Some features of the ultrastructure of the coenecium of Rhabdopleura compacta.

The coenecium of Rhabdopleura consists of a series of tubes, some erect and some repent. These tubes are composed of rings, one stacked within another. The rings are smooth on the inside surface and rough outside. Newly laid down rings are thin and smooth on both surfaces, fibrous material is laid down on the external surface during growth in thickness by the cephalic shield of the zooid. The erect tubes remain discrete, but the repent tubes, which are attached to the substratum can become incorporated in a mass of secreted material. The external vertical fibres cross several rings and probably serve to anchor the stack. Besides these fibrils that run for several segments, there are other shorter fibres that run along the length of each cylindrical ring, and are not continuous across the rings. These long and short fibres have features in common with those found in the graptolites.