A b s t r a c t
The main characters of morphology, onto- and astogeny, and the chemical composition of the skeleton of graptolites are reviewed and discussed. Although the morphology of the colony is variable in different order of graptolites, their essential characters, such as the initial stages of development, the fine structure and chemical composition of the skeleton are the same in all representatives of this great group of Paleozoic animals. This denotes that the graptolites constitute one coherent group. A comparison of graptolites with coelenterates indicates that their similarities are only an expression of convergence. Moreover they concern almost exclusively the general aspect of the colony. The arguments advanced by Bohlin (1950) supporting the relationships of graptolites with coelenterates are decidedly rejected. The observations of Decker (1956, 1957), who seemed to distinguished on the graptolites rhabdosomes preserved zooids morphologically similar to hydroid polyps, are considered as wholly erroneous. The fine structure of the skeleton, its chemical composition (protein substances) and the manner of zooid budding are in graptolites the same as in pterobranchs. All this demonstrates conclusively that the Graptolithina and the Pterobranchia correspond to two closely related groups representing the phylum Hemichorda. Both were individualized at the beginning of the Ordovician period. |
