of chains of organisms and the chamber's shape. The orders were subsequently revised to conform better to Linnean taxonomy, placing related organisms more closely together. This was made possible as scientific advances permitted the identification of distinctive traits in organisms across Eisenack's groups. Features of the base and neck, the presence of spines, and perforations or connections are now considered the most useful diagnostic features. Chitinozoans are placed into two orders. The order Operculatifera includes those with an operculum over the aperture and no distinct neck. The order Prosomatifera includes those with a clearly discernable neck and an internal prosome. Affinities Young graptolites The graptolites are colonial organic walled fossils which also occurred from the Ordovician to the Devonian; only part of their life cycle is known and it is not clear how they reproduced. It has been suggested that the Chitinozoa may represent the pre-sicula stages of graptolites - the period between the colony's sexual reproduction, and the formation of a new colony. This hypothesis appears to be supported by the co-occurrence of graptolite and chitinozoan fossils, whose abundances appear to mirror one another. The similar chemical composition of the fossils has been seized by both sides of the argument. Proponents suggest that the use of the same chemical framework is an indicator that the two may be related. However, this factor means that situations favouring the preservation of one will also tend to preserve the other - and the preparation techniques used to extract the fossils will also favour or disfavour the two groups equally. Therefore, the apparent co-occurrence of the two fossils may merely be an artifact of their similar composition. The hypothesis struggles to explain the continuing abundance of chitinozoans after the middle Devonian, when graptolites became increasingly rare. Eggs The test of the Chitinozoa was fixed - there was no scope for any parts of it to move or rotate. This makes it seem likely that the tests were containers, to protect whatever was inside - whether that was a "hibernating" or encysted organism, or a clutch of hatching eggs. There are several arguments behind an association of the chitinozoans with annelids or gastropods, and it is not impossible that the chitinozoans are a convergent phenomenon laid by both groups. In fact, the spirally coiled nature of chitinozoan chains has been used to suggest that they were laid by a spirally coiled organism, such as