The morphology of the colony in the different graptolite orders is variable but 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 the group. A comparison of graptolites with coelenterates indicates that their similarities are only an expression of convergence. The arguments of Bohlin and observations of Decker are considered erroneous. The fine structure of the skeleton, its chemical composition (protein substances), and the manner of zooid budding are the same in graptolites and in pterobranchs demonstrating that the Graptolithina and the Pterobranchia are closely related groups of the phylum Hemichorda.

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