The cyphonautes larva of the marine bryozoan Membranipora membranacea. I. General morphology, body wall, and gut

Abstract

The cyphonautes larva of Membranipora membranacea (phylum Bryozoa, order Cheilostomata) is a laterally compressed, planktotrophic larva covered by a triangular, bivalved shell. In this paper, the general morphology of the larva and the cytology of the body wall and gut are examined by light and electron microscopy. The outer part of the body wall underlying the shell consists of an essentially nonciliated layer, called the aboral epithelium. At the larval apex, the aboral epithelium forms a knob-shaped apical organ. The apical organ contains putative sensory cells, a basal nerve plexus, and a group of undifferentiated epithelial cells that probably corresponds to a blastema. The body wall at the base of the larva constitutes the larval locomotory organ or corona. The corona consists of two rows of multiciliated cells, which are flanked by myoepithelial cells and monociliated cells. A large mantle cavity, referred to as the vestibule, occupies the central region of the larva. The vestibule is partially divided into an anterior inhalant chamber and a posterior exhalant chamber by two ciliary ridges that run along the sides of the mantle cavity. At the base of the larva, the inhalant chamber is surrounded by a horseshoe-shaped flap of tissue, called the velum. Apically, the inhalant chamber forms a densely ciliated preoral funnel. Food particles filtered by the ciliary ridges are conveyed to the larval mouth by cilia in the preoral funnel. The mouth in turn leads into a tripartite gut consisting of an esophagus, stomach, and intestine. Two types of ciliated lining cells occur in the esophagus and stomach. The intestine appears to be lined by nonciliated epithelial cells and empties into the exhalant chamber by way of an anus. Morphological features of the cyphonautes larva are compared with those described for other bryozoan larvae in an attempt to define homologous structures among the various larvae of bryozoans.