128 



J.D. TAYLOR, Y.I. KANTOR AND A.V. SYSOEV 



Characters of the rhynchocoel 



In all toxoglossans there is a permanent cavity in the anterior 

 part of the body called the rhynchodeal cavity or rhynchocoel 

 (Fig. 1). It contains the proboscis and is maintained even 

 when the proboscis is extended. The rhynchodeal cavity 

 opens to the exterior via the rhynchostome, which is situated 

 at the ventral margin of the head. The walls of the rhynchoc- 

 oel (rhynchodeum) are usually thick and muscular. 



Rhynchostomal sphincter 



This an annular, muscular sphincter which encircles the 

 mouth of the rhynchocoel (Fig. 1). It is present in most 

 species of Turridae, Terebridae, Pervicaciidae and Conidae, 

 but absent in the turrids Clavatula diadema and Tomopleura 

 violacea and the pervicaciids Pervicacia tristis, 'Terebra' nas- 

 soides, and 'T. ' capen-sis. In these latter pervicaciids and 

 some turrids without a prominent sphincter, for example 

 Tomopleura, the anterior part of the rhynchodeum is very 

 muscular. 



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, e.g. Philbertia linearis, P. leufroyi, 

 P. gracilis, Cenodagreutes, Daphnella reeveana (Smith, 1967; 

 Sheridan et ai, 1973; unpublished observations), in Hemi- 

 lienardia mailed (Mangeliinae) and in all Terebridae and 

 Pervicaciidae (Miller, 1975, 1980; Taylor, 1990). We have not 

 seen an introvert in any other subfamily of Turridae (except 

 perhaps for Ophiodermella, see above), or in the Conidae. In 

 species of Daphnellinae the introvert is fairly short, but in 

 some terebrids, for example Terebra maculata, the introvert 



Fig. 3 Hemilienardia malleti; extended rhynchodeal introvert, 

 forming a pseudoproboscis in a relaxed, critical-point dried 

 specimen. Scale bar = 100 um. 



is very long, and when retracted, lies coiled in the rhynchoc- 

 oel (Miller, 1970). 



In those animals possessing a rhynchodeal introvert, the 

 outer and inner walls are joined by radial muscles (Fig. 5). In 

 Turridae, the possession of an introvert is associated with a 

 reduction in size or complete loss of the proboscis. However, 

 within the Terebridae, even those species with a well- 

 developed proboscis possess an introvert. 



Epithelium of the rhynchodeum 



In some Turridae, there is a distinct division in the character 

 of the epithelium lining the inner wall of the rhynchocoel. In 

 the anterior part of the cavity the epithelial cells are high and 



