On metamorphosis and ancestrula formation in cyclostomatous bryozoans

Abstract

Abstract Studies on larvae and ancestrula formation in species of Crisia, Crisiella, Tubulipora, Berenicea, and Disporella, representing three of the five major groups within the Cyclostomata, have revealed a uniform pattern of development within this group. The larvae, which are ciliated on the whole surface, show an anterior invagination covered by a fine cuticle and a posterior invagination known as the adhesive sac. The larvae swim for a rather short period and settle on a substratum by everting the adhesive sac; the upper invagination becomes everted simultaneously and its cuticle comes to cover the whole upper side of the primary disc; the ciliated epithelium becomes enclosed in a ring-shaped lumen within the primary disc. A layer of cells below the epithelium of the upper invagination grows rapidly, folds together into a sack-shape and becomes two-layered; the organ thus formed is the polypide bud. The lower part of the polypide bud elongates and forms the alimentary canal while the upper part gives rise to the lophophore; the ganglion is formed as a narrow invagination from the inner, basal part of the lophophore. An invagination of the epithelium of the cuticle forms the vestibule. The outer layer of the polypide bud differentiates into the mesodenn of the polypide and the membranous sac. The anatomy of adults of Crisia and Crisiella has been investigated and it is concluded that the membranous sac represents the mesoderm of the cystid. The acellular part of this sac is interpreted as the basement membrane. The entosaccal cavity is, thus, the only coelomic space.