. Bulletin of the British Museum (Natural History) Zoology. 128 J. A. ALLEN & F. J. HANNAH. Fig. 7 L. pustulosa pustulosa: a) Transverse section through the combined siphon; b) Transverse section through the siphonal tentacle. (Scale = mm). A narrow band of longitudinal muscle fibres lie immediately internal to the basal membrane of the outer epithelium. Internal to the longitudinal layer, the muscles consist of short circular fibres with the occasional transverse fibre between inner and outer basal membranes. Scattered throughout muscle layer are subepithelial gland cells (Fig. 7a). A
. Bulletin of the British Museum (Natural History) Zoology. 128 J. A. ALLEN & F. J. HANNAH. Fig. 7 L. pustulosa pustulosa: a) Transverse section through the combined siphon; b) Transverse section through the siphonal tentacle. (Scale = mm). A narrow band of longitudinal muscle fibres lie immediately internal to the basal membrane of the outer epithelium. Internal to the longitudinal layer, the muscles consist of short circular fibres with the occasional transverse fibre between inner and outer basal membranes. Scattered throughout muscle layer are subepithelial gland cells (Fig. 7a). Although considerably contracted and difficult to discern, there is some evidence to suggest that longitudinal muscle blocks are separated by haemocoelic channels. The faecal rod takes up all of the contracted siphonal space (Fig. 7a). We have no evidence of a functional separation formed by a pair of opposing mid-lateral ridges into upper exhalent and a lower inhalent parts as occurs in some protobranchs. The gill axes posteriorly join with the midlateral section of the siphons (Fig. 6) and the posterior gill plates possibly act to channel the faeces into the siphon. The anus lies close to the anterior limit of the siphon. The siphonal tentacle, derived from the middle sensory fold of the mantle, is a fine thread-like structure that tapers to a point. It originates from a pocket in the right ventral side of the siphonal embayment. In transverse section, it comprises a large central nerve surrounded by fine muscle fibres inter- spersed with gland cells and connective tissue covered by an outer layer of glandular epithelial cells (Fig. 7b). A specialized region of the mantle, the feeding aperture, lies ventral to the siphon. Here the inner muscular mantle fold is greatly extended, a secondary fold is also present that forms an arch defining the internal margin of the feeding aperture (Figs. 4a & 7a). A large concentration of acidophilic gland cells are found on this innermost
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