution. The sensory cells of the mol- luscan osphradia seemingly fit well into this model. But there are exceptions like the patellid limpets, which are considered primitive prosobranchs. The cell bodies of their osphradial sensory neurons are situated underneath the epithelium, as in the pulmonates (Haszprunar, 1985). Comparative studies of osphradia might help to elucidate the factors that govern the behavior of neuroepithelial cells, which sometimes migrate from their sites of origin and sometimes, on the contrary, firmly retain their epi- thelial position. Reports reviewed by Haszprunar (1985) indicate that, at least in primitive prosobranchs, osphradia are acti
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