Fig. 16

Schemes of several hypothetical steps of bryozoan evolution. I) Hypothetical bryozoan ancestor with three body parts: epistome, lophophore with tentacles, and the trunk. Each part has paired ganglion. The cerebral ganglion of recent bryozoans forms as a result of aggregation of these three paired ganglia. II) Possible scenario of evolution of the bryozoan lophophore. Both directions of evolution are correct. a Horseshoe-shaped lophophore known in some recent phylactolaemates. b Bell-shaped lophophore with numerous tentacles. c Lophophore of A. gracilis. III) Gross anatomy of the nervous system of the lophophore in the lophophorates: phoronids (a), brachiopods (b), and bryozoans (c). The hypothetical bryozoan ancestor had a horseshoe-shaped lophophore, six tentacular nerves, and an outer nerve. Abbreviations: abn - accessory brachial nerve; an - anus; bt - “border” (“frontier”) tentacle; cg - cerebral ganglion; cm - commissure; con - circum-oral nerve ring; dg - dorsal ganglion; eg - epistomal ganglion corresponds to distal zone of cerebral ganglion; epi - epistome; laf - latero-abfrontal nerve of tentacle; lbn - lower brachial nerve; ldl = lh - lophophoral dorso-lateral nerves homologous to the “lophophoral horns”; lf - latero-frontal nerve of tentacle; lg - lophophoral ganglion corresponds to central zone of the cerebral ganglion; lh - “lophophoral horns” (main nerve tract of the lophophore arm); lph - lophophore; maf - medio-abfrontal nerve of tentacle; m - mouth; mbn - main brachial nerve; mf - medio-frontal nerve of tentacle; mnr - minor nerve ring; on - outer nerve ring; tg - trunk ganglion corresponds to the proximal zone of the cerebral ganglion; tnr - tentacular nerve ring; trk - trunk; tsn - tentacle sheath nerve; tsr - tentacle sheath nerve ring