Tentacle structure and filter-feeding in Crisia eburnea and other cyclostomatous bryozoans, with a review of upstream-collecting mechanisms

The upstream-collecting filter-feeding mechanisms occurring in many aquatic organisms are not adequately descnbed.Tentacles of Crisia eburnea and other cyclostome bryozoans, with only lateral and laterofrontal ciliary bands, are among the least complicated upstream-collecting systems among metazoans.SEM and TEM revealed that the tentacles have 2 rows of very closely set lateral cilia and 1 row of stiff laterofrontal cilia on each side.The shape of the basal membrane and the longitudinal muscles indicate that the tentacles are specialized for flicking movements directed towards the centre of the tentacle crown.Video observations of C. eburnea feeding on Rhodomonas cells showed charactenstic velocity gradients around the tentacle crown.Particles in the central current galn the highest velocity at the entrance to the tentacle crown from where the speed decreases to zero in front of the mouth.Usually the path of a particle deviates from the downward course to a more outwards directed course (between the tentacies), where they may be irclppeci by iile iiiiel iarnied by the stiff Ieteiofron:~!cilia; the tentacle then makes a flick that brings the particle into the central current and further down towards the mouth.A survey of the literature shows that a similar mechanical filter mechanism occurs also in gymnolaemate bryozoans and their cyphonautes larvae, but that the particle-collecting mechanism of larvae and adults of phoronids, brachiopods, pterobranchs, and enteropneusts is different.The differences in structure and functlon between the tentacles of ectoprocts and those of phoronids, brachiopods and pterobranchs support the idea that the 2 types of tentacle crowns are not homologous.