Ultrastructure of the coenecium of Cephalodiscus (Hemichordata: Pterobranchia)

The ultrastructure of the coenecia of Cephalodiscus (Cephalodiscus) hodgsoni Ridewood, 1907, Cephalodiscus (Idiothecia) nigrescens Lankester, 1905, and Cephalodiscus (Orthoecus) densus Andersson, 1907 was characterized using light microscopy, transmission electron microscopy, and scanning electron microscopy. The coenecium of Cephalodiscus is composed of layers of coenecial material of variable thickness laid down one upon the next and separated by sheets. Thick fusellar-like layers (up to 160 mu m thick) and thin cortical-like layers (down to 15 nm thick) are present, but do not form two distinct components. Instead, a continuum exists in the thickness and shape of these layers. At the ultrastructural level, both fusellar-like and cortical-like layers are composed of thin (16-23 nm) long and straight fibrils, similar to the fibrils described in extant Rhabdopleura Allman, 1869. In C. densus, fibrils in the outer secondary deposits show a parallel arrangement, similar to the arrangement of fibrils in the graptolite eucortex. Although similarities in the shape and arrangement of growth increments between Cephalodiscus, Rhabdopleura, and graptolites probably reflect homologous zooidal behaviors and secretion mechanisms, differences at the ultrastructural level show that fibril types and fibril arrangement can evolve independently from larger scale features of the coenecium.