Body Wall Morphology of the Sertellid Cheilostome Bryozoan,Reteporellina evelinae

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Abstract
Autozoids in R. evelinae are budded as in most cheilostomes, but some lateral zoid buds systematically become empty kenozoids that wrap around onto the basal side of the colony. Each kenozoid contributes partly to the formation of a younger autozoid. Bud suppression and kenozoid formation lead to an erect, branching, stony colony. Kenozoids calcify heavily and may bud other kenozoids or autozoids in old colony parts; these probably are adaptations to 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 the avicularium contact the frontal cuticle (epitheca), from which the mandible differentiates. The hypostegal cavity of the autozoid remains in communication with the perigastric cavity only through avicularia. Rare lateral avicularia similarly differentiate from pores connecting autozoids and basal kenozoids. All zoids heavily thicken secondarily; cuticles between adjacent zoids simultaneously are thrown into interlocking longitudinal folds which probably strengthen the colony mechanically. Despite continuing carbonate secretion, the cystid epidermis is tenuous and incomplete; the peritoneum is absent or poorly-developed. This situation is usual among gymnolaemate Bryozoa. The bryozoan body cavity is therefore usually a pseudocoel in the morphological sense, but developmental evidence indicates it represents a degenerate coelom. The ovicell develops as a compressed evagination from the body cavity of the autozoid distal to the zoid which deposits its egg in the ovicell. Both inner and outer walls calcify, th outer more heavily than the inner. Secondary calcification is achieved by engulfment of the ovicell by a fold of hypostegal coelom which calcifies basally. There is no nutritive median vesicle, so embryos develop without food. The polypide is unique in lacking a vestibule; this may be associated with the absense of a medial vesicle.

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