184 DE. J. GILCHRIST ON THE MINUTE STRTTCTUEE OF 



Dr. Bethe in other groups ; and surface sections would doubt- 

 less throw light on this point. 



The osphradium o£ Aplysia was found to be a simple sensory 

 organ peculiarly suited to the method of investigation adopted. 

 It is a small circular patch of epithelium surrounded by a ring 

 of dark pigment situated at the anterior end of the ctenidium. 

 Immediately under this epithelium is a ganglion, which is little 

 more than a collection of cells on the nerve coming from the 

 right visceral ganglion (fig. 9, g). 



Various attempts have been made to discover the connexion 

 between the osphradial epithelium and the underlying ganglion 

 in the Mollusca, and peculiar small cells have been described 

 in the former, A successful methylene-blue preparation demon- 

 strated without doubt the existence of intra-epithelial struc- 

 tures (fig. 9, a) and showed their true nature, viz., that they are 

 the peripheral endings of sensory cells, the nucleated bodies of 

 which lie at a greater or less depth under the epithelium, and the 

 lower offshoots of which penetrate the ganglion. Certain of the 

 basal fibres of these cells M-ere observed (fig. 9, I) to pass out 

 at the side of the ganglion, sometimes in bundles, sometimes 

 singly. The corresponding peripheral sensory processes of 

 their parent cells were found within the neighbouring epithelium, 

 beyond the region of the osphradial pit proper; and they pre- 

 sent a structural condition intermediate between the short cells 

 of the epithelium over the ganglion and the longer sensory cells 

 of the general epithelium afore described. Perhaps these are 

 to be regarded as indicative of a former condition, in which 

 the osphradial epithelium was of much greater extent than 

 now *. 



The sensory cells seemed to break up into fine branches within 

 the ganglion (fig. 9, c), and the majority of the cells of the latter 

 are similar to the elements already described for other ganglia. 

 The outer extremities of the sensory cells, as will be seen from 

 tig. 10, project a considerable distance beyond the epithelium, 

 and present a bent and swollen condition, probably the result 

 of treatment in staining and fixation, suggestive of Plemming's 

 " pinselformige Zelien." In one case the nucleated body of a 

 sensory cell appeared to lie within the epithelium ; but this was 

 doubtful, and may have been due to the fact that the ordinary 



* I liaTe elsewhere giTCu reasons for believing that we have here to do with 

 a degenerate organ. Cf. Jenaische Zeitschr. Bd. xxviii. p. 40S. 



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