. Bulletin of the Natural History Museum Zoology. Fig. 2 Ophiodermella inermis; longitudinal section of the anterior rhynchodaeum showing the posteriorly-situated, rhynchostomal sphincter located on an introvert-like structure. Abbreviations: in, introvert; m, mouth; p, proboscis; r, rhynchostome; s, sphincter. Position of rhynchostomal sphincter In the normal condition, the sphincter is usually situated around the rhynchostome, but in some turrids (for example in Glyphostoma, Borsonia, Lophiotoma, Pontiothauma and Thatcheria) it is situated more posteriorly. In Ophiodermella inermis (but not
. Bulletin of the Natural History Museum Zoology. Fig. 2 Ophiodermella inermis; longitudinal section of the anterior rhynchodaeum showing the posteriorly-situated, rhynchostomal sphincter located on an introvert-like structure. Abbreviations: in, introvert; m, mouth; p, proboscis; r, rhynchostome; s, sphincter. Position of rhynchostomal sphincter In the normal condition, the sphincter is usually situated around the rhynchostome, but in some turrids (for example in Glyphostoma, Borsonia, Lophiotoma, Pontiothauma and Thatcheria) it is situated more posteriorly. In Ophiodermella inermis (but not O. ogurana) and Suavodrillia kennicotti the moderately large, posteriorly situated, rhynchostomal sphinc- ter is probably able to evert, forming a sort of 'rhynchostomal introvert' but situated in the middle part of the rhynchocoel (Fig. 2). The ability to evert is indicated by the presence of a well-developed layer of longitudinal muscles underlying the epithelium and by the existence of free space between the sphincter and the longitudinal muscle layers. This structure may demonstrate the possible origin of the true rhynchodeal introvert (see below) or alternatively be an autapomorphy for the species. Rostrum In the some fish-feeding species of Conus, the anterior part of the rhynchocoel is elastic and can be greatly extended to accomodate large food items during preliminary digestion. This extensible feature, known as the rostrum, cannot be inverted into the rhynchocoel. Rhynchodeal introvert (= labial tube or pseudoproboscis) In this structure, the rhynchostomal lips are mobile and can be retracted into the rhynchocoel by infolding, or extended as a tube (Figs 3 & 4). The introvert is found in nearly all the species which we and others have studied from the turrid sub-family Daphnellinae, Philbertia linearis, P. leufroyi, P. gracilis, Cenodagreutes, Daphnella reeveana (Smith, 1967; Sheridan et ai, 1973; unpublished observations), in Hemi- lienardia mailed (Mangelii
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