Body Wall Morphology of the Sertellid Cheilostome Bryozoan,Reteporellina evelinae

Autozoids in R. evelinae are budded as in most cheilostomes,but some lateral zoid buds systematically become empty kenozoidsthat wrap around onto the basal side of the colony. Each kenozoidcontributes partly to the formation of a younger autozoid. Budsuppression and kenozoid formation lead to an erect, branching,stony colony. Kenozoids calcify heavily and may bud other kenozoidsor autozoids in old colony parts; these probably are adaptationsto protect the otherwise delicate basal sides of autozoids. Large frontal avicularia are produced from two areolae (pores)in frontal walls of autozoids. Interior walls delimiting theavicularium contact the frontal cuticle (epitheca), from whichthe mandible differentiates. The hypostegal cavity of the autozoidremains in communication with the perigastric cavity only throughavicularia. Rare lateral avicularia similarly differentiatefrom pores connecting autozoids and basal kenozoids. All zoids heavily thicken secondarily; cuticles between adjacentzoids simultaneously are thrown into interlocking longitudinalfolds which probably strengthen the colony mechanically. Despitecontinuing carbonate secretion, the cystid epidermis is tenuousand incomplete; the peritoneum is absent or poorly-developed.This situation is usual among gymnolaemate Bryozoa. The bryozoanbody cavity is therefore usually a pseudocoel in the morphologicalsense, but developmental evidence indicates it represents adegenerate coelom. The ovicell develops as a compressed evagination from the bodycavity of the autozoid distal to the zoid which deposits itsegg in the ovicell. Both inner and outer walls calcify, th outermore heavily than the inner. Secondary calcification is achievedby engulfment of the ovicell by a fold of hypostegal coelomwhich calcifies basally. There is no nutritive median vesicle,so embryos develop without food. The polypide is unique in lackinga vestibule; this may be associated with the absense of a medialvesicle.