a–d Gross anatomy of Indosphenia kayalum a left side after removal of the shell b left side after removal of shell and mantle c exterior view of the siphonal apertures d interior view of the siphonal apertures e Gross anatomy of Sphenia binghami from the left side after removal of the shell and anterior mantle. Abbreviations: aa, anterior adductor muscle. exh, exhalant siphon. ft, foot. id, inner demibranch. inh, inhalant siphon. irt, inner ring of tentacles. lp, labial palps. od, outer demibranch. ort, outer ring of tentacles. pa, posterior adductor muscle. per, periostracum. pg, pedal gape. ppr, posterior pedal retractor muscle. rod, reflected portion of outer demibranch. sph, siphons. sphm, siphonal muscles.