Fig. 2

Borehole apertures with apertural rim in Penetrantia japonica from Japan A–D and Penetrantia clionoides from Guam E–F. A–B Stereomicroscopic images of P. japonica with prominent apertural rims sitting in the sutures of shells of the gastropod Tegula rugata. C–D Scanning electron microscopic (SEM) images of apertural rims in P. japonica. E SEM images of the apertural rim in P. clionoides. F SEM image of developing borehole aperture of an autozooid in P. clionoides. Abbreviations anc – ancestrula, apr – apertural rim, bha – bore hole aperture, bt – boring trace, exc – exterior cuticle, op – operculum, su – suture of gastropod shell