Fig. 3

Schematic representations of the gonozooid, nutritive structures and embryonic development in crisiid cyclostomes. a Early primary embryo surrounded by multilayered ‘envelope’ inside of non-altered membranous sac (corresponding to Figs. 4b, c, 7a, b); b Formation of coenocytic elements inside and outside of the multilayered ‘envelope’; membranous sac wall is partially incorporated into the developing nutritive tissue (corresponding to Figs. 4d, 7c‒e). c Early nutritive tissue (corresponding to Figs. 4e, 5a, insert) (in a‒c distal part of growing gonozooid is not depicted). d Primary embryo producing secondary embryos via fission inside growing nutritive tissue (corresponding to Figs. 4f, 5b, insert, c); developing coenocytic elements of the membranous sac roof are surrounded by the ‘upper cell complex’ (see Fig. 7f). e Fully-formed gonozooid filled with secondary embryos and young larvae embedded into the nutritive tissue (corresponding to Figs. 4g, H, 6i, 9i, j); dark solitary cells are visible on the periphery of the nutritive tissue (see Fig. 9g, h). In all images, basal lamina shown by red line. c, coelom; cw, cystid wall; e, embryo; la, larva; mc, ‘mesothelial’ cells; ms, membranous sac; nt, nutritive tissue; pc, pseudocoel; uc, ‘upper cell complex’