The cyphonautes larva of the marine bryozoan Membranipora membranacea. II. Internal sac, musculature, and pyriform organ

Abstract

The internal sac, muscles, and pyriform organ of the cyphonautes larva of Membranipora membranacea (phylum Bryozoa, order Cheilostomata) were examined by light and electron microscopy. The internal sac is an ovoid, tripartite organ that lies near the larval gut. The posterior and lateral parts of the sac are composed of nonglandular epithelial cells and are referred to as the roof and wall regions, respectively. The neck region occurring at the anterior end of the sac is packed with large secretory granules and is folded posteriorly into the lumen surrounded by the roof and wall regions. A previously undescribed network of ciliated cells, which may constitute part of an excretory organ, occurs between the internal sac and the gut. Four major sets of muscles are present in the larva: (i) a single adductor muscle that attaches to the two valves of the larval shell and contains non-striated myofibers with conspicuous dense bodies, (ii) a pair of nonstriated sac muscles that extend between the shell and the roof region of the internal sac, (iii) two sets of striated lateral muscles which originate near the center of the shell and ramify along the sides of the larva, and (iv) a median band of striated myofibers located along both the anterior and posterior margins of the larva. The pyriform organ is a large neuroglandular complex located at the anterior end of the larva. The organ consists of (i) a median ciliated groove with an anteriorly positioned tuft of long cilia, (ii) numerous glandular cells surrounding the ciliated groove, and (iii) several discrete tracts of nerves with accompanying muscles. The cytological features of the internal sac, larval muscles, and pyriform organ are compared with those described for other bryozoan larvae.