or protrusions, which are sometimes as large as the chamber itself. The range and complexity of ornament increased with time, against a backdrop of decreasing organism size. The earliest Ordovician species were large and smooth-walled; by the mid-Ordovician a large and expanding variety of ornament, and of hollow appendages, was evident. While shorter appendages are generally solid, larger protrusions tend to be hollow, with some of the largest displaying a spongy internal structure. However, even hollow appendages leave no mark on the inner wall of the organisms: this may suggest that they were secreted or attached from the outside. There is some debate about the number of layers present in the organisms' walls: up to three layers have been reported, with the internal wall often ornamented; some specimens only appear to display one such wall layer. The multitude of walls may indeed reflect the construction of the organism, but could be a result of the preservational process. "Immature" or juvenile examples of chitinozoans have not been found; this may suggest that either they did not "grow", that they were moults (unlikely), or that the fossilisable parts of the organism only formed after the developmental process was complete. However in 2019 a study found that morphological variation of specimens of Desmochitina likely represented a growth series. Many chitinozoans are found as isolated fossils, but chains of multiple tests, joined from aperture to base, have been reported for all genera. Very long chains twist into helical (spring-shaped) forms. Occasionally, clusters or condensed chains are found, packed in an organic "cocoon". Classification Alfred Eisenack's original description of the Chitinozoans placed them in three families, spanning seven genera, based on morphological grounds. Further genera were identified, at first on an annual basis, as time progressed. Since its publication in 1931, Eisenack's original classification has been much honed by these additional discoveries, as well as advances in microscopy. The advent of the scanning electron microscope in the 1970s allowed the improved detection of surface ornamentation which is hugely important in identification - as can be appreciated by a comparison of the images on this page. Even the light microscope image here is of far greater quality than could have been achieved earlier in the century, using poorly preserved specimens and less advanced microscopes. The original three families proposed by Eisenack represented the best classification possible with available data, based largely on the presence or absence