The body wall of cheilostome bryozoa. I. The ectocyst of Watersipora nigra (Canu and Bassler)

The cuticle of Watersipora nigra is at first translucent, but it later becomes black and differentiates into two layers. It is composed, at least in part, of a protein‐polysaccharide complex. Calcified parts are three‐layered: (1) an outer, cuticular layer, (2) a calcium carbonate skeleton deposited on a matrix of acid mucopolysaccharide, and (3) a “skeletal membrane.” The relationships of these layers indicate that the skeleton is intracuticular. A layer of cuticular material, the “intercalary cuticle” is present in lateral walls, but not transverse walls; it may become calcified in some species. The cuticles of calcified and uncalcified parts of cheilostomes are not necessarily homologous.