To be a transit link: Similarity in the structure of colonial system of integration and communication pores in autozooids and avicularia of Terminoflustra membranaceotruncata (Bryozoa: Cheilostomata)

Bryozoan colonies consist of zooids, which can differ in structure and function. Most heteromorphic zooids are unable to feed and autozooids supply them with nutrients. The structure of the tissues providing nutrient transfer is poorly investigated. Here, I present a detailed description of the colonial system of integration (CSI) and communication pores in autozooids and avicularia of the cheilosome bryozoan Terminoflustra membranaceotruncata. The CSI is the nutrient transport and distribution system in the colony. In both autozooids and avicularia it consists of a single cell type, that is, elongated cells, and has a variable branching pattern, except for the presence of a peripheral cord. The general similarity in the CSI structure in avicularia and autozooids is probably due to the interzooidal type of the avicularium. Interzooidal avicularia are likely to consume only a part of the nutrients delivered to them by the CSI, and they transit the rest of the nutrients further. The variability and irregularity of branching pattern of the CSI may be explained by the presence of single communication pores and their varying number. The structure of communication pores is similar regardless of their location (in the transverse or lateral wall) and the type of zooid in contact. Rosette complexes include a cincture cell, a few special cells, and a few limiting cells. Along each zooidal wall, there are communication pores with both unidirectional and bidirectional polarity of special cells. However, the total number of nucleus-containing lobes of special cells is approximately the same on each side of any zooidal wall. Supposing the polarity of special cells reflects the direction of nutrient transport, the pattern of special cells polarity is probably related to the need for bidirectional transport through each zooidal wall. The possibility for such transport is important in large perennial colonies with wide zones of autozooids undergoing polypide degeneration.