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Department of Anatomy, St George’s Hospital Medical School, Cranmer Terrace, London SW17 0RE, U.K.
There is a considerable difference in the ultrastructure of fusellar tissue between fossil graptolites and modern pterobranchs. The phylogenetic significance of these differences is uncertain. It was thought that the ultrastructure of the earliest, and therefore probably the simplest, stage of formation of the pterobranch prosiculum might provide additional relevant clues. The larva escapes from the original dome-shaped seal by producing a hole, and immediately produces a series of fuselli around the aperture. The zooid uses the cephalic shield to construct the tube. The size of the original fuselli is very variable, but soon achieves adult size. The fuselli are very similar to those of adult erect tubes, and have oblique sutures. There is much extraneous material incorporated into the wall. It is proposed that the fibrils of the stroma of the coenecium somehow mature after they have been secreted. Histochemical tests suggest that the stroma contains a considerable amount of acid and sulphated mucopolysaccharides. Tests for chitin and keratin have been uniformly negative. Similar positive histochemical reactions have been achieved on the cephalic shield. Adult tube building and repair are reported for the first time.
