Cephalopod Adaptations-The Record and Its Interpretation

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Abstract
Three subclasses of cephalopods should be recognized[long dash]Nautiloidea, Ammonoidea, and Coleoidea (Dibranchiata). The keynote of evolution of the cephalopods is adaptation to various habits of life. Their phylogeny is characterized by progressive coiling. The usually empty chambers of their shells were sealed off from the siphuncle and thus served to buoy up the end of the shell. This tipped the animal up, led him to secrete shell more rapidly on his ventral side, and resulted in the production of a curved, and finally of a coiled, shell. Some, at least, of the straight shelled forms lived in a horizontal position, as proved by the color patterns on their shells; and most of them probably swam above the surface, rather than crawled on the bottom. Cyrtoceras probably crawled on the bottom, with the shell pointing upward. Other types floated and swam at a greater or less distance above the bottom, with the shell upward. The complex septa of some forms were probably developed to give a better attachment of the body to the shell.

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