FOREGUT ANATOMY AND CLASSIFICATION OF CONOIDEA 



149 



radular membrane are extremely diverse compared with the 

 'lower' conoideans. Moreover, this relative diversity has 

 steadily increased throughout the Cenozoic (Sysoev, 1991). 

 This suggests that higher conoideans may possess some 

 adaptive advantages. In our opinion these advantages lie in 

 the features of the morphology of the radular diverticulum. 



The higher Conoidea lack a subradular membrane, and the 

 radular diverticulum is divided into two different parts; the 

 radular sac and radular caecum (also known as long and short 

 arms). The caecum serves for the storage of the fully-formed, 

 marginal teeth. Many teeth can be stored; for example, in a 

 specimen of Mitromorpha (Mitrolumna) sp. there were 106 

 teeth in the radular sac compared with 64 in the caecum 

 (Kantor & Sysoev, 1990). Species of higher Conoidea can 

 probably use several teeth in each feeding act. For example, 

 observations on the feeding of Conus textile showed that up to 

 17 teeth can be used in the same attack (Schoenberg, 1981). 

 By contrast, in lower turrids, there is no caecum and probably 

 no more than a single tooth can be used in each feeding act. 

 Predatory attacks by higher Conoidea are thus likely to be 

 more successful, and the mechanism of prey capture probably 

 more efficient. This may explain the relative success of the 

 higher Conoidea. 



The feeding and diets of gastropods of this functional type 

 are well known (Oliviera et al. 1990) and it is unnecessary to 

 describe the process in detail. Only the most important 

 morphological features should be noted. These are the vesti- 

 gial, or completely reduced, radular membrane; the absence 

 of an odontophore; the presence of a radular caecum where 

 the fully-formed marginal teeth are stored, and a well- 

 developed oral sphincter for gripping the teeth. The radula is 

 represented by hollow, marginal teeth. The tooth ligament 

 (long flexible stalk attached to the tooth base) is probably the 

 rudiment of the radular membrane (Fig. 23c). Also the 

 gastropods of this group often have enlarged rhynchostomal 

 lips. In some species, the lips are able to invert (i.e. to form 

 an introvert or pseudoproboscis) and this is also used in prey 

 capture. It should be noted, that some vermivorous species of 

 Conus (Marsh, 1970) and the fish-feeding C. geographus 

 (Johnson & Stablum, 1971) do not stab they prey in every 

 feeding act. This is possibly an initial stage of transition to 

 feeding mechanism type 5. 



Usually, teeth are gripped at the proboscis tip by the buccal 

 tube sphincter, but in some turrids the buccal tube introvert 

 (valvule of Sheridan et al., 1973) is involved (Fig. 9). This 

 structure has been reported so far in Mangelia nebula 

 (Sheridan et al., 1973; Delaunois & Sheridan, 1989) and in 

 Eucithara stromboides (Fig. 10). It is also possible, that the 

 buccal tube introvert can be everted through the mouth 

 opening and have a role in holding the prey. 



After envenomation, the prey may be held by the tooth 

 itself, as occurs in many vermivorous species of Conus 

 (Kohn, 1959), or with the mouth. The buccal lips may play a 

 role in the transport of prey to the buccal cavity. These are 

 highly protrusive in many Mangeliinae, and at least in M. 

 nebula (Fig. 9) can be retracted into the buccal cavity 

 (Delaunois & Sheridan, 1989). A similar possibility was 

 described for Oenopota by Bogdanov (1990), who suggested 

 that the buccal lips and the proboscis itself might be inverted 

 into the buccal cavity. 



II. Venom gland absent 



Feeding mechanism Type 4 



Gastropods of this group have a radula with a well-developed 

 radular membrane and a proboscis may be either present or 

 reduced. According to the position of the buccal mass they 

 can be divided into two sub-types. 



Conoideans of the first sub-type which at present includes 

 only Strictispira and probably Cleospira, have the buccal mass 

 located at the tip of a well-developed proboscis (Fig. 13). The 

 buccal mass and radular apparatus are large, with two large 

 odontophoral cartilages and massive odontophoral and pro- 

 boscis retractor muscles. The radula has a strong membrane 

 with two rows of solid, awl-shaped, marginal teeth. The 

 buccal tube is very short and there are no oral sphincters. 

 Apart from the record of polychaete setae in two individuals 

 of Strictispira paxillus (Maes, 1983), nothing is known of the 

 habits of this group. 



The terminal position of the buccal mass on the muscular 

 proboscis, the short buccal tube and the massive radular 

 apparatus, suggest that when the gastropod is feeding the 

 radula is protracted out of the extended proboscis tip. The 

 solid teeth and absence of venom apparatus suggest that the 

 radula is involved in biting and tearing rather than stabbing. 

 The feeding mechanism is thus probably more similar to 

 other neogastropods such as the Buccinidae rather than to 

 other conoideans. 



Conoideans of the second sub-type differ from these of the 

 first one in possessing a basal buccal mass. The radula is 

 well-developed, whilst the proboscis is either absent or highly 

 reduced, and a rhynchostomal introvert is usually present. 

 This feeding mode is found in the Pervicaciidae. The diet of 

 this family is largely unknown, except for 'Terebra' nassoides 

 which feeds on capitellid polychaetes (Taylor, 1990). 



In the Pervicaciidae, the absence of a proboscis means that 

 the rhynchodeal introvert becomes the main organ of prey 

 capture, as occurs in some proboscis-less terebrids such as T. 

 gouldi (Miller, 1975). Prey are presumably pulled into the 

 rhynchocoel by the introvert. In Duplicaria spectabilis there 

 are large muscular buccal lips and probably a protrusive 

 odontophore (Taylor, 1990, fig. 7). However, in Pervicacia 

 tristis and Duplicaria kieneri there is a septum with a narrow 

 aperture dividing the rhynchocoel and it is very unlikely that 

 the odontophore can be protruded through the septum. 

 Although we have no direct evidence, it is possible that the 

 septum functions to hold prey during swallowing and perhaps 

 early digestion. 



Feeding mechanism Type 5 



Finally, there are many conoideans which lack a radula, 

 venom and salivary glands. Gastropods of this group include 

 some Daphnellinae, Taraninae and some Terebridae. In 

 addition to the absence of foregut glands and radula, a 

 characteristic feature of these species is the very reduced size 

 or complete absence of the proboscis. Radula-less Conoidea 

 either have well-developed, rhynchostomal lips or a large 

 rhynchostomal introvert, as for example, in the Terebridae 

 (Miller, 1975) or Philbertia linearis (Sheridan et al., 1973). It 

 is possible, that a rhynchostomal introvert is also present in 

 Teretiopsis, although all sectioned specimens have it in the 

 extended position and it was overlooked during the original 

 description (Kantor & Sysoev, 1989). In some turrids, such as 



